ci: update publish workflow

.github/workflows/ci.yml

@@ -9,112 +9,30 @@ env:
   RUSTFLAGS: --deny warnings
   RUSTDOCFLAGS: --deny warnings
 jobs:
-  # Run tests
-  test:
-    name: Tests
+  build-and-test:
     runs-on: ubuntu-latest
-    timeout-minutes: 30
+    env:
+      SCCACHE_GHA_ENABLED: "true"
+      RUSTC_WRAPPER: "sccache"
     steps:
-      - name: Checkout repository
-        uses: actions/checkout@v4
-      - name: Install Rust toolchain
+      - name: Checkout code
+        uses: actions/checkout@v5
+      - name: Install Rust
         uses: dtolnay/rust-toolchain@stable
-      - name: Install dependencies
-        run: |
-          sudo apt-get update
-          sudo apt-get install --no-install-recommends \
-            libasound2-dev libudev-dev libwayland-dev \
-            libxkbcommon-dev
-      - name: Populate target directory from cache
-        uses: Leafwing-Studios/cargo-cache@v2
         with:
-          sweep-cache: true
+          toolchain: stable
+          components: clippy, rustfmt
+      - name: Run sccache-cache
+        uses: mozilla-actions/sccache-action@v0.0.9
       - name: Install cargo-nextest
-        run: cargo install cargo-nextest --locked
-      - name: Run tests with nextest
+        uses: taiki-e/install-action@cargo-nextest
+      - name: Run Clippy
+        run: cargo clippy --locked --workspace --all-targets --all-features -- -D warnings
+      - name: Run formatting
+        run: cargo fmt --all --check
+      - name: Run Tests
         run: |
-          cargo nextest run \
-            --all-features \
-            --all-targets
-          # Workaround for https://github.com/rust-lang/cargo/issues/6669
-          cargo test \
-            --locked \
-            --workspace \
-            --all-features \
-            --doc
-  # Run clippy lints
-  clippy:
-    name: Clippy
-    runs-on: ubuntu-latest
-    timeout-minutes: 30
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v4
-      - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
-        with:
-          components: clippy
-      - name: Install dependencies
-        run: |
-          sudo apt-get update
-          sudo apt-get install --no-install-recommends \
-            libasound2-dev libudev-dev libwayland-dev \
-            libxkbcommon-dev
-      - name: Populate target directory from cache
-        uses: Leafwing-Studios/cargo-cache@v2
-        with:
-          sweep-cache: true
-      - name: Run clippy lints
-        run: |
-          cargo clippy \
-            --locked \
-            --workspace \
-            --all-features \
-            -- \
-            --deny warnings
-  # Check formatting
-  format:
-    name: Format
-    runs-on: ubuntu-latest
-    timeout-minutes: 30
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v4
-      - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
-        with:
-          components: rustfmt
-      - name: Run cargo fmt
-        run: |
-          cargo fmt \
-            --all \
-            -- \
-            --check
-  # Check documentation
-  doc:
-    name: Docs
-    runs-on: ubuntu-latest
-    timeout-minutes: 30
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v4
-      - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
-      - name: Install dependencies
-        run: |
-          sudo apt-get update
-          sudo apt-get install --no-install-recommends \
-            libasound2-dev libudev-dev libwayland-dev \
-            libxkbcommon-dev
-      - name: Populate target directory from cache
-        uses: Leafwing-Studios/cargo-cache@v2
-        with:
-          sweep-cache: true
-      - name: Check documentation
-        run: |
-          cargo doc \
-            --locked \
-            --workspace \
-            --all-features \
-            --document-private-items \
-            --no-deps
+          cargo nextest run --all-features --all-targets
+          cargo test --locked --workspace --all-features --doc
+      - name: Check Documentation
+        run: cargo doc --locked --workspace --all-features --document-private-items --no-deps

.github/workflows/publish.yml

@@ -4,7 +4,7 @@ on:
   push:
     tags:
       # Pattern syntax: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#filter-pattern-cheat-sheet
-      - "v[0-9]+.[0-9]+.[0-9]+*"
+      - "v[0-9]*.[0-9]*.[0-9]*"
   # Trigger this workflow manually via workflow dispatch.
   workflow_dispatch:
     inputs:
@@ -15,10 +15,9 @@ on:
 jobs:
   audit:
     name: Audit
-    runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
       - uses: actions-rust-lang/audit@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
@@ -28,9 +27,17 @@ jobs:
       - audit
     runs-on: ubuntu-latest
     timeout-minutes: 25
+    env:
+      SCCACHE_GHA_ENABLED: "true"
+      RUSTC_WRAPPER: "sccache"
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
       - uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: stable
+          components: clippy, rustfmt
+      - name: Run sccache-cache
+        uses: mozilla-actions/sccache-action@v0.0.9
       - name: cargo-release Cache
         id: cargo_release_cache
         uses: actions/cache@v4
@@ -57,12 +64,5 @@ jobs:
         # allow-branch HEAD is because GitHub actions switches
         # to the tag while building, which is a detached head
 
-        run: |-
-          cargo release \
-            publish \
-            --workspace \
-            --all-features \
-            --allow-branch HEAD \
-            --no-confirm \
-            --no-verify \
-            --execute
+        run: |
+          cargo release publish --workspace --all-features --allow-branch HEAD --no-confirm --no-verify --execute

Cargo.lock

@@ -50,9 +50,23 @@ checksum = "37909eebbb50d72f9059c3b6d82c0463f2ff062c9e95845c43a6c9c0355411be"
 
 [[package]]
 name = "filecaster"
-version = "0.1.0"
+version = "0.2.3"
+dependencies = [
+ "filecaster-derive",
+ "merge",
+ "serde",
+ "serde_json",
+ "tempfile",
+ "toml",
+ "trybuild",
+]
+
+[[package]]
+name = "filecaster-derive"
+version = "0.2.3"
 dependencies = [
  "claims",
+ "filecaster",
  "merge",
  "proc-macro-error2",
  "proc-macro2",
@@ -60,8 +74,6 @@ dependencies = [
  "serde",
  "serde_json",
  "syn",
- "tempfile",
- "toml",
 ]
 
 [[package]]
@@ -76,6 +88,12 @@ dependencies = [
  "wasi",
 ]
 
+[[package]]
+name = "glob"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0cc23270f6e1808e30a928bdc84dea0b9b4136a8bc82338574f23baf47bbd280"
+
 [[package]]
 name = "hashbrown"
 version = "0.15.4"
@@ -270,6 +288,12 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "target-triple"
+version = "0.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1ac9aa371f599d22256307c24a9d748c041e548cbf599f35d890f9d365361790"
+
 [[package]]
 name = "tempfile"
 version = "3.20.0"
@@ -283,6 +307,15 @@ dependencies = [
  "windows-sys 0.59.0",
 ]
 
+[[package]]
+name = "termcolor"
+version = "1.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "06794f8f6c5c898b3275aebefa6b8a1cb24cd2c6c79397ab15774837a0bc5755"
+dependencies = [
+ "winapi-util",
+]
+
 [[package]]
 name = "toml"
 version = "0.9.2"
@@ -322,6 +355,21 @@ version = "1.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fcc842091f2def52017664b53082ecbbeb5c7731092bad69d2c63050401dfd64"
 
+[[package]]
+name = "trybuild"
+version = "1.0.110"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "32e257d7246e7a9fd015fb0b28b330a8d4142151a33f03e6a497754f4b1f6a8e"
+dependencies = [
+ "glob",
+ "serde",
+ "serde_derive",
+ "serde_json",
+ "target-triple",
+ "termcolor",
+ "toml",
+]
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.18"
@@ -337,6 +385,15 @@ dependencies = [
  "wit-bindgen-rt",
 ]
 
