fix misleading cannot infer type for type parameter error

Author: TaKO8Ki

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -866,6 +866,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 }
             }
 
+            self.report_ambiguous_type_parameter(&mut err, arg);
             err.span_label(
                 span,
                 arg_data.cannot_infer_msg(use_diag.filter(|d| d.applies_to(span))),
@@ -933,6 +934,27 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
     }
 
+    fn report_ambiguous_type_parameter(&self, err: &mut Diagnostic, arg: GenericArg<'tcx>) {
+        if let GenericArgKind::Type(ty) = arg.unpack()
+            && let ty::Infer(ty::TyVar(ty_vid)) = *ty.kind()
+        {
+            let mut inner = self.inner.borrow_mut();
+            let ty_vars = &inner.type_variables();
+            let var_origin = ty_vars.var_origin(ty_vid);
+            if let TypeVariableOriginKind::TypeParameterDefinition(name, Some(def_id)) =
+                var_origin.kind
+                && let Some(parent_def_id) = self.tcx.parent(def_id).as_local()
+                && let Some(hir::Node::Item(item)) = self.tcx.hir().find_by_def_id(parent_def_id)
+                && let hir::ItemKind::Impl(impl_item) = item.kind
+                && let Some(trait_ref) = &impl_item.of_trait
+                && let Some(did) = trait_ref.trait_def_id()
+                && self.tcx.generics_of(did).params.iter().any(|param| param.name == name)
+            {
+                err.span_help(self.tcx.def_span(def_id), "type parameter declared here");
+            }
+        }
+    }
+
     pub fn need_type_info_err_in_generator(
         &self,
         kind: hir::GeneratorKind,

src/test/ui/inference/ambiguous_type_parameter.rs

@@ -0,0 +1,17 @@
+use std::collections::HashMap;
+
+trait Store<K, V> {
+    fn get_raw(&self, key: &K) -> Option<()>;
+}
+
+struct InMemoryStore;
+
+impl<K> Store<String, HashMap<K, String>> for InMemoryStore {
+    fn get_raw(&self, key: &String) -> Option<()> {
+        None
+    }
+}
+
+fn main() {
+    InMemoryStore.get_raw(&String::default()); //~ ERROR type annotations needed
+}

src/test/ui/inference/ambiguous_type_parameter.stderr

@@ -0,0 +1,15 @@
+error[E0282]: type annotations needed
+  --> $DIR/ambiguous_type_parameter.rs:16:19
+   |
+LL |     InMemoryStore.get_raw(&String::default());
+   |                   ^^^^^^^ cannot infer type for type parameter `K`
+   |
+help: type parameter declared here
+  --> $DIR/ambiguous_type_parameter.rs:9:6
+   |
+LL | impl<K> Store<String, HashMap<K, String>> for InMemoryStore {
+   |      ^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/traits/not-suggest-non-existing-fully-qualified-path.stderr

@@ -16,6 +16,11 @@ LL |     a.method();
    |     | cannot infer type for type parameter `U`
    |     this method call resolves to `U`
    |
+help: type parameter declared here
+  --> $DIR/not-suggest-non-existing-fully-qualified-path.rs:12:9
+   |
+LL | impl<T, U> V<U> for A<T>
+   |         ^
 note: multiple `impl`s satisfying `B: I<_>` found
   --> $DIR/not-suggest-non-existing-fully-qualified-path.rs:5:1
    |