refactor: add better messages

Cargo.lock

@@ -58,6 +58,7 @@ dependencies = [
  "serde_json",
  "tempfile",
  "toml",
+ "trybuild",
 ]
 
 [[package]]
@@ -87,6 +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"
@@ -281,6 +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"
@@ -294,6 +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"
@@ -333,6 +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"
@@ -348,6 +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

@@ -12,6 +12,7 @@ claims = "0.8"
 serde_json = "1.0"
 tempfile = "3.10"
 toml = "0.9"
+trybuild = "1.0"
 
 [workspace.lints.clippy]
 pedantic = "warn"

filecaster-derive/src/from_file.rs

@@ -60,12 +60,14 @@ fn extract_named_fields(input: &DeriveInput) -> Result<&FieldsNamed> {
             Fields::Named(fields) => Ok(fields),
             _ => Err(Error::new_spanned(
                 &input.ident,
-                "FromFile can only be derived for structs with named fields",
+                r#"FromFile only works on structs with *named* fields.
+Tuple structs and unit structs are not supported."#,
             )),
         },
         _ => Err(Error::new_spanned(
             &input.ident,
-            "FromFile can only be derived for structs",
+            r#"FromFile only works on structs.
+Enums are not supported."#,
         )),
     }
 }

filecaster/Cargo.toml

@@ -28,3 +28,4 @@ merge = { workspace = true, optional = true }
 serde_json.workspace = true
 tempfile.workspace = true
 toml.workspace = true
+trybuild.workspace = true

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/invalid_attr.rs

@@ -0,0 +1,9 @@
+use filecaster::FromFile;
+
+#[derive(FromFile)]
+struct MyStruct {
+    #[from_file(unknown)]
+    field: i32,
+}
+
+fn main() {}

filecaster/tests/ui/tuple_struct_not_supported.rs

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