+[[package]]
+name = "winapi-util"
+version = "0.1.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cf221c93e13a30d793f7645a0e7762c55d169dbb0a49671918a2319d289b10bb"
+dependencies = [
+ "windows-sys 0.59.0",
+]
+
 [[package]]
 name = "windows-sys"
 version = "0.59.0"

Cargo.toml

@@ -1,40 +1,20 @@
-[package]
-name = "filecaster"
-version = "0.1.0"
-edition = "2024"
-authors = ["Kristofers Solo <dev@kristofers.xyz>"]
-description = "Procedural macro to derive configuration from files, with optional merging capabilities."
-repository = "https://github.com/kristoferssolo/filecaster"
-documentation = "https://docs.rs/filecaster"
-homepage = "https://github.com/kristoferssolo/filecaster"
-license = "MIT OR Apache-2.0"
-keywords = ["proc-macro", "derive", "configuration", "file-parsing"]
-categories = ["rust-patterns", "parsing", "config"]
-exclude = ["/.github", "/.gitignore", "/tests", "*.png", "*.md"]
-readme = "README.md"
+[workspace]
+resolver = "2"
+members = ["filecaster", "filecaster-derive"]
 
-[features]
-default = []
-merge = ["dep:merge"]
-
-[dependencies]
-proc-macro2 = "1.0"
-quote = "1.0"
-proc-macro-error2 = "2.0"
-syn = { version = "2.0", features = ["extra-traits", "parsing"] }
-serde = { version = "1.0", features = ["derive"] }
-merge = { version = "0.2", optional = true }
-
-[dev-dependencies]
+[workspace.dependencies]
+filecaster-derive = { version = "0.2", path = "filecaster-derive" }
+serde = { version = "1.0", features = ["derive"], default-features = false }
+merge = "0.2"
+# dev-dependencies
+filecaster = { path = "filecaster" }
 claims = "0.8"
 serde_json = "1.0"
-tempfile = "3.20"
+tempfile = "3.10"
 toml = "0.9"
+trybuild = "1.0"
 
-[lib]
-proc-macro = true
-
-[lints.clippy]
+[workspace.lints.clippy]
 pedantic = "warn"
 nursery = "warn"
 unwrap_used = "warn"

README.md

@@ -5,45 +5,71 @@ Procedural macro to derive configuration from files, with optional merging capab
 ## Features
 
 - **Derive Configuration:** Easily load configuration from files into your Rust structs.
-- **Default Values:** Specify default values for struct fields using the `#[default = "..."]` attribute.
+- **Default Values:** Specify default values for struct fields using the `#[from_file(default = "...")]` attribute.
 - **Optional Merging:** When the `merge` feature is enabled, allows merging multiple configuration sources.
 
 ## Usage
 
 ```toml
 [dependencies]
-filecaster = "0.1"
+filecaster = "0.2"
 ```
 
 ```rust
 use filecaster::FromFile;
 
-#[derive(Debug, Clone, FromFile)]
-pub struct MyConfig {
-    #[from_file(default = "localhost")]
-    pub host: String,
+#[derive(Debug, Clone, PartialEq, FromFile)]
+struct AppConfig {
+    /// If the user does not specify a host, use `"127.0.0.1"`.
+    #[from_file(default = "127.0.0.1")]
+    host: String,
+
+    /// Port number; defaults to `8080`.
     #[from_file(default = 8080)]
-    pub port: u16,
-    #[from_file(default = false)]
-    pub enabled: bool,
+    port: u16,
+
+    /// If not set, use `false`. Requires `bool: Default`.
+    auto_reload: bool,
 }
 
 fn main() {
-    // Simulate loading from a file (e.g., JSON, YAML, TOML)
-    let file_content = r#"
-        {
-            "host": "localhost"
-        }
-    "#;
+    // Simulate file content (e.g., from a JSON file)
+    let file_content = r#"{ "host": "localhost", "port": 3000 }"#;
 
-    let config_from_file: MyConfig = serde_json::from_str(file_content).unwrap();
-    let config = MyConfig::from_file(Some(config_from_file));
+    // The `AppConfigFile` struct is automatically generated by `#[derive(FromFile)]`.
+    // It has all fields as `Option<T>`.
+    let partial_config: AppConfigFile = serde_json::from_str(file_content).unwrap();
+    let partial_config2 = partial_config.clone();
 
-    println!("Config: {:?}", config);
-    // Expected output: Config { host: "localhost", port: 8080, enabled: false }
+    // Use the generated `from_file` method to get the final config.
+    // Default values are applied for missing fields.
+    let config = AppConfig::from_file(Some(partial_config));
+    // or
+    let config: AppConfig = partial_config2.into();
+
+    assert_eq!(config.host, "localhost");
+    assert_eq!(config.port, 3000);
+    assert_eq!(config.auto_reload, false); // `bool::default()` is `false`
+
+    println!("Final Config: {:#?}", config);
+
+    // Example with no file content (all defaults)
+    let default_config = AppConfig::from_file(None);
+    assert_eq!(default_config.host, "127.0.0.1");
+    assert_eq!(default_config.port, 8080);
+    assert_eq!(default_config.auto_reload, false);
 }
 ```
 
+## Examples
+
+Use `cargo run --example <example_name>` to execute a specific example. For example:
+
+```bash
+cargo run --example simple
+cargo run --example nested
+```
+
 ## Documentation
 
 Full documentation is available at [docs.rs](https://docs.rs/filecaster).

filecaster-derive/Cargo.toml

@@ -0,0 +1,35 @@
+[package]
+name = "filecaster-derive"
+version = "0.2.3"
+edition = "2024"
+authors = ["Kristofers Solo <dev@kristofers.xyz>"]
+description = "Procedural derive macro for `filecaster`: automatically implement `FromFile` for your structs."
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/kristoferssolo/filecaster"
+homepage = "https://github.com/kristoferssolo/filecaster"
+documentation = "https://docs.rs/filecaster-derive"
+readme = "../README.md"
+keywords = ["proc-macro", "derive", "configuration", "file-parsing"]
+categories = ["rust-patterns", "parsing", "config"]
+exclude = ["/.github", "/.gitignore", "/tests", "*.png", "*.md"]
+
+[lib]
+proc-macro = true
+
+[features]
+default = ["serde"]
+serde = ["dep:serde"]
+merge = ["dep:merge"]
+
+[dependencies]
+proc-macro2 = "1.0"
+quote = "1.0"
+proc-macro-error2 = "2.0"
+syn = { version = "2.0", features = ["extra-traits", "parsing"] }
+serde = { workspace = true, optional = true }
+merge = { workspace = true, optional = true }
+
+[dev-dependencies]
+claims.workspace = true
+serde_json.workspace = true
+filecaster.workspace = true

