Suggest correct syntax when writing type arg instead of assoc type

When confusing an associated type with a type argument, suggest the
appropriate syntax.

Given `Iterator<isize>`, suggest `Iterator<Item = isize>`.

src/librustc_typeck/astconv.rs

@@ -182,7 +182,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         item_segment: &hir::PathSegment)
         -> &'tcx Substs<'tcx>
     {
-        let (substs, assoc_bindings) = item_segment.with_generic_args(|generic_args| {
+        let (substs, assoc_bindings, _) = item_segment.with_generic_args(|generic_args| {
             self.create_substs_for_ast_path(
                 span,
                 def_id,
@@ -256,7 +256,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
             },
             def.parent.is_none() && def.has_self, // `has_self`
             seg.infer_types || suppress_mismatch, // `infer_types`
-        )
+        ).0
     }
 
     /// Check that the correct number of generic arguments have been provided.
@@ -269,7 +269,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         position: GenericArgPosition,
         has_self: bool,
         infer_types: bool,
-    ) -> bool {
+    ) -> (bool, Option<Vec<Span>>) {
         // At this stage we are guaranteed that the generic arguments are in the correct order, e.g.
         // that lifetimes will proceed types. So it suffices to check the number of each generic
         // arguments in order to validate them with respect to the generic parameters.
@@ -303,13 +303,13 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
                     let mut err = tcx.sess.struct_span_err(span, msg);
                     err.span_note(span_late, note);
                     err.emit();
-                    return true;
+                    return (true, None);
                 } else {
                     let mut multispan = MultiSpan::from_span(span);
                     multispan.push_span_label(span_late, note.to_string());
                     tcx.lint_node(lint::builtin::LATE_BOUND_LIFETIME_ARGUMENTS,
                                   args.args[0].id(), multispan, msg);
-                    return false;
+                    return (false, None);
                 }
             }
         }
@@ -323,7 +323,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
             // For kinds without defaults (i.e. lifetimes), `required == permitted`.
             // For other kinds (i.e. types), `permitted` may be greater than `required`.
             if required <= provided && provided <= permitted {
-                return false;
+                return (false, None);
             }
 
             // Unfortunately lifetime and type parameter mismatches are typically styled
@@ -338,19 +338,16 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
                 (required, "")
             };
 
+            let mut potential_assoc_types: Option<Vec<Span>> = None;
             let (spans, label) = if required == permitted && provided > permitted {
                 // In the case when the user has provided too many arguments,
                 // we want to point to the unexpected arguments.
-                (
-                    args.args[offset+permitted .. offset+provided]
+                let spans: Vec<Span> = args.args[offset+permitted .. offset+provided]
                         .iter()
                         .map(|arg| arg.span())
-                        .collect(),
-                    format!(
-                        "unexpected {} argument",
-                        kind,
-                    ),
-                )
+                        .collect();
+                potential_assoc_types = Some(spans.clone());
+                (spans, format!( "unexpected {} argument", kind))
             } else {
                 (vec![span], format!(
                     "expected {}{} {} argument{}",
@@ -377,7 +374,8 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
             }
             err.emit();
 
-            provided > required // `suppress_error`
+            (provided > required, // `suppress_error`
+             potential_assoc_types)
         };
 
         if !infer_lifetimes || arg_counts.lifetimes > param_counts.lifetimes {
@@ -399,7 +397,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
                 arg_counts.lifetimes,
             )
         } else {
-            false
+            (false, None)
         }
     }
 
@@ -557,7 +555,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         generic_args: &hir::GenericArgs,
         infer_types: bool,
         self_ty: Option<Ty<'tcx>>)
-        -> (&'tcx Substs<'tcx>, Vec<ConvertedBinding<'tcx>>)
+        -> (&'tcx Substs<'tcx>, Vec<ConvertedBinding<'tcx>>, Option<Vec<Span>>)
     {
         // If the type is parameterized by this region, then replace this
         // region with the current anon region binding (in other words,
@@ -573,7 +571,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         assert_eq!(generic_params.has_self, self_ty.is_some());
 
         let has_self = generic_params.has_self;
-        Self::check_generic_arg_count(
+        let (_, potential_assoc_types) = Self::check_generic_arg_count(
             self.tcx(),
             span,
             &generic_params,
@@ -678,7 +676,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         debug!("create_substs_for_ast_path(generic_params={:?}, self_ty={:?}) -> {:?}",
                generic_params, self_ty, substs);
 
-        (substs, assoc_bindings)
+        (substs, assoc_bindings, potential_assoc_types)
     }
 
     /// Instantiates the path for the given trait reference, assuming that it's
@@ -720,7 +718,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         self_ty: Ty<'tcx>,
         poly