Suggest replacing _ in type signature of impl for Trait

compiler/rustc_typeck/src/astconv/mod.rs

@@ -2563,12 +2563,29 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         // We proactively collect all the inferred type params to emit a single error per fn def.
         let mut visitor = HirPlaceholderCollector::default();
-        for ty in decl.inputs {
-            visitor.visit_ty(ty);
-        }
+        let mut infer_replacements = vec![];
+
         walk_generics(&mut visitor, generics);
 
-        let input_tys = decl.inputs.iter().map(|a| self.ty_of_arg(a, None));
+        let input_tys: Vec<_> = decl
+            .inputs
+            .iter()
+            .enumerate()
+            .map(|(i, a)| {
+                if let hir::TyKind::Infer = a.kind && !self.allow_ty_infer() {
+                    if let Some(suggested_ty) =
+                      self.suggest_trait_fn_ty_for_impl_fn_infer(hir_id, i) {
+                        infer_replacements.push((a.span, suggested_ty.to_string()));
+                        return suggested_ty;
+                    }
+                }
+
+                // Only visit the type looking for `_` if we didn't fix the type above
+                visitor.visit_ty(a);
+                self.ty_of_arg(a, None)
+            })
+            .collect();
+
         let output_ty = match decl.output {
             hir::FnRetTy::Return(output) => {
                 visitor.visit_ty(output);
@@ -2579,24 +2596,34 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         debug!("ty_of_fn: output_ty={:?}", output_ty);
 
-        let fn_ty = tcx.mk_fn_sig(input_tys, output_ty, decl.c_variadic, unsafety, abi);
+        let fn_ty = tcx.mk_fn_sig(input_tys.into_iter(), output_ty, decl.c_variadic, unsafety, abi);
         let bare_fn_ty = ty::Binder::bind_with_vars(fn_ty, bound_vars);
 
-        if !self.allow_ty_infer() {
+        if !self.allow_ty_infer() && !(visitor.0.is_empty() && infer_replacements.is_empty()) {
             // We always collect the spans for placeholder types when evaluating `fn`s, but we
             // only want to emit an error complaining about them if infer types (`_`) are not
             // allowed. `allow_ty_infer` gates this behavior. We check for the presence of
             // `ident_span` to not emit an error twice when we have `fn foo(_: fn() -> _)`.
 
-            crate::collect::placeholder_type_error(
+            let mut diag = crate::collect::placeholder_type_error_diag(
                 tcx,
                 ident_span.map(|sp| sp.shrink_to_hi()),
                 generics.params,
                 visitor.0,
+                infer_replacements.iter().map(|(s, _)| *s).collect(),
                 true,
                 hir_ty,
                 "function",
             );
+
+            if !infer_replacements.is_empty() {
+                diag.multipart_suggestion(&format!(
+                    "try replacing `_` with the type{} in the corresponding trait method signature",
+                    if infer_replacements.len() > 1 { "s" } else { "" }
+                ), infer_replacements, Applicability::MachineApplicable);
+            }
+
+            diag.emit();
         }
 
         // Find any late-bound regions declared in return type that do
@@ -2624,6 +2651,39 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         bare_fn_ty
     }
 
+    /// Given a fn_hir_id for a impl function, suggest the type that is found on the
+    /// corresponding function in the trait that the impl implements, if it exists.
+    fn suggest_trait_fn_ty_for_impl_fn_infer(
+        &self,
+        fn_hir_id: hir::HirId,
+        arg_idx: usize,
+    ) -> Option<Ty<'tcx>> {
+        let tcx = self.tcx();
+        let hir = tcx.hir();
+
+        let hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), ident, .. }) =
+            hir.get(fn_hir_id) else { return None };
+        let hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(i), .. }) =
+                hir.get(hir.get_parent_node(fn_hir_id)) else { return None };
+
+        let trait_ref =
+            self.instantiate_mono_trait_ref(i.of_trait.as_ref()?, self.ast_ty_to_ty(i.self_ty));
+
+        let x: &ty::AssocItem = tcx.associated_items(trait_ref.def_id).find_by_name_and_kind(
+            tcx,
+            *ident,
+            ty::AssocKind::Fn,
+            trait_ref.def_id,
+        )?;
+
+        let fn_sig = tcx.fn_sig(x.def_id).subst(
+            tcx,
+            trait_ref.substs.extend_to(tcx, x.def_id, |param, _| tcx.mk_param_from_def(param)),
+        );
+
+        Some(tcx.erase_late_bound_regions(fn_sig.input(arg_idx)))
+    }
+
     fn validate_late_bound_regions(
         &self,
         constrained_regions: FxHashSet<ty::BoundRegionKind>,

compiler/rustc_typeck/src/collect.rs

@@ -160,6 +160,33 @@ crate fn placeholder_type_error<'tcx>(
         return;
     }
 