filecaster-derive/src/from_file.rs

@@ -0,0 +1,258 @@
+use proc_macro2::TokenStream;
+use quote::{format_ident, quote};
+use syn::{
+    Attribute, Data, DeriveInput, Error, Expr, Field, Fields, FieldsNamed, GenericParam, Generics,
+    Ident, Lit, Meta, MetaList, Result, Type, parse_quote,
+};
+
+const WITH_MERGE: bool = cfg!(feature = "merge");
+const WITH_SERDE: bool = cfg!(feature = "serde");
+
+/// Entry point: generate the shadow struct + `FromFile` impls.
+pub fn impl_from_file(input: &DeriveInput) -> Result<TokenStream> {
+    let name = &input.ident;
+    let vis = &input.vis;
+    let generics = add_trait_bounds(input.generics.clone());
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+    let file_ident = format_ident!("{name}File");
+
+    let fields = extract_named_fields(input)?;
+    let (field_assignments, file_fields) = process_fields(fields)?;
+
+    let derive_clause = build_derive_clause();
+
+    Ok(quote! {
+        #derive_clause
+        #vis struct #file_ident #ty_generics #where_clause {
+            #(#file_fields),*
+        }
+
+        impl #impl_generics filecaster::FromFile for #name #ty_generics #where_clause {
+            type Shadow = #file_ident #ty_generics;
+
+            fn from_file(file: Option<Self::Shadow>) -> Self {
+                let file = file.unwrap_or_default();
+                Self {
+                    #(#field_assignments),*
+                }
+            }
+        }
+
+        impl #impl_generics From<Option<#file_ident #ty_generics>> for #name #ty_generics #where_clause {
+            fn from(value: Option<#file_ident #ty_generics>) -> Self {
+                <Self as filecaster::FromFile>::from_file(value)
+            }
+        }
+
+        impl #impl_generics From<#file_ident #ty_generics> for #name #ty_generics #where_clause {
+            fn from(value: #file_ident #ty_generics) -> Self {
+                <Self as filecaster::FromFile>::from_file(Some(value))
+            }
+        }
+    })
+}
+
+/// Ensure we only work on named-field structs
+fn extract_named_fields(input: &DeriveInput) -> Result<&FieldsNamed> {
+    match &input.data {
+        Data::Struct(ds) => match &ds.fields {
+            Fields::Named(fields) => Ok(fields),
+            _ => Err(Error::new_spanned(
+                &input.ident,
+                r#"FromFile only works on structs with *named* fields.
+Tuple structs and unit structs are not supported."#,
+            )),
+        },
+        _ => Err(Error::new_spanned(
+            &input.ident,
+            r#"FromFile only works on structs.
+Enums are not supported."#,
+        )),
+    }
+}
+
+/// Build the shadow field + assignment for one original field
+fn build_file_field(field: &Field) -> Result<(TokenStream, TokenStream)> {
+    let ident = field
+        .ident
+        .as_ref()
+        .ok_or_else(|| Error::new_spanned(field, "Expected named fields"))?;
+    let ty = &field.ty;
+
+    let default_override = parse_from_file_default_attr(&field.attrs)?;
+
+    let field_attrs = if WITH_MERGE {
+        quote! { #[merge(strategy = merge::option::overwrite_none)] }
+    } else {
+        quote! {}
+    };
+
+    // Nested struct -> delegate to its own `FromFile` impl
+    let shadow_ty = quote! { <#ty as filecaster::FromFile>::Shadow };
+    let field_decl = quote! {
+        #field_attrs
+        pub #ident: Option<#shadow_ty>
+    };
+
+    let assign = build_file_assing(ident, ty, default_override);
+
+    Ok((field_decl, assign))
+}
+
+fn build_file_assing(ident: &Ident, ty: &Type, default_override: Option<Expr>) -> TokenStream {
+    if let Some(expr) = default_override {
+        return quote! {
+            #ident: file.#ident.map(|inner| <#ty as filecaster::FromFile>::from_file(Some(inner))).unwrap_or(#expr)
+        };
+    }
+    quote! {
+        #ident: <#ty as filecaster::FromFile>::from_file(file.#ident)
+    }
+}
+
+/// Process all fields
+fn process_fields(fields: &FieldsNamed) -> Result<(Vec<TokenStream>, Vec<TokenStream>)> {
+    fields.named.iter().try_fold(
+        (Vec::new(), Vec::new()),
+        |(mut assignments, mut file_fields), field| {
+            let (file_field, assignment) = build_file_field(field)?;
+            file_fields.push(file_field);
+            assignments.push(assignment);
+            Ok((assignments, file_fields))
+        },
+    )
+}
+
+/// Derive clause for the shadow struct
+fn build_derive_clause() -> TokenStream {
+    let mut traits = vec![quote! {Debug}, quote! {Clone}, quote! {Default}];
+    if WITH_SERDE {
+        traits.extend([quote! { serde::Deserialize }, quote! { serde::Serialize }]);
+    }
+
+    if WITH_MERGE {
+        traits.push(quote! { merge::Merge });
+    }
+
+    quote! { #[derive( #(#traits),* )] }
+}
+
+/// Add Default bound to every generic parameter
+fn add_trait_bounds(mut generics: Generics) -> Generics {
+    for param in &mut generics.params {
+        if let GenericParam::Type(ty) = param {
+            ty.bounds.push(parse_quote!(Default));
+        }
+    }
+    generics
+}
+
+/// Attribute parsing: `#[from_file(default = ...)]`
+fn parse_from_file_default_attr(attrs: &[Attribute]) -> Result<Option<Expr>> {
+    for attr in attrs {
+        if !attr.path().is_ident("from_file") {
+            continue; // Not a #[from_file] attribute, skip it
+        }
+
+        // Parse the content inside the parentheses of #[from_file(...)]
+        return match &attr.meta {
+            Meta::List(meta_list) => parse_default(meta_list),
+            _ => Err(Error::new_spanned(
+                attr,
+                "Expected #[from_file(default = \"literal\")] or similar",
+            )),
+        };
+    }
+    Ok(None)
+}
+
+fn parse_default(list: &MetaList) -> Result<Option<Expr>> {
+    let mut default_expr = None;
+    list.parse_nested_meta(|meta| {
+        if meta.path.is_ident("default") {
+            let value = meta.value()?;
+            let expr = value.parse::<Expr>()?;
+
+            if let Expr::Lit(expr_lit) = &expr
+                && let Lit::Str(lit_str) = &expr_lit.lit
+            {
+                default_expr = Some(parse_quote! {
+                    #lit_str.to_string()
+                });
+                return Ok(());
+            }
+            default_expr = Some(expr);
+        }
+        Ok(())
+    })?;
+    Ok(default_expr)
+}
+
+#[cfg(test)]
+mod tests {
+    use claims::{assert_err, assert_none};
+    use quote::ToTokens;
+
+    use super::*;
+
+    #[test]
+    fn extract_named_fields_success() {
+        let input: DeriveInput = parse_quote! {
+            struct S { x: i32, y: String }
+        };
+        let fields = extract_named_fields(&input).unwrap();
+        let names = fields
+            .named
+            .iter()
+            .map(|f| f.ident.as_ref().unwrap().to_string())
+            .collect::<Vec<_>>();
+        assert_eq!(names, vec!["x", "y"]);
+    }
+
+    #[test]
+    fn extract_named_fields_err_on_enum() {
+        let input: DeriveInput = parse_quote! {
+            enum E { A, B }
+        };
+        assert_err!(extract_named_fields(&input));
+    }
+
+    #[test]
+    fn extract_named_fields_err_on_tuple_struct() {
+        let input: DeriveInput = parse_quote! {
+            struct T(i32, String);
+        };
+        assert_err!(extract_named_fields(&input));
+    }
+
+    #[test]
+    fn parse_default_attrs_none() {
+        let attrs: Vec<Attribute> = vec![parse_quote!(#[foo])];
+        assert_none!(parse_from_file_default_attr(&attrs).unwrap());
+    }
+
+    #[test]
+    fn process_fields_mixed() {
+        let fields: FieldsNamed = parse_quote! {
+            {
+                #[from_file(default = 1)]
+                a: u32,
+                b: String,
+            }
+        };
+        let (assign, file_fields) = process_fields(&fields).unwrap();
+        // two fields
+        assert_eq!(assign.len(), 2);
+        assert_eq!(file_fields.len(), 2);
+    }
+
+    #[test]
+    fn add_trait_bouds_appends_default() {
+        let gens: Generics = parse_quote!(<T, U>);
+        let new = add_trait_bounds(gens);
+        let s = new.to_token_stream().to_string();
+        assert!(s.contains("T : Default"));
+        assert!(s.contains("U : Default"));
+    }
+}

