Auto merge of #68447 - estebank:sugg-type-param, r=petrochenkov

Suggest defining type parameter when appropriate

```
error[E0412]: cannot find type `T` in this scope
 --> file.rs:3:12
  |
3 | impl Trait<T> for Struct {}
  |     -      ^ not found in this scope
  |     |
  |     help: you might be missing a type parameter: `<T>`
```

Fix #64298.

src/librustc_resolve/diagnostics.rs

@@ -1415,11 +1415,15 @@ crate fn show_candidates(
     better: bool,
     found_use: bool,
 ) {
+    if candidates.is_empty() {
+        return;
+    }
     // we want consistent results across executions, but candidates are produced
     // by iterating through a hash map, so make sure they are ordered:
     let mut path_strings: Vec<_> =
         candidates.into_iter().map(|c| path_names_to_string(&c.path)).collect();
     path_strings.sort();
+    path_strings.dedup();
 
     let better = if better { "better " } else { "" };
     let msg_diff = match path_strings.len() {
@@ -1444,6 +1448,7 @@ crate fn show_candidates(
             msg.push('\n');
             msg.push_str(&candidate);
         }
+        err.note(&msg);
     }
 }
 

src/librustc_resolve/late.rs

@@ -323,7 +323,7 @@ impl<'a> PathSource<'a> {
 }
 
 #[derive(Default)]
-struct DiagnosticMetadata {
+struct DiagnosticMetadata<'ast> {
     /// The current trait's associated types' ident, used for diagnostic suggestions.
     current_trait_assoc_types: Vec<Ident>,
 
@@ -333,6 +333,13 @@ struct DiagnosticMetadata {
     /// The current self item if inside an ADT (used for better errors).
     current_self_item: Option<NodeId>,
 
+    /// The current trait (used to suggest).
+    current_item: Option<&'ast Item>,
+
+    /// When processing generics and encountering a type not found, suggest introducing a type
+    /// param.
+    currently_processing_generics: bool,
+
     /// The current enclosing function (used for better errors).
     current_function: Option<Span>,
 
@@ -347,7 +354,7 @@ struct DiagnosticMetadata {
     current_let_binding: Option<(Span, Option<Span>, Option<Span>)>,
 }
 
-struct LateResolutionVisitor<'a, 'b> {
+struct LateResolutionVisitor<'a, 'b, 'ast> {
     r: &'b mut Resolver<'a>,
 
     /// The module that represents the current item scope.
@@ -364,30 +371,32 @@ struct LateResolutionVisitor<'a, 'b> {
     current_trait_ref: Option<(Module<'a>, TraitRef)>,
 
     /// Fields used to add information to diagnostic errors.
-    diagnostic_metadata: DiagnosticMetadata,
+    diagnostic_metadata: DiagnosticMetadata<'ast>,
 }
 
