Don't refer to 'this tail expression' in expansion.

The user has no clue what tail expression the compiler is talking
about: it is an implementation detail of the macro that it uses a block
with tail expression.

Author: m-ou-se

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -3841,7 +3841,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     .expr_ty_adjusted_opt(inner_expr)
                     .unwrap_or(Ty::new_misc_error(tcx));
                 let span = inner_expr.span;
-                if Some(span) != err.span.primary_span() {
+                if Some(span) != err.span.primary_span()
+                    && !span.in_external_macro(tcx.sess.source_map())
+                {
                     err.span_label(
                         span,
                         if ty.references_error() {

tests/ui/never_type/feature-gate-never_type_fallback.stderr

@@ -2,10 +2,8 @@ error[E0277]: the trait bound `(): T` is not satisfied
   --> $DIR/feature-gate-never_type_fallback.rs:10:9
    |
 LL |     foo(panic!())
-   |     --- ^^^^^^^^
-   |     |   |
-   |     |   the trait `T` is not implemented for `()`
-   |     |   this tail expression is of type `()`
+   |     --- ^^^^^^^^ the trait `T` is not implemented for `()`
+   |     |
    |     required by a bound introduced by this call
    |
 help: this trait has no implementations, consider adding one