filecaster-derive/src/lib.rs

@@ -0,0 +1,115 @@
+//! # filecaster-derive
+//!
+//! `filecaster-derive` is the procedural macro crate for `filecaster`. It provides the
+//! `#[derive(FromFile)]` macro, which automates the process of loading partial
+//! configurations from files, merging them with default values, and constructing
+//! fully-populated Rust structs.
+//!
+//! This crate significantly simplifies configuration management by generating
+//! the necessary boilerplate code for the `FromFile` trait (defined in the
+//! `filecaster` crate).
+//!
+//! ## What it does
+//!
+//! For any struct with named fields, `#[derive(FromFile)]` generates:
+//!
+//! 1.  A companion "shadow" struct (e.g., `YourStructFile` for `YourStruct`)
+//!     where each field is wrapped in `Option<T>`. This shadow struct is
+//!     designed for deserialization from configuration files (e.g., JSON, TOML, YAML).
+//! 2.  An implementation of the `FromFile` trait for your original struct. This
+//!     includes the `from_file` method, which takes an `Option<YourStructFile>`
+//!     and constructs your final `YourStruct`. It intelligently fills in `None`
+//!     fields with either:
+//!     -   An expression you supply via `#[from_file(default = ...)]`.
+//!     -   `Default::default()` (if no `default` attribute is provided, requiring `T: Default`).
+//!
+//! ## Optional per-field defaults
+//!
+//! Use a `#[from_file(default = <expr>)]` attribute on any field to override
+//! the fallback value. You may supply any expression valid in that struct’s
+//! context. If you omit it, the macro will require the field's type to implement
+//! `Default` and will call `Default::default()`.
+//!
+//! ## Example
+//!
+//! ```rust
+//! use filecaster::FromFile;
+//!
+//! #[derive(Debug, Clone, PartialEq, FromFile)]
+//! struct AppConfig {
+//!     /// If the user does not specify a host, use `"127.0.0.1"`.
+//!     #[from_file(default = "127.0.0.1")]
+//!     host: String,
+//!
+//!     /// Port number; defaults to `8080`.
+//!     #[from_file(default = 8080)]
+//!     port: u16,
+//!
+//!     /// If not set, use `false`. Requires `bool: Default`.
+//!     auto_reload: bool,
+//! }
+//!
+//! fn example() {
+//!     // Simulate file content (e.g., from a JSON file)
+//!     let file_content = r#"{ "host": "localhost", "port": 3000 }"#;
+//!
+//!     // The `AppConfigFile` struct is automatically generated by `#[derive(FromFile)]`.
+//!     // It has all fields as `Option<T>`.
+//!     let partial_config: AppConfigFile = serde_json::from_str(file_content).unwrap();
+//!     let partial_config2 = partial_config.clone();
+//!
+//!     // Use the generated `from_file` method to get the final config.
+//!     // Default values are applied for missing fields.
+//!     let config = AppConfig::from_file(Some(partial_config));
+//!     // or
+//!     let config: AppConfig = partial_config2.into();
+//!
+//!     assert_eq!(config.host, "localhost");
+//!     assert_eq!(config.port, 3000);
+//!     assert_eq!(config.auto_reload, false); // `Default::default()` for bool is `false`
+//!
+//!     println!("Final Config: {:#?}", config);
+//!
+//!     // Example with no file content (all defaults)
+//!     let default_config = AppConfig::from_file(None);
+//!     assert_eq!(default_config.host, "127.0.0.1");
+//!     assert_eq!(default_config.port, 8080);
+//!     assert_eq!(default_config.auto_reload, false);
+//! }
+//! ```
+//!
+//! ## Feature flags
+//!
+//! -   `serde`: Enables `serde` serialization/deserialization support for the
+//!     generated shadow structs. This is typically required to deserialize
+//!     your configuration from file formats like JSON, TOML, or YAML.
+//! -   `merge`: If enabled, the generated shadow struct will also derive
+//!     `merge::Merge`. This allows you to layer multiple partial configuration
+//!     files together before calling `.from_file(...)`. Any field-level
+//!     `#[merge(...)]` attributes will be respected.
+//!
+//! ## Limitations
+//!
+//! -   Only works on structs with _named_ fields (no tuple structs or enums).
+//! -   All fields without a `#[from_file(default = ...)]` attribute must
+//!     implement the `Default` trait.
+
+mod from_file;
+
+pub(crate) use from_file::impl_from_file;
+use proc_macro::TokenStream;
+use proc_macro_error2::proc_macro_error;
+use syn::{DeriveInput, parse_macro_input};
+
+/// Implements the [`FromFile`] trait.
+///
+/// This macro processes the `#[from_file]` attribute on structs to generate
+/// code for loading data from files.
+#[proc_macro_error]
+#[proc_macro_derive(FromFile, attributes(from_file))]
+pub fn derive_from_file(input: TokenStream) -> TokenStream {
+    let inp = parse_macro_input!(input as DeriveInput);
+    impl_from_file(&inp)
+        .unwrap_or_else(|e| e.to_compile_error())
+        .into()
+}

filecaster/Cargo.toml

@@ -0,0 +1,31 @@
+[package]
+name = "filecaster"
+version = "0.2.3"
+edition = "2024"
+authors = ["Kristofers Solo <dev@kristofers.xyz>"]
+description = "Procedural macro to derive configuration from files, with optional merging capabilities."
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/kristoferssolo/filecaster"
+homepage = "https://github.com/kristoferssolo/filecaster"
+documentation = "https://docs.rs/filecaster"
+readme = "../README.md"
+keywords = ["configuration", "file-parsing"]
+categories = ["rust-patterns", "parsing", "config"]
+exclude = ["/.github", "/.gitignore", "/tests", "*.png", "*.md"]
+
+[features]
+default = ["serde", "derive"]
+derive = ["dep:filecaster-derive"]
+serde = ["dep:serde", "filecaster-derive/serde"]
+merge = ["dep:merge", "filecaster-derive/merge"]
+
+[dependencies]
+filecaster-derive = { workspace = true, optional = true }
+serde = { workspace = true, optional = true }
+merge = { workspace = true, optional = true }
+
+[dev-dependencies]
+serde_json.workspace = true
+tempfile.workspace = true
+toml.workspace = true
+trybuild.workspace = true

filecaster/examples/data/nested.json

@@ -0,0 +1,7 @@
+{
+  "key": "json key",
+  "number": 123,
+  "nested": {
+    "inner_number": 42
+  }
+}

filecaster/examples/data/nested.toml

@@ -0,0 +1,6 @@
+key = "toml key"
+number = 456
+
+[nested]
+inner_key = "inner toml key"
+inner_number = 99