 /// Walks the whole crate in DFS order, visiting each item, resolving names as it goes.
-impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
-    fn visit_item(&mut self, item: &'tcx Item) {
+impl<'a, 'ast> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast> {
+    fn visit_item(&mut self, item: &'ast Item) {
+        let prev = replace(&mut self.diagnostic_metadata.current_item, Some(item));
         self.resolve_item(item);
+        self.diagnostic_metadata.current_item = prev;
     }
-    fn visit_arm(&mut self, arm: &'tcx Arm) {
+    fn visit_arm(&mut self, arm: &'ast Arm) {
         self.resolve_arm(arm);
     }
-    fn visit_block(&mut self, block: &'tcx Block) {
+    fn visit_block(&mut self, block: &'ast Block) {
         self.resolve_block(block);
     }
-    fn visit_anon_const(&mut self, constant: &'tcx AnonConst) {
+    fn visit_anon_const(&mut self, constant: &'ast AnonConst) {
         debug!("visit_anon_const {:?}", constant);
         self.with_constant_rib(|this| {
             visit::walk_anon_const(this, constant);
         });
     }
-    fn visit_expr(&mut self, expr: &'tcx Expr) {
+    fn visit_expr(&mut self, expr: &'ast Expr) {
         self.resolve_expr(expr, None);
     }
-    fn visit_local(&mut self, local: &'tcx Local) {
+    fn visit_local(&mut self, local: &'ast Local) {
         let local_spans = match local.pat.kind {
             // We check for this to avoid tuple struct fields.
             PatKind::Wild => None,
@@ -401,7 +410,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
         self.resolve_local(local);
         self.diagnostic_metadata.current_let_binding = original;
     }
-    fn visit_ty(&mut self, ty: &'tcx Ty) {
+    fn visit_ty(&mut self, ty: &'ast Ty) {
         match ty.kind {
             TyKind::Path(ref qself, ref path) => {
                 self.smart_resolve_path(ty.id, qself.as_ref(), path, PathSource::Type);
@@ -417,7 +426,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
         }
         visit::walk_ty(self, ty);
     }
-    fn visit_poly_trait_ref(&mut self, tref: &'tcx PolyTraitRef, m: &'tcx TraitBoundModifier) {
+    fn visit_poly_trait_ref(&mut self, tref: &'ast PolyTraitRef, m: &'ast TraitBoundModifier) {
         self.smart_resolve_path(
             tref.trait_ref.ref_id,
             None,
@@ -426,7 +435,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
         );
         visit::walk_poly_trait_ref(self, tref, m);
     }
-    fn visit_foreign_item(&mut self, foreign_item: &'tcx ForeignItem) {
+    fn visit_foreign_item(&mut self, foreign_item: &'ast ForeignItem) {
         match foreign_item.kind {
             ForeignItemKind::Fn(_, ref generics) => {
                 self.with_generic_param_rib(generics, ItemRibKind(HasGenericParams::Yes), |this| {
@@ -443,7 +452,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
             }
         }
     }
-    fn visit_fn(&mut self, fn_kind: FnKind<'tcx>, declaration: &'tcx FnDecl, sp: Span, _: NodeId) {
+    fn visit_fn(&mut self, fn_kind: FnKind<'ast>, declaration: &'ast FnDecl, sp: Span, _: NodeId) {
         let previous_value = replace(&mut self.diagnostic_metadata.current_function, Some(sp));
         debug!("(resolving function) entering function");
         let rib_kind = match fn_kind {
@@ -472,7 +481,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
         self.diagnostic_metadata.current_function = previous_value;
     }
 
-    fn visit_generics(&mut self, generics: &'tcx Generics) {
+    fn visit_generics(&mut self, generics: &'ast Generics) {
         // For type parameter defaults, we have to ban access
         // to following type parameters, as the InternalSubsts can only
         // provide previous type parameters as they're built. We
@@ -534,11 +543,12 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
         }
     }
 
-    fn visit_generic_arg(&mut self, arg: &'tcx GenericArg) {
+    fn visit_generic_arg(&mut self, arg: &'ast GenericArg) {
         debug!("visit_generic_arg({:?})", arg);
+        let prev = replace(&mut self.diagnostic_metadata.currently_processing_generics, true);
         match arg {
             GenericArg::Type(ref ty) => {
-                // We parse const arguments as path types as we cannot distiguish them durring
+                // We parse const arguments as path types as we cannot distiguish them during
                 // parsing. We try to resolve that ambiguity by attempting resolution the type
                 // namespace first, and if that fails we try again in the value namespace. If
                 // resolution in the value namespace succeeds, we have an generic const argument on
@@ -556,7 +566,6 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
                             )
                             .is_some()
                         };
-
                         if !check_ns(TypeNS) && check_ns(ValueNS) {
                             // This must be equivalent to `visit_anon_const`, but we cannot call it
                             // directly due to visitor lifetimes so we have to copy-paste some code.
@@ -574,6 +583,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
                                 this.visit_path(path, ty.id);
                             });
 
