Adjust error message

Author: kiendang

Cargo.toml

@@ -253,7 +253,7 @@ required-features = ["unwrap"]
 [[test]]
 name = "compile_fail"
 path = "tests/compile_fail/mod.rs"
-required-features = ["as_ref", "debug", "display", "from", "into", "is_variant", "try_from"]
+required-features = ["as_ref", "debug", "display", "from", "into", "is_variant", "try_from", "try_into"]
 
 [[test]]
 name = "no_std"

impl/src/try_into.rs

@@ -4,7 +4,7 @@ use crate::utils::{
 };
 use proc_macro2::{Span, TokenStream};
 use quote::{quote, ToTokens};
-use syn::{Attribute, DeriveInput, Ident, Result};
+use syn::{Attribute, DeriveInput, Error, Ident, Result};
 
 use crate::utils::{
     attr::{self, ParseMultiple as _},
@@ -52,7 +52,20 @@ pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStre
             struct_: vec!["ignore", "owned", "ref", "ref_mut"],
             field: vec!["ignore"],
         },
-    )?;
+    ).map_err(|err| {
+        let msg = &err.to_string();
+        if msg == "Only a single attribute is allowed" {
+            Error::new(
+                err.span(),
+                "Only a single attribute is allowed.\n\
+                For the top-level attribute, an additional `#[try_into(error(...))]` may be provided.",
+            )
+        } else if msg.starts_with("Attribute parameter not supported. Supported attribute parameters are:") {
+            Error::new(err.span(), format!("{msg}, error"))
+        } else {
+            err
+        }
+    })?;
     assert!(
         state.derive_type == DeriveType::Enum,
         "Only enums can derive TryInto"

tests/compile_fail/try_into/invalid_attr.rs

@@ -0,0 +1,8 @@
+#[derive(derive_more::TryInto)]
+#[try_into(foo)]
+enum MixedData {
+    Int(u32),
+    String(String),
+}
+
+fn main() {}

tests/compile_fail/try_into/invalid_attr.stderr

@@ -0,0 +1,5 @@
+error: Attribute parameter not supported. Supported attribute parameters are: ignore, owned, ref, ref_mut, error
+ --> tests/compile_fail/try_into/invalid_attr.rs:2:12
+  |
+2 | #[try_into(foo)]
+  |            ^^^

tests/compile_fail/try_into/surplus_attrs.rs

@@ -0,0 +1,9 @@
+#[derive(derive_more::TryInto)]
+#[try_into(owned)]
+#[try_into(foo)]
+enum MixedData {
+    Int(u32),
+    String(String),
+}
+
+fn main() {}

tests/compile_fail/try_into/surplus_attrs.stderr

@@ -0,0 +1,6 @@
+error: Only a single attribute is allowed.
+       For the top-level attribute, an additional `#[try_into(error(...))]` may be provided.
+ --> tests/compile_fail/try_into/surplus_attrs.rs:3:1
+  |
+3 | #[try_into(foo)]
+  | ^