filecaster/examples/data/simple.json

@@ -0,0 +1,4 @@
+{
+  "key": "json key",
+  "number": 123
+}

filecaster/examples/data/simple.toml

@@ -0,0 +1,2 @@
+number = 456
+exists = true

filecaster/examples/nested.rs

@@ -0,0 +1,59 @@
+use filecaster::FromFile;
+use std::fs;
+
+#[derive(Debug, FromFile)]
+pub struct InnerData {
+    #[from_file(default = "inner default")]
+    pub inner_key: String,
+    #[from_file(default = 42)]
+    pub inner_number: i32,
+}
+
+#[derive(Debug, FromFile)]
+pub struct MyData {
+    #[from_file(default = "default key")]
+    pub key: String,
+    #[from_file(default = 0)]
+    pub number: i32,
+    pub nested: InnerData,
+}
+
+fn main() {
+    // Get the absolute current directory
+    let current_dir = std::env::current_dir().expect("Failed to get current directory");
+    // Path to the data directory
+    let data_dir = current_dir.join("filecaster/examples/data");
+
+    // Paths to JSON and TOML files
+    let json_path = data_dir.join("nested.json");
+    let toml_path = data_dir.join("nested.toml");
+
+    // Read and parse JSON file
+    let json_content = fs::read_to_string(&json_path)
+        .unwrap_or_else(|e| panic!("Failed to read JSON file at {:?}: {}", json_path, e));
+    let json_data: MyData = serde_json::from_str::<MyDataFile>(&json_content)
+        .unwrap_or_else(|e| panic!("Failed to parse JSON in {:?}: {}", json_path, e))
+        .into();
+
+    // Read and parse TOML file
+    let toml_content = fs::read_to_string(&toml_path)
+        .unwrap_or_else(|e| panic!("Failed to read TOML file at {:?}: {}", toml_path, e));
+    let toml_data: MyData = toml::from_str::<MyDataFile>(&toml_content)
+        .unwrap_or_else(|e| panic!("Failed to parse TOML in {:?}: {}", toml_path, e))
+        .into();
+
+    // Output the parsed data
+    dbg!(&json_data);
+    dbg!(&toml_data);
+
+    // Example assertions (adjust based on your actual file contents)
+    assert_eq!(json_data.key, "json key");
+    assert_eq!(json_data.number, 123);
+    assert_eq!(json_data.nested.inner_key, "inner default");
+    assert_eq!(json_data.nested.inner_number, 42);
+
+    assert_eq!(toml_data.key, "toml key");
+    assert_eq!(toml_data.number, 456);
+    assert_eq!(toml_data.nested.inner_key, "inner toml key");
+    assert_eq!(toml_data.nested.inner_number, 99);
+}

filecaster/examples/simple.rs

@@ -0,0 +1,48 @@
+use filecaster::FromFile;
+use std::fs;
+
+#[derive(Debug, FromFile)]
+pub struct MyData {
+    #[from_file(default = "default key")]
+    pub key: String,
+    pub number: i32,
+    pub exists: bool,
+}
+
+fn main() {
+    // Get the absolute current directory
+    let current_dir = std::env::current_dir().expect("Failed to get current directory");
+    // Path to the data directory
+    let data_dir = current_dir.join("filecaster/examples/data");
+
+    // Paths to JSON and TOML files
+    let json_path = data_dir.join("simple.json");
+    let toml_path = data_dir.join("simple.toml");
+
+    // Read and parse JSON file
+    let json_content = fs::read_to_string(&json_path)
+        .unwrap_or_else(|e| panic!("Failed to read JSON file at {:?}: {}", json_path, e));
+    let json_data: MyData = serde_json::from_str::<MyDataFile>(&json_content)
+        .unwrap_or_else(|e| panic!("Failed to parse JSON in {:?}: {}", json_path, e))
+        .into();
+
+    // Read and parse TOML file
+    let toml_content = fs::read_to_string(&toml_path)
+        .unwrap_or_else(|e| panic!("Failed to read TOML file at {:?}: {}", toml_path, e));
+    let toml_data: MyData = toml::from_str::<MyDataFile>(&toml_content)
+        .unwrap_or_else(|e| panic!("Failed to parse TOML in {:?}: {}", toml_path, e))
+        .into();
+
+    // Output the parsed data
+    dbg!(&json_data);
+    dbg!(&toml_data);
+
+    // Example assertions (adjust based on your actual file contents)
+    assert_eq!(json_data.key, "json key".to_string());
+    assert_eq!(json_data.number, 123);
+    assert!(!json_data.exists); // `bool::default()` is `false`
+
+    assert_eq!(toml_data.key, "default key".to_string());
+    assert_eq!(toml_data.number, 456);
+    assert!(toml_data.exists);
+}

filecaster/src/lib.rs

@@ -0,0 +1,151 @@
+//! # filecaster
+//!
+//! `filecaster` provides the core `FromFile` trait, which is used in conjunction with the
+//! `filecaster-derive` crate to enable automatic deserialization and merging of
+//! configuration from various file formats into Rust structs.
+//!
+//! This crate defines the fundamental interface for types that can be constructed
+//! from an optional "shadow" representation, typically deserialized from a file.
+//! The `filecaster-derive` crate provides a procedural macro to automatically
+//! implement this trait for your structs, handling default values and merging logic.
+//!
+//! ## How it works
+//!
+//! The `FromFile` trait defines how a final configuration struct (`Self`) can be
+//! constructed from an optional intermediate "shadow" struct (`Self::Shadow`).
+//! The `filecaster-derive` macro generates this `Shadow` struct and the
+//! `from_file` implementation for your configuration types.
+//!
+//! When you derive `FromFile` for a struct, `filecaster-derive` creates a
+//! corresponding `YourStructFile` (the `Shadow` type) where all fields are
+//! wrapped in `Option<T>`. This `YourStructFile` can then be deserialized
+//! from a file (e.g., JSON, TOML, YAML) using `serde`.
+//!
+//! The `from_file` method then takes this `Option<YourStructFile>` and
+//! constructs your final `YourStruct`, applying default values for any fields
+//! that were `None` in the `YourStructFile`.
+//!
+//! ## Example
+//!
+//! While the `FromFile` trait is implemented via the `filecaster-derive` macro,
+//! here's a conceptual example of how it's used:
+//!
+//! ```rust,ignore
+//! use filecaster::FromFile;
+//! use serde::{Deserialize, Serialize};
+//!
+//! // This struct would typically have `#[derive(FromFile)]`
+//! // from the `filecaster-derive` crate.
+//! #[derive(Debug, Default, PartialEq, Serialize, Deserialize)]
+//! struct AppConfig {
+//!     host: String,
+//!     port: u16,
+//!     auto_reload: bool,
+//! }
+//!
+//! // The `Shadow` type is automatically generated by `filecaster-derive`
+//! // and would look something like this:
+//! #[derive(Debug, Default, PartialEq, Serialize, Deserialize)]
+//! struct AppConfigFile {
+//!     host: Option<String>,
+//!     port: Option<u16>,
+//!     auto_reload: Option<bool>,
+//! }
+//!
+//! // The `FromFile` implementation is also automatically generated.
+//! // For demonstration, here's a simplified manual implementation:
+//! impl FromFile for AppConfig {
+//!     type Shadow = AppConfigFile;
+//!
+//!     fn from_file(file: Option<Self::Shadow>) -> Self {
+//!         let file = file.unwrap_or_default();
+//!         AppConfig {
+//!             host: file.host.unwrap_or_else(|| "127.0.0.1".to_string()),
+//!             port: file.port.unwrap_or(8080),
+//!             auto_reload: file.auto_reload.unwrap_or(true),
+//!         }
+//!     }
+//! }
+//!
+//! fn example() {
+//!     // Simulate deserializing from a file
+//!     let file_content = r#"{ "host": "localhost", "port": 3000 }"#;
+//!     let partial_config: AppConfigFile = serde_json::from_str(file_content).unwrap();
+//!
+//!     // Construct the final config using the FromFile trait
+//!     let config = AppConfig::from_file(Some(partial_config));
+//!
+//!     assert_eq!(config.host, "localhost");
+//!     assert_eq!(config.port, 3000);
+//!     assert_eq!(config.auto_reload, false); // `Default::default()` for bool is `false`
+//!
+//!     println!("Final Config: {:#?}", config);
+//!
+//!     // Example with no file content (all defaults)
+//!     let default_config = AppConfig::from_file(None);
+//!     assert_eq!(default_config.host, "127.0.0.1");
+//!     assert_eq!(default_config.port, 8080);
+//!     assert_eq!(default_config.auto_reload, false);
+//! }
+//! ```
+//!
+//! ## Feature flags
+//!
+//! - `derive`: Enables the `filecaster-derive` crate, allowing you to use `#[derive(FromFile)]`.
+//! - `serde`: Enables `serde` serialization/deserialization support for the `FromFile` trait.
+//! - `merge`: Enables `merge` crate support, allowing for merging multiple partial configurations.
+
+pub use filecaster_derive::FromFile;
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+/// Marker for types that can be built from an [`Option<Shadow>`] produced by the macro.
+///
+/// The `FromFile` trait is the core interface for `filecaster`. It defines how a
+/// final configuration struct (`Self`) can be constructed from an optional
+/// intermediate "shadow" struct (`Self::Shadow`).
+///
+/// The `Self::Shadow` associated type represents the intermediate structure
+/// that is typically deserialized from a configuration file. All fields in
+/// `Self::Shadow` are usually `Option<T>`, allowing for partial configurations.
+///
+/// The `from_file` method takes an `Option<Self::Shadow>` and is responsible
+/// for producing a fully-populated `Self` instance. This involves applying
+/// default values for any fields that were `None` in the `Shadow` instance.
+///
+/// This trait is primarily designed to be implemented automatically via the
+/// `#[derive(FromFile)]` procedural macro provided by the `filecaster-derive` crate.
+pub trait FromFile: Sized {
+    /// The intermediate "shadow" type that is typically deserialized from a file.
+    ///
+    /// This type usually mirrors the main struct but with all fields wrapped in `Option<T>`.
+    type Shadow: Default;
+    /// Constructs the final struct from an optional shadow representation.
+    ///
+    /// If `file` is `None`, a default `Shadow` instance should be used.
+    /// The implementation should then populate `Self` by taking values from
+    /// `file` where present, and applying defaults otherwise.
+    fn from_file(file: Option<Self::Shadow>) -> Self;
+}
+
+#[cfg(not(feature = "serde"))]
+impl<T> FromFile for T
+where
+    T: Default,
+{
+    type Shadow = T;
+    fn from_file(file: Option<Self>) -> Self {
+        file.unwrap_or_default()
+    }
+}
+
+#[cfg(feature = "serde")]
+impl<T> FromFile for T
+where
+    T: Default + Serialize + for<'de> Deserialize<'de>,
+{
+    type Shadow = T;
+    fn from_file(file: Option<Self>) -> Self {
+        file.unwrap_or_default()
+    }
+}

filecaster/tests/nested_structure.rs

@@ -0,0 +1,141 @@
+use filecaster::FromFile;
+
+#[derive(Debug, Clone, PartialEq, FromFile)]
+pub struct Coordinates {
+    x: i32,
+    y: i32,
+}
+
+impl Coordinates {
+    fn new(x: i32, y: i32) -> Self {
+        Self { x, y }
+    }
+}
+
+impl CoordinatesFile {
+    fn new(x: i32, y: i32) -> Self {
+        Self {
+            x: Some(x),
+            y: Some(y),
+        }
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, FromFile)]
+struct Wrapper {
+    parent: Parent,
+}
+
+// And one more level
+#[derive(Debug, Clone, PartialEq, FromFile)]
+struct DoubleWrapper {
+    wrapper: Wrapper,
+}
+
+#[derive(Debug, Clone, PartialEq, FromFile)]
+pub struct Parent {
+    #[from_file(default = "Foo")]
+    name: String,
+    coordinates: Coordinates,
+}
+
+#[test]
+fn parent_all_defaults() {
+    let p = Parent::from_file(None);
+    assert_eq!(p.name, "Foo".to_string());
+    assert_eq!(p.coordinates, Coordinates::new(0, 0));
+}
+
+#[test]
+fn parent_partial_shadow_merges_defaults() {
+    let shadow = ParentFile {
+        name: None,
+        coordinates: Some(CoordinatesFile::new(1, 2)),
+    };
+    let p = Parent::from_file(Some(shadow));
+    assert_eq!(p.name, "Foo".to_string());
+    assert_eq!(p.coordinates, Coordinates::new(1, 2));
+}
+
+#[test]
+fn parent_full_shadow_overrides_everything() {
+    let shadow = ParentFile {
+        name: Some("Bar".into()),
+        coordinates: Some(CoordinatesFile::new(42, 24)),
+    };
+    let p = Parent::from_file(Some(shadow));
+    assert_eq!(p.name, "Bar".to_string());
+    assert_eq!(p.coordinates, Coordinates::new(42, 24));
+}
+
+#[test]
+fn wrapper_all_defaults() {
+    // None → WrapperFile::default() → parent = Parent::from_file(None)
+    let w = Wrapper::from_file(None);
+    assert_eq!(w.parent.name, "Foo".to_string());
+    assert_eq!(w.parent.coordinates, Coordinates::new(0, 0));
+}
+
+#[test]
+fn wrapper_partial_parent() {
+    // We supply only coordinates
+    let shadow = WrapperFile {
+        parent: Some(ParentFile {
+            name: None,
+            coordinates: Some(CoordinatesFile::new(5, -2)),
+        }),
+    };
+    let w = Wrapper::from_file(Some(shadow));
+    assert_eq!(w.parent.name, "Foo".to_string());
+    assert_eq!(w.parent.coordinates, Coordinates::new(5, -2));
+}
+
+#[test]
+fn wrapper_full_parent_override() {
+    let shadow = WrapperFile {
+        parent: Some(ParentFile {
+            name: Some("Baz".into()),
+            coordinates: Some(CoordinatesFile::new(1, 1)),
+        }),
+    };
+    let w = Wrapper::from_file(Some(shadow));
+    assert_eq!(w.parent.name, "Baz".to_string());
+    assert_eq!(w.parent.coordinates, Coordinates::new(1, 1));
+}
+
+#[test]
+fn double_wrapper_all_defaults() {
+    let dw = DoubleWrapper::from_file(None);
+    assert_eq!(dw.wrapper.parent.name, "Foo".to_string());
+    assert_eq!(dw.wrapper.parent.coordinates, Coordinates::new(0, 0));
+}
+
+#[test]
+fn double_wrapper_partial_deep() {
+    let shadow = DoubleWrapperFile {
+        wrapper: Some(WrapperFile {
+            parent: Some(ParentFile {
+                name: None,
+                coordinates: Some(CoordinatesFile::new(10, 20)),
+            }),
+        }),
+    };
+    let dw = DoubleWrapper::from_file(Some(shadow));
+    assert_eq!(dw.wrapper.parent.name, "Foo".to_string());
+    assert_eq!(dw.wrapper.parent.coordinates, Coordinates::new(10, 20));
+}
+
+#[test]
+fn double_wrapper_full_override_deep() {
+    let shadow = DoubleWrapperFile {
+        wrapper: Some(WrapperFile {
+            parent: Some(ParentFile {
+                name: Some("Deep".into()),
+                coordinates: Some(CoordinatesFile::new(3, 4)),
+            }),
+        }),
+    };
+    let dw = DoubleWrapper::from_file(Some(shadow));
+    assert_eq!(dw.wrapper.parent.name, "Deep".to_string());
+    assert_eq!(dw.wrapper.parent.coordinates, Coordinates::new(3, 4));
+}

filecaster/tests/ui.rs

@@ -0,0 +1,7 @@
+use trybuild::TestCases;
+
+#[test]
+fn ui() {
+    let t = TestCases::new();
+    t.compile_fail("tests/ui/*.rs");
+}

filecaster/tests/ui/enum_not_supported.rs

@@ -0,0 +1,9 @@
+use filecaster::FromFile;
+
+#[derive(FromFile)]
+enum MyEnum {
+    A,
+    B,
+}
+
+fn main() {}