+                            self.diagnostic_metadata.currently_processing_generics = prev;
                             return;
                         }
                     }
@@ -584,11 +594,12 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
             GenericArg::Lifetime(lt) => self.visit_lifetime(lt),
             GenericArg::Const(ct) => self.visit_anon_const(ct),
         }
+        self.diagnostic_metadata.currently_processing_generics = prev;
     }
 }
 
-impl<'a, 'b> LateResolutionVisitor<'a, '_> {
-    fn new(resolver: &'b mut Resolver<'a>) -> LateResolutionVisitor<'a, 'b> {
+impl<'a, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
+    fn new(resolver: &'b mut Resolver<'a>) -> LateResolutionVisitor<'a, 'b, 'ast> {
         // During late resolution we only track the module component of the parent scope,
         // although it may be useful to track other components as well for diagnostics.
         let graph_root = resolver.graph_root;
@@ -724,7 +735,7 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         None
     }
 
-    fn resolve_adt(&mut self, item: &Item, generics: &Generics) {
+    fn resolve_adt(&mut self, item: &'ast Item, generics: &'ast Generics) {
         debug!("resolve_adt");
         self.with_current_self_item(item, |this| {
             this.with_generic_param_rib(generics, ItemRibKind(HasGenericParams::Yes), |this| {
@@ -778,7 +789,7 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         }
     }
 
-    fn resolve_item(&mut self, item: &Item) {
+    fn resolve_item(&mut self, item: &'ast Item) {
         let name = item.ident.name;
         debug!("(resolving item) resolving {} ({:?})", name, item.kind);
 
@@ -1024,16 +1035,15 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         let mut new_id = None;
         if let Some(trait_ref) = opt_trait_ref {
             let path: Vec<_> = Segment::from_path(&trait_ref.path);
-            let res = self
-                .smart_resolve_path_fragment(
-                    trait_ref.ref_id,
-                    None,
-                    &path,
-                    trait_ref.path.span,
-                    PathSource::Trait(AliasPossibility::No),
-                    CrateLint::SimplePath(trait_ref.ref_id),
-                )
-                .base_res();
+            let res = self.smart_resolve_path_fragment(
+                trait_ref.ref_id,
+                None,
+                &path,
+                trait_ref.path.span,
+                PathSource::Trait(AliasPossibility::No),
+                CrateLint::SimplePath(trait_ref.ref_id),
+            );
+            let res = res.base_res();
             if res != Res::Err {
                 new_id = Some(res.def_id());
                 let span = trait_ref.path.span;
@@ -1070,11 +1080,11 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
 
     fn resolve_implementation(
         &mut self,
-        generics: &Generics,
-        opt_trait_reference: &Option<TraitRef>,
-        self_type: &Ty,
+        generics: &'ast Generics,
+        opt_trait_reference: &'ast Option<TraitRef>,
+        self_type: &'ast Ty,
         item_id: NodeId,
-        impl_items: &[AssocItem],
+        impl_items: &'ast [AssocItem],
     ) {
         debug!("resolve_implementation");
         // If applicable, create a rib for the type parameters.
@@ -1179,7 +1189,7 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         }
     }
 
-    fn resolve_params(&mut self, params: &[Param]) {
+    fn resolve_params(&mut self, params: &'ast [Param]) {
         let mut bindings = smallvec![(PatBoundCtx::Product, Default::default())];
         for Param { pat, ty, .. } in params {
             self.resolve_pattern(pat, PatternSource::FnParam, &mut bindings);
@@ -1188,7 +1198,7 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         }
     }
 
-    fn resolve_local(&mut self, local: &Local) {
+    fn resolve_local(&mut self, local: &'ast Local) {
         // Resolve the type.
         walk_list!(self, visit_ty, &local.ty);
 
@@ -1307,7 +1317,7 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
     }
 
     /// Check the consistency of the outermost or-patterns.
-    fn check_consistent_bindings_top(&mut self, pat: &Pat) {
+    fn check_consistent_bindings_top(&mut self, pat: &'ast Pat) {
         pat.walk(&mut |pat| match pat.kind {
             PatKind::Or(ref ps) => {
                 self.check_consistent_bindings(ps);
@@ -1317,7 +1327,7 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         })
     }
 
-    fn resolve_arm(&mut self, arm: &Arm) {
+    fn resolve_arm(&mut self, arm: &'ast Arm) {
         self.with_rib(ValueNS, NormalRibKind, |this| {
             this.resolve_pattern_top(&arm.pat, PatternSource::Match);
             walk_list!(this, visit_expr, &arm.guard);
@@ -1326,14 +1336,14 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
     }
 
     /// Arising from `source`, resolve a top level pattern.
-    fn resolve_pattern_top(&mut self, pat: &Pat, pat_src: PatternSource) {
+    fn resolve_pattern_top(&mut self, pat: &'ast Pat, pat_src: PatternSource) {
         let mut bindings = smallvec![(PatBoundCtx::Product, Default::default())];
         self.resolve_pattern(pat, pat_src, &mut bindings);
     }
 
     fn resolve_pattern(
         &mut self,
-        pat: &Pat,
+        pat: &'ast Pat,
         pat_src: PatternSource,
         bindings: &mut SmallVec<[(PatBoundCtx, FxHashSet<Ident>); 1]>,
     ) {
@@ -1544,7 +1554,7 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         id: NodeId,
         qself: Option<&QSelf>,
         path: &Path,
-        source: PathSource<'_>,
+        source: PathSource<'ast>,
     ) {
         self.smart_resolve_path_fragment(
             id,
@@ -1562,7 +1572,7 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         qself: Option<&QSelf>,
         path: &[Segment],
         span: Span,
-        source: PathSource<'_>,
+        source: PathSource<'ast>,
         crate_lint: CrateLint,
     ) -> PartialRes {
         let ns = source.namespace();
@@ -1573,7 +1583,9 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
             let def_id = this.parent_scope.module.normal_ancestor_id;
             let node_id = this.r.definitions.as_local_node_id(def_id).unwrap();
             let better = res.is_some();
-            this.r.use_injections.push(UseError { err, candidates, node_id, better });
+            let suggestion =
+                if res.is_none() { this.report_missing_type_error(path) } else { None };
+            this.r.use_injections.push(UseError { err, candidates, node_id, better, suggestion });
             PartialRes::new(Res::Err)
         };
 
@@ -1838,11 +1850,11 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         }
     }
 
-    fn resolve_labeled_block(&mut self, label: Option<Label>, id: NodeId, block: &Block) {
+    fn resolve_labeled_block(&mut self, label: Option<Label>, id: NodeId, block: &'ast Block) {
         self.with_resolved_label(label, id, |this| this.visit_block(block));
     }
 
-    fn resolve_block(&mut self, block: &Block) {
+    fn resolve_block(&mut self, block: &'ast Block) {
         debug!("(resolving block) entering block");
         // Move down in the graph, if there's an anonymous module rooted here.
         let orig_module = self.parent_scope.module;
@@ -1885,7 +1897,7 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         debug!("(resolving block) leaving block");
     }
 
-    fn resolve_expr(&mut self, expr: &Expr, parent: Option<&Expr>) {
+    fn resolve_expr(&mut self, expr: &'ast Expr, parent: Option<&'ast Expr>) {
         // First, record candidate traits for this expression if it could
         // result in the invocation of a method call.
 
@@ -2023,7 +2035,7 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
         }
     }
 
-    fn record_candidate_traits_for_expr_if_necessary(&mut self, expr: &Expr) {
+    fn record_candidate_traits_for_expr_if_necessary(&mut self, expr: &'ast Expr) {
         match expr.kind {
             ExprKind::Field(_, ident) => {
                 // FIXME(#6890): Even though you can't treat a method like a