Change span of as_dyn_error() to point compile error at attribute.

impl/src/expand.rs

@@ -23,14 +23,14 @@ fn impl_struct(input: Struct) -> TokenStream {
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
     let mut error_inferred_bounds = InferredBounds::new();
 
-    let source_body = if input.attrs.transparent.is_some() {
+    let source_body = if let Some(transparent_attr) = &input.attrs.transparent {
         let only_field = &input.fields[0];
         if only_field.contains_generic {
             error_inferred_bounds.insert(only_field.ty, quote!(std::error::Error));
         }
         let member = &only_field.member;
-        Some(quote! {
-            std::error::Error::source(self.#member.as_dyn_error())
+        Some(quote_spanned! {
+            transparent_attr.span => std::error::Error::source(self.#member.as_dyn_error())
         })
     } else if let Some(source_field) = input.source_field() {
         let source = &source_field.member;
@@ -43,7 +43,8 @@ fn impl_struct(input: Struct) -> TokenStream {
         } else {
             None
         };
-        let dyn_error = quote_spanned!(source.span()=> self.#source #asref.as_dyn_error());
+        let dyn_error =
+            quote_spanned!(source_field.source_span() => self.#source #asref.as_dyn_error());
         Some(quote! {
             ::core::option::Option::Some(#dyn_error)
         })
@@ -193,13 +194,13 @@ fn impl_enum(input: Enum) -> TokenStream {
     let source_method = if input.has_source() {
         let arms = input.variants.iter().map(|variant| {
             let ident = &variant.ident;
-            if variant.attrs.transparent.is_some() {
+            if let Some(transparent_attr) = &variant.attrs.transparent {
                 let only_field = &variant.fields[0];
                 if only_field.contains_generic {
                     error_inferred_bounds.insert(only_field.ty, quote!(std::error::Error));
                 }
                 let member = &only_field.member;
-                let source = quote!(std::error::Error::source(transparent.as_dyn_error()));
+                let source = quote_spanned!(transparent_attr.span => std::error::Error::source(transparent.as_dyn_error()));
                 quote! {
                     #ty::#ident {#member: transparent} => #source,
                 }
@@ -215,7 +216,7 @@ fn impl_enum(input: Enum) -> TokenStream {
                     None
                 };
                 let varsource = quote!(source);
-                let dyn_error = quote_spanned!(source.span()=> #varsource #asref.as_dyn_error());
+                let dyn_error = quote_spanned!(source_field.source_span()=> #varsource #asref.as_dyn_error());
                 quote! {
                     #ty::#ident {#source: #varsource, ..} => ::core::option::Option::Some(#dyn_error),
                 }

impl/src/prop.rs

@@ -1,5 +1,6 @@
 use crate::ast::{Enum, Field, Struct, Variant};
-use syn::{Member, Type};
+use proc_macro2::Span;
+use syn::{spanned::Spanned, Member, Type};
 
 impl Struct<'_> {
     pub(crate) fn from_field(&self) -> Option<&Field> {
@@ -70,6 +71,16 @@ impl Field<'_> {
     pub(crate) fn is_backtrace(&self) -> bool {
         type_is_backtrace(self.ty)
     }
+
+    pub(crate) fn source_span(&self) -> Span {
+        if let Some(source_attr) = &self.attrs.source {
+            source_attr.path().span()
+        } else if let Some(from_attr) = &self.attrs.from {
+            from_attr.path().span()
+        } else {
+            self.member.span()
+        }
+    }
 }
 
 fn from_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {

tests/ui/source-enum-unnamed-field-not-error.rs

@@ -0,0 +1,12 @@
+use thiserror::Error;
+
+#[derive(Debug)]
+pub struct NotError;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub enum ErrorEnum {
+    Broken(#[source] NotError),
+}
+
+fn main() {}

tests/ui/source-enum-unnamed-field-not-error.stderr

@@ -0,0 +1,22 @@
+error[E0599]: the method `as_dyn_error` exists for reference `&NotError`, but its trait bounds were not satisfied
+ --> tests/ui/source-enum-unnamed-field-not-error.rs:9:14
+  |
+4 | pub struct NotError;
+  | -------------------
+  | |
+  | doesn't satisfy `NotError: AsDynError<'_>`
+  | doesn't satisfy `NotError: std::error::Error`
+...
+9 |     Broken(#[source] NotError),
+  |              ^^^^^^ method cannot be called on `&NotError` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `NotError: std::error::Error`
+          which is required by `NotError: AsDynError<'_>`
+          `&NotError: std::error::Error`
+          which is required by `&NotError: AsDynError<'_>`
+note: the trait `std::error::Error` must be implemented
+ --> $RUST/core/src/error.rs
+  |
+  | pub trait Error: Debug + Display {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

tests/ui/source-struct-unnamed-field-not-error.rs

@@ -0,0 +1,10 @@
+use thiserror::Error;
+
+#[derive(Debug)]
+struct NotError;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct ErrorStruct(#[source] NotError);
+
+fn main() {}

