Tweak extern Rust Result spans to work with nightly-2021-08-12

Author: dtolnay

macro/src/expand.rs

@@ -1127,9 +1127,11 @@ fn expand_rust_function_shim_super(
             Some(ret) => quote!(#ret),
             None => quote!(()),
         };
-        let impl_trait = quote_spanned!(result.span=> impl);
-        let display = quote_spanned!(rangle.span=> ::std::fmt::Display);
-        quote!(-> ::std::result::Result<#ok, #impl_trait #display>)
+        // Set spans that result in the `Result<...>` written by the user being
+        // highlighted as the cause if their error type has no Display impl.
+        let result_begin = quote_spanned!(result.span=> ::std::result::Result<#ok, impl);
+        let result_end = quote_spanned!(rangle.span=> ::std::fmt::Display>);
+        quote!(-> #result_begin #result_end)
     } else {
         expand_return_type(&sig.ret)
     };

tests/ui/result_no_display.stderr

@@ -1,9 +1,8 @@
 error[E0277]: `NonError` doesn't implement `std::fmt::Display`
- --> $DIR/result_no_display.rs:1:1
+ --> $DIR/result_no_display.rs:4:19
   |
-1 | #[cxx::bridge]
-  | ^^^^^^^^^^^^^^ `NonError` cannot be formatted with the default formatter
+4 |         fn f() -> Result<()>;
+  |                   ^^^^^^^^^^ `NonError` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `NonError`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-  = note: this error originates in the attribute macro `cxx::bridge` (in Nightly builds, run with -Z macro-backtrace for more info)