Add "in this derive macro expansion" to missing Display errors

Author: dtolnay

impl/src/expand.rs

@@ -496,7 +496,7 @@ fn impl_enum(input: Enum) -> TokenStream {
 // deprecated type without triggering deprecation warning on the generated impl.
 fn call_site_ident(ident: &Ident) -> Ident {
     let mut ident = ident.clone();
-    ident.set_span(Span::call_site());
+    ident.set_span(ident.span().resolved_at(Span::call_site()));
     ident
 }
 

tests/ui/missing-display.stderr

@@ -1,8 +1,10 @@
 error[E0277]: `MyError` doesn't implement `std::fmt::Display`
- --> tests/ui/missing-display.rs:3:10
+ --> tests/ui/missing-display.rs:4:10
   |
 3 | #[derive(Error, Debug)]
-  |          ^^^^^ `MyError` cannot be formatted with the default formatter
+  |          ----- in this derive macro expansion
+4 | pub enum MyError {
+  |          ^^^^^^^ `MyError` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `MyError`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead