Make error message for malformed fn/fn_mut lang item more specific

Author: FabianWolff

Diff for: compiler/rustc_typeck/src/check/wfcheck.rs

@@ -203,33 +203,52 @@ pub fn check_trait_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
 
     let encl_trait_hir_id = tcx.hir().get_parent_item(hir_id);
     let encl_trait = tcx.hir().expect_item(encl_trait_hir_id);
-    if [tcx.lang_items().fn_trait(), tcx.lang_items().fn_mut_trait()]
-        .contains(&Some(encl_trait.def_id.to_def_id()))
-        && trait_item.ident.name.to_ident_string() == "call"
+    let encl_trait_def_id = encl_trait.def_id.to_def_id();
+    let fn_lang_item_name = if Some(encl_trait_def_id) == tcx.lang_items().fn_trait() {
+        Some("fn")
+    } else if Some(encl_trait_def_id) == tcx.lang_items().fn_mut_trait() {
+        Some("fn_mut")
+    } else {
+        None
+    };
+
+    if let (Some(fn_lang_item_name), "call") =
+        (fn_lang_item_name, trait_item.ident.name.to_ident_string().as_str())
     {
         // We are looking at the `call` function of the `fn` or `fn_mut` lang item.
         // Do some rudimentary sanity checking to avoid an ICE later (issue #83471).
-        if let Some(method_sig @ hir::FnSig { decl, .. }) = method_sig {
+        if let Some(hir::FnSig { decl, span, .. }) = method_sig {
             if let &[self_ty, _] = &decl.inputs {
                 if !matches!(self_ty.kind, hir::TyKind::Rptr(_, _)) {
-                    tcx.sess.struct_span_err(
-                        self_ty.span,
-                        "first argument of `call` in `fn`/`fn_mut` lang item must be a reference",
-                    ).emit();
+                    tcx.sess
+                        .struct_span_err(
+                            self_ty.span,
+                            &format!(
+                                "first argument of `call` in `{}` lang item must be a reference",
+                                fn_lang_item_name
+                            ),
+                        )
+                        .emit();
                 }
             } else {
                 tcx.sess
                     .struct_span_err(
-                        method_sig.span,
-                        "`call` function in `fn`/`fn_mut` lang item takes exactly two arguments",
+                        *span,
+                        &format!(
+                            "`call` function in `{}` lang item takes exactly two arguments",
+                            fn_lang_item_name
+                        ),
                     )
                     .emit();
             }
         } else {
             tcx.sess
                 .struct_span_err(
                     trait_item.span,
-                    "`call` trait item in `fn`/`fn_mut` lang item must be a function",
+                    &format!(
+                        "`call` trait item in `{}` lang item must be a function",
+                        fn_lang_item_name
+                    ),
                 )
                 .emit();
         }

Diff for: src/test/ui/lang-items/fn-fn_mut-call-ill-formed.rs

@@ -8,13 +8,13 @@
 #[lang = "fn"]
 trait MyFn<T> {
     const call: i32 = 42;
-    //~^ ERROR: `call` trait item in `fn`/`fn_mut` lang item must be a function
+    //~^ ERROR: `call` trait item in `fn` lang item must be a function
 }
 
 #[lang = "fn_mut"]
 trait MyFnMut<T> {
     fn call(i: i32, j: i32) -> i32 { i + j }
-    //~^ ERROR: first argument of `call` in `fn`/`fn_mut` lang item must be a reference
+    //~^ ERROR: first argument of `call` in `fn_mut` lang item must be a reference
 }
 
 fn main() {

Diff for: src/test/ui/lang-items/fn-fn_mut-call-ill-formed.stderr

@@ -1,10 +1,10 @@
-error: `call` trait item in `fn`/`fn_mut` lang item must be a function
+error: `call` trait item in `fn` lang item must be a function
   --> $DIR/fn-fn_mut-call-ill-formed.rs:10:5
    |
 LL |     const call: i32 = 42;
    |     ^^^^^^^^^^^^^^^^^^^^^
 
-error: first argument of `call` in `fn`/`fn_mut` lang item must be a reference
+error: first argument of `call` in `fn_mut` lang item must be a reference
   --> $DIR/fn-fn_mut-call-ill-formed.rs:16:16
    |
 LL |     fn call(i: i32, j: i32) -> i32 { i + j }