Rollup merge of #116296 - compiler-errors:default-return, r=estebank

More accurately point to where default return type should go

When getting the "default return type" span, instead of pointing to the low span of the next token, point to the high span of the previous token. This:

1. Makes forming return type suggestions more uniform, since we expect them all in the same place.
2. Arguably makes labels easier to understand, since we're pointing to where the implicit `-> ()` would've gone, rather than the starting brace or the semicolon.

r? ```@estebank```

Author: workingjubilee

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -1189,7 +1189,7 @@ fn report_trait_method_mismatch<'tcx>(
                     let ap = Applicability::MachineApplicable;
                     match sig.decl.output {
                         hir::FnRetTy::DefaultReturn(sp) => {
-                            let sugg = format!("-> {} ", trait_sig.output());
+                            let sugg = format!(" -> {}", trait_sig.output());
                             diag.span_suggestion_verbose(sp, msg, sugg, ap);
                         }
                         hir::FnRetTy::Return(hir_ty) => {

compiler/rustc_hir_typeck/src/check.rs

@@ -113,7 +113,11 @@ pub(super) fn check_fn<'a, 'tcx>(
 
     fcx.typeck_results.borrow_mut().liberated_fn_sigs_mut().insert(fn_id, fn_sig);
 
-    fcx.require_type_is_sized(declared_ret_ty, decl.output.span(), traits::SizedReturnType);
+    let return_or_body_span = match decl.output {
+        hir::FnRetTy::DefaultReturn(_) => body.value.span,
+        hir::FnRetTy::Return(ty) => ty.span,
+    };
+    fcx.require_type_is_sized(declared_ret_ty, return_or_body_span, traits::SizedReturnType);
     fcx.check_return_expr(&body.value, false);
 
     // We insert the deferred_generator_interiors entry after visiting the body.