filecaster/tests/ui/enum_not_supported.stderr

@@ -0,0 +1,6 @@
+error: FromFile only works on structs.
+       Enums are not supported.
+ --> tests/ui/enum_not_supported.rs:4:6
+  |
+4 | enum MyEnum {
+  |      ^^^^^^

filecaster/tests/ui/tuple_struct_not_supported.rs

@@ -0,0 +1,6 @@
+use filecaster::FromFile;
+
+#[derive(FromFile)]
+struct MyTuple(i32, String);
+
+fn main() {}

filecaster/tests/ui/tuple_struct_not_supported.stderr

@@ -0,0 +1,6 @@
+error: FromFile only works on structs with *named* fields.
+       Tuple structs and unit structs are not supported.
+ --> tests/ui/tuple_struct_not_supported.rs:4:8
+  |
+4 | struct MyTuple(i32, String);
+  |        ^^^^^^^

src/from_file.rs

@@ -1,324 +0,0 @@
-use proc_macro2::TokenStream;
-use quote::{format_ident, quote};
-use syn::{
-    Attribute, Data, DeriveInput, Error, Expr, Fields, FieldsNamed, GenericParam, Generics, Lit,
-    Meta, MetaList, Result, WhereClause, WherePredicate, parse_quote, parse2,
-};
-
-const WITH_MERGE: bool = cfg!(feature = "merge");
-
-pub fn impl_from_file(input: &DeriveInput) -> Result<TokenStream> {
-    let name = &input.ident;
-    let vis = &input.vis;
-    let generics = add_trait_bouds(input.generics.clone());
-    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
-
-    let file_ident = format_ident!("{name}File");
-
-    let fields = extract_named_fields(input)?;
-    let (field_assignments, file_fields, default_bounds) = process_fields(fields)?;
-
-    let where_clause = build_where_clause(where_clause.cloned(), default_bounds)?;
-
-    let derive_clause = build_derive_clause();
-
-    Ok(quote! {
-        #derive_clause
-        #vis struct #file_ident #where_clause {
-            #(#file_fields),*
-        }
-
-        impl #impl_generics #name #ty_generics #where_clause {
-            pub fn from_file(file: Option<#file_ident #ty_generics>) -> Self {
-                let file = file.unwrap_or_default();
-                Self {
-                    #(#field_assignments),*
-                }
-            }
-        }
-
-        impl #impl_generics From<Option<#file_ident #ty_generics>> for #name #ty_generics #where_clause {
-            fn from(value: Option<#file_ident #ty_generics>) -> Self {
-                Self::from_file(value)
-            }
-        }
-    })
-}
-
-fn extract_named_fields(input: &DeriveInput) -> Result<&FieldsNamed> {
-    match &input.data {
-        Data::Struct(ds) => match &ds.fields {
-            Fields::Named(fields) => Ok(fields),
-            _ => Err(Error::new_spanned(
-                &input.ident,
-                "FromFile can only be derived for structs with named fields",
-            )),
-        },
-        _ => Err(Error::new_spanned(
-            &input.ident,
-            "FromFile can only be derived for structs",
-        )),
-    }
-}
-
-fn process_fields(
-    fields: &FieldsNamed,
-) -> Result<(Vec<TokenStream>, Vec<TokenStream>, Vec<TokenStream>)> {
-    let mut field_assignments = Vec::new();
-    let mut file_fields = Vec::new();
-    let mut default_bounds = Vec::new();
-
-    for field in &fields.named {
-        let ident = field
-            .ident
-            .as_ref()
-            .ok_or_else(|| Error::new_spanned(field, "Expected named fields"))?;
-        let ty = &field.ty;
-
-        let default_expr = parse_from_file_default_attr(&field.attrs)?;
-
-        let field_attrs = if WITH_MERGE {
-            quote! {
-                #[merge(strategy = merge::option::overwrite_none)]
-            }
-        } else {
-            quote! {}
-        };
-        file_fields.push(quote! {
-            #field_attrs
-            pub #ident: Option<#ty>
-        });
-
-        if let Some(expr) = default_expr {
-            field_assignments.push(quote! {
-                #ident: file.#ident.unwrap_or_else(|| #expr)
-            });
-        } else {
-            default_bounds.push(quote! { #ty: Default });
-            field_assignments.push(quote! {
-                #ident: file.#ident.unwrap_or_default()
-            });
-        }
-    }
-
-    Ok((field_assignments, file_fields, default_bounds))
-}
-
-fn build_where_clause(
-    where_clause: Option<WhereClause>,
-    default_bounds: Vec<TokenStream>,
-) -> Result<Option<WhereClause>> {
-    if default_bounds.is_empty() {
-        return Ok(where_clause);
-    }
-
-    let mut where_clause = where_clause;
-    if let Some(wc) = &mut where_clause {
-        for bound in default_bounds {
-            let predicate = parse2::<WherePredicate>(bound.clone())
-                .map_err(|_| Error::new_spanned(&bound, "Failed to parse where predicate"))?;
-            wc.predicates.push(predicate);
-        }
-    } else {
-        where_clause = Some(parse_quote!(where #(#default_bounds),*));
-    }
-    Ok(where_clause)
-}
-
-fn build_derive_clause() -> TokenStream {
-    if WITH_MERGE {
-        return quote! {
-            #[derive(Debug, Clone, Default, serde::Deserialize, serde::Serialize, merge::Merge)]
-        };
-    }
-
-    quote! {
-        #[derive(Debug, Clone, Default, serde::Deserialize, serde::Serialize)]
-    }
-}
-
-fn add_trait_bouds(mut generics: Generics) -> Generics {
-    for param in &mut generics.params {
-        if let GenericParam::Type(type_param) = param {
-            type_param.bounds.push(parse_quote!(Default));
-        }
-    }
-    generics
-}
-
-/// Parses attributes for `#[from_file(default = ...)]`
-fn parse_from_file_default_attr(attrs: &[Attribute]) -> Result<Option<Expr>> {
-    for attr in attrs {
-        if !attr.path().is_ident("from_file") {
-            continue; // Not a #[from_file] attribute, skip it
-        }
-
-        // Parse the content inside the parentheses of #[from_file(...)]
-        return match &attr.meta {
-            Meta::List(meta_list) => parse_default(meta_list),
-            _ => Err(Error::new_spanned(
-                attr,
-                "Expected #[from_file(default = \"literal\")] or similar",
-            )),
-        };
-    }
-    Ok(None)
-}
-
-fn parse_default(list: &MetaList) -> Result<Option<Expr>> {
-    let mut default_expr = None;
-    list.parse_nested_meta(|meta| {
-        if meta.path.is_ident("default") {
-            let value = meta.value()?;
-            let expr = value.parse::<Expr>()?;
-
-            if let Expr::Lit(expr_lit) = &expr {
-                if let Lit::Str(lit_str) = &expr_lit.lit {
-                    default_expr = Some(parse_quote! {
-                        #lit_str.to_string()
-                    });
-                    return Ok(());
-                }
-            }
-            default_expr = Some(expr);
-        }
-        Ok(())
-    })?;
-    Ok(default_expr)
-}
-
-#[cfg(test)]
-mod tests {
-    use claims::{assert_err, assert_none};
-    use quote::ToTokens;
-
-    use super::*;
-
-    #[test]
-    fn extract_named_fields_success() {
-        let input: DeriveInput = parse_quote! {
-            struct S { x: i32, y: String }
-        };
-        let fields = extract_named_fields(&input).unwrap();
-        let names = fields
-            .named
-            .iter()
-            .map(|f| f.ident.as_ref().unwrap().to_string())
-            .collect::<Vec<_>>();
-        assert_eq!(names, vec!["x", "y"]);
-    }
-
-    #[test]
-    fn extract_named_fields_err_on_enum() {
-        let input: DeriveInput = parse_quote! {
-            enum E { A, B }
-        };
-        assert_err!(extract_named_fields(&input));
-    }
-
-    #[test]
-    fn extract_named_fields_err_on_tuple_struct() {
-        let input: DeriveInput = parse_quote! {
-            struct T(i32, String);
-        };
-        assert_err!(extract_named_fields(&input));
-    }
-
-    #[test]
-    fn parse_default_attrs_none() {
-        let attrs: Vec<Attribute> = vec![parse_quote!(#[foo])];
-        assert_none!(parse_from_file_default_attr(&attrs).unwrap());
-    }
-
-    #[test]
-    fn process_fields_mixed() {
-        let fields: FieldsNamed = parse_quote! {
-            {
-                #[from_file(default = 1)]
-                a: u32,
-                b: String,
-            }
-        };
-        let (assign, file_fields, bounds) = process_fields(&fields).unwrap();
-        // two fields
-        assert_eq!(assign.len(), 2);
-        assert_eq!(file_fields.len(), 2);
-        // a uses unwrap_or_else
-        assert!(
-            assign[0]
-                .to_string()
-                .contains("a : file . a . unwrap_or_else")
-        );
-        // b uses unwrap_or_default
-        assert!(
-            assign[1]
-                .to_string()
-                .contains("b : file . b . unwrap_or_default")
-        );
-        // default-bound should only mention String
-        assert_eq!(bounds.len(), 1);
-        assert!(bounds[0].to_string().contains("String : Default"));
-    }
-
-    #[test]
-    fn build_where_clause_to_new() {
-        let bounds = vec![quote! { A: Default }, quote! { B: Default }];
-        let wc = build_where_clause(None, bounds).unwrap().unwrap();
-        let s = wc.to_token_stream().to_string();
-        assert!(s.contains("where A : Default , B : Default"));
-    }
-
-    #[test]
-    fn build_where_clause_append_existing() {
-        let orig: WhereClause = parse_quote!(where X: Clone);
-        let bounds = vec![quote! { Y: Default }];
-        let wc = build_where_clause(Some(orig.clone()), bounds)
-            .unwrap()
-            .unwrap();
-        let preds: Vec<_> = wc
-            .predicates
-            .iter()
-            .map(|p| p.to_token_stream().to_string())
-            .collect();
-        assert!(preds.contains(&"X : Clone".to_string()));
-        assert!(preds.contains(&"Y : Default".to_string()));
-    }
-
-    #[test]
-    fn build_where_clause_no_bounds_keeps_original() {
-        let orig: WhereClause = parse_quote!(where Z: Eq);
-        let wc = build_where_clause(Some(orig.clone()), vec![])
-            .unwrap()
-            .unwrap();
-        let preds: Vec<_> = wc
-            .predicates
-            .iter()
-            .map(|p| p.to_token_stream().to_string())
-            .collect();
-        assert_eq!(preds, vec!["Z : Eq".to_string()]);
-    }
-
-    #[test]
-    fn build_derive_clause_defaults() {
-        let derive_ts = build_derive_clause();
-        let s = derive_ts.to_string();
-        if WITH_MERGE {
-            assert!(s.contains(
-                "derive (Debug , Clone , Default , serde :: Deserialize , serde :: Serialize , merge :: Merge)"
-            ));
-        } else {
-            assert!(s.contains(
-                "derive (Debug , Clone , Default , serde :: Deserialize , serde :: Serialize)"
-            ));
-        }
-    }
-
-    #[test]
-    fn add_trait_bouds_appends_default() {
-        let gens: Generics = parse_quote!(<T, U>);
-        let new = add_trait_bouds(gens);
-        let s = new.to_token_stream().to_string();
-        assert!(s.contains("T : Default"));
-        assert!(s.contains("U : Default"));
-    }
-}