tests/ui/source-struct-unnamed-field-not-error.stderr

@@ -0,0 +1,21 @@
+error[E0599]: the method `as_dyn_error` exists for struct `NotError`, but its trait bounds were not satisfied
+ --> tests/ui/source-struct-unnamed-field-not-error.rs:8:26
+  |
+4 | struct NotError;
+  | ---------------
+  | |
+  | method `as_dyn_error` not found for this struct
+  | doesn't satisfy `NotError: AsDynError<'_>`
+  | doesn't satisfy `NotError: std::error::Error`
+...
+8 | pub struct ErrorStruct(#[source] NotError);
+  |                          ^^^^^^ method cannot be called on `NotError` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `NotError: std::error::Error`
+          which is required by `NotError: AsDynError<'_>`
+note: the trait `std::error::Error` must be implemented
+ --> $RUST/core/src/error.rs
+  |
+  | pub trait Error: Debug + Display {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

tests/ui/transparent-enum-not-error.rs

@@ -0,0 +1,11 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error(transparent)]
+    Other {
+        message: String,
+    }
+}
+
+fn main() {}

tests/ui/transparent-enum-not-error.stderr

@@ -0,0 +1,23 @@
+error[E0599]: the method `as_dyn_error` exists for reference `&String`, but its trait bounds were not satisfied
+ --> tests/ui/transparent-enum-not-error.rs:5:13
+  |
+5 |     #[error(transparent)]
+  |             ^^^^^^^^^^^ method cannot be called on `&String` due to unsatisfied trait bounds
+  |
+ ::: $RUST/alloc/src/string.rs
+  |
+  | pub struct String {
+  | -----------------
+  | |
+  | doesn't satisfy `String: AsDynError<'_>`
+  | doesn't satisfy `String: std::error::Error`
+  |
+  = note: the following trait bounds were not satisfied:
+          `String: std::error::Error`
+          which is required by `String: AsDynError<'_>`
+          `&String: std::error::Error`
+          which is required by `&String: AsDynError<'_>`
+          `str: Sized`
+          which is required by `str: AsDynError<'_>`
+          `str: std::error::Error`
+          which is required by `str: AsDynError<'_>`

tests/ui/transparent-enum-unnamed-field-not-error.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error(transparent)]
+    Other(String),
+}
+
+fn main() {}

tests/ui/transparent-enum-unnamed-field-not-error.stderr

@@ -0,0 +1,23 @@
+error[E0599]: the method `as_dyn_error` exists for reference `&String`, but its trait bounds were not satisfied
+ --> tests/ui/transparent-enum-unnamed-field-not-error.rs:5:13
+  |
+5 |     #[error(transparent)]
+  |             ^^^^^^^^^^^ method cannot be called on `&String` due to unsatisfied trait bounds
+  |
+ ::: $RUST/alloc/src/string.rs
+  |
+  | pub struct String {
+  | -----------------
+  | |
+  | doesn't satisfy `String: AsDynError<'_>`
+  | doesn't satisfy `String: std::error::Error`
+  |
+  = note: the following trait bounds were not satisfied:
+          `String: std::error::Error`
+          which is required by `String: AsDynError<'_>`
+          `&String: std::error::Error`
+          which is required by `&String: AsDynError<'_>`
+          `str: Sized`
+          which is required by `str: AsDynError<'_>`
+          `str: std::error::Error`
+          which is required by `str: AsDynError<'_>`

tests/ui/transparent-struct-not-error.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error {
+    message: String,
+}
+
+fn main() {}

tests/ui/transparent-struct-not-error.stderr

@@ -0,0 +1,21 @@
+error[E0599]: the method `as_dyn_error` exists for struct `String`, but its trait bounds were not satisfied
+ --> tests/ui/transparent-struct-not-error.rs:4:9
+  |
+4 | #[error(transparent)]
+  |         ^^^^^^^^^^^ method cannot be called on `String` due to unsatisfied trait bounds
+  |
+ ::: $RUST/alloc/src/string.rs
+  |
+  | pub struct String {
+  | -----------------
+  | |
+  | doesn't satisfy `String: AsDynError<'_>`
+  | doesn't satisfy `String: std::error::Error`
+  |
+  = note: the following trait bounds were not satisfied:
+          `String: std::error::Error`
+          which is required by `String: AsDynError<'_>`
+          `str: Sized`
+          which is required by `str: AsDynError<'_>`
+          `str: std::error::Error`
+          which is required by `str: AsDynError<'_>`

tests/ui/transparent-struct-unnamed-field-not-error.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error(String);
+
+fn main() {}

tests/ui/transparent-struct-unnamed-field-not-error.stderr

@@ -0,0 +1,21 @@
+error[E0599]: the method `as_dyn_error` exists for struct `String`, but its trait bounds were not satisfied
+ --> tests/ui/transparent-struct-unnamed-field-not-error.rs:4:9
+  |
+4 | #[error(transparent)]
+  |         ^^^^^^^^^^^ method cannot be called on `String` due to unsatisfied trait bounds
+  |
+ ::: $RUST/alloc/src/string.rs
+  |
+  | pub struct String {
+  | -----------------
+  | |
+  | doesn't satisfy `String: AsDynError<'_>`
+  | doesn't satisfy `String: std::error::Error`
+  |
+  = note: the following trait bounds were not satisfied:
+          `String: std::error::Error`
+          which is required by `String: AsDynError<'_>`
+          `str: Sized`
+          which is required by `str: AsDynError<'_>`
+          `str: std::error::Error`
+          which is required by `str: AsDynError<'_>`