+    placeholder_type_error_diag(
+        tcx,
+        span,
+        generics,
+        placeholder_types,
+        vec![],
+        suggest,
+        hir_ty,
+        kind,
+    )
+    .emit();
+}
+
+crate fn placeholder_type_error_diag<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    span: Option<Span>,
+    generics: &[hir::GenericParam<'_>],
+    placeholder_types: Vec<Span>,
+    additional_spans: Vec<Span>,
+    suggest: bool,
+    hir_ty: Option<&hir::Ty<'_>>,
+    kind: &'static str,
+) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+    if placeholder_types.is_empty() {
+        return bad_placeholder(tcx, additional_spans, kind);
+    }
+
     let type_name = generics.next_type_param_name(None);
     let mut sugg: Vec<_> =
         placeholder_types.iter().map(|sp| (*sp, (*type_name).to_string())).collect();
@@ -182,7 +209,8 @@ crate fn placeholder_type_error<'tcx>(
         sugg.push((span, format!(", {}", type_name)));
     }
 
-    let mut err = bad_placeholder(tcx, placeholder_types, kind);
+    let mut err =
+        bad_placeholder(tcx, placeholder_types.into_iter().chain(additional_spans).collect(), kind);
 
     // Suggest, but only if it is not a function in const or static
     if suggest {
@@ -218,7 +246,8 @@ crate fn placeholder_type_error<'tcx>(
             );
         }
     }
-    err.emit();
+
+    err
 }
 
 fn reject_placeholder_type_signatures_in_item<'tcx>(

src/test/ui/did_you_mean/replace-impl-infer-ty-from-trait.fixed

@@ -0,0 +1,13 @@
+// run-rustfix
+#![allow(unused)]
+
+trait Foo<T>: Sized {
+    fn bar(i: i32, t: T, s: &Self) {}
+}
+
+impl Foo<usize> for () {
+    fn bar(i: i32, t: usize, s: &()) {}
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for functions
+}
+
+fn main() {}

src/test/ui/did_you_mean/replace-impl-infer-ty-from-trait.rs

@@ -0,0 +1,13 @@
+// run-rustfix
+#![allow(unused)]
+
+trait Foo<T>: Sized {
+    fn bar(i: i32, t: T, s: &Self) {}
+}
+
+impl Foo<usize> for () {
+    fn bar(i: _, t: _, s: _) {}
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for functions
+}
+
+fn main() {}

src/test/ui/did_you_mean/replace-impl-infer-ty-from-trait.stderr

@@ -0,0 +1,17 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions
+  --> $DIR/replace-impl-infer-ty-from-trait.rs:9:15
+   |
+LL |     fn bar(i: _, t: _, s: _) {}
+   |               ^     ^     ^ not allowed in type signatures
+   |               |     |
+   |               |     not allowed in type signatures
+   |               not allowed in type signatures
+   |
+help: try replacing `_` with the types in the corresponding trait method signature
+   |
+LL |     fn bar(i: i32, t: usize, s: &()) {}
+   |               ~~~     ~~~~~     ~~~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0121`.

src/test/ui/typeck/typeck_type_placeholder_item.stderr

@@ -560,10 +560,10 @@ error[E0121]: the placeholder `_` is not allowed within types on item signatures
 LL |     fn clone_from(&mut self, other: _) { *self = Test9; }
    |                                     ^ not allowed in type signatures
    |
-help: use type parameters instead
+help: try replacing `_` with the type in the corresponding trait method signature
    |
-LL |     fn clone_from<T>(&mut self, other: T) { *self = Test9; }
-   |                  +++                   ~
+LL |     fn clone_from(&mut self, other: &Test9) { *self = Test9; }
+   |                                     ~~~~~~
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
   --> $DIR/typeck_type_placeholder_item.rs:107:31
@@ -600,10 +600,10 @@ error[E0121]: the placeholder `_` is not allowed within types on item signatures
 LL |         fn clone_from(&mut self, other: _) { *self = FnTest9; }
    |                                         ^ not allowed in type signatures
    |
-help: use type parameters instead
+help: try replacing `_` with the type in the corresponding trait method signature
    |
-LL |         fn clone_from<T>(&mut self, other: T) { *self = FnTest9; }
-   |                      +++                   ~
+LL |         fn clone_from(&mut self, other: &FnTest9) { *self = FnTest9; }
+   |                                         ~~~~~~~~
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for associated types
   --> $DIR/typeck_type_placeholder_item.rs:201:14