src/lib.rs

@@ -1,97 +0,0 @@
-//! # filecaster
-//!
-//! `filecaster` is a small `proc-macro` crate that provides a derive‐macro
-//! `#[derive(FromFile)]` to make it trivial to load partial configurations
-//! from files, merge them with defaults, and get a fully‐populated struct.
-//!
-//! ## What it does
-//!
-//! For any struct with named fields, `#[derive(FromFile)]` generates:
-//!
-//! 1. A companion `<YourStruct>NameFile` struct in which each field is wrapped
-//!    in `Option<...>`.
-//! 2. A constructor `YourStruct::from_file(file: Option<YourStructFile>) -> YourStruct`
-//!    that takes your partially‐filled file struct, fills in `None` fields
-//!    with either:
-//!    - an expression you supply via `#[from_file(default = ...)]`, or
-//!    - `Default::default()` (requires `T: Default`)
-//! 3. An implementation of `From<Option<YourStructFile>> for YourStruct`.
-//!
-//! Because each field in the file‐struct is optional, you can deserialize
-//! e.g. JSON, YAML or TOML into it via Serde, then call `.from_file(...)`
-//! to get your final struct.
-//!
-//! ## Optional per‐field defaults
-//!
-//! Use a `#[from_file(default = <expr>)]` attribute on any field to override
-//! the fallback value. You may supply any expression valid in that struct’s
-//! context. If you omit it, the macro will require `T: Default` and call
-//! `unwrap_or_default()`.
-//!
-//! Example:
-//!
-//! ```rust
-//! use filecaster::FromFile;
-//!
-//! #[derive(Debug, Clone, FromFile)]
-//! struct AppConfig {
-//!     /// If the user does not specify a host, use `"127.0.0.1"`.
-//!     #[from_file(default = "127.0.0.1")]
-//!     host: String,
-//!
-//!     /// Number of worker threads; defaults to `4`.
-//!     #[from_file(default = 4)]
-//!     workers: usize,
-//!
-//!     /// If not set, use `false`.
-//!     auto_reload: bool,  // requires `bool: Default`
-//! }
-//!
-//! let file_content = r#"
-//!     {
-//!         "host": "localhost"
-//!     }
-//! "#;
-//!
-//! let config_from_file = serde_json::from_str::<AppConfigFile>(file_content).unwrap();
-//! // After deserializing the partial config from disk (e.g. with Serde):
-//! let cfg = AppConfig::from_file(Some(config_from_file));
-//! println!("{cfg:#?}");
-//! ```
-//!
-//! ## Feature flags
-//!
-//! - `merge`
-//!   If you enable the `merge` feature, the generated `<Name>File` struct will
-//!   also derive `merge::Merge`, and you can layer multiple partial files
-//!   together before calling `.from_file(...)`. Any field‐level merge strategy
-//!   annotations (`#[merge(...)]`) are applied automatically.
-//!
-//! ## Limitations
-//!  
-//! - Only works on structs with _named_ fields (no tuple‐structs or enums).
-//! - All fields without a `#[from_file(default = ...)]` must implement `Default`.
-//!  
-//! ## License
-//!  
-//! MIT OR Apache-2.0
-
-mod from_file;
-
-pub(crate) use from_file::impl_from_file;
-use proc_macro::TokenStream;
-use proc_macro_error2::proc_macro_error;
-use syn::{DeriveInput, parse_macro_input};
-
-/// Implements the `FromFile` derive macro.
-///
-/// This macro processes the `#[from_file]` attribute on structs to generate
-/// code for loading data from files.
-#[proc_macro_error]
-#[proc_macro_derive(FromFile, attributes(from_file))]
-pub fn derive_from_file(input: TokenStream) -> TokenStream {
-    let inp = parse_macro_input!(input as DeriveInput);
-    impl_from_file(&inp)
-        .unwrap_or_else(|e| e.to_compile_error())
-        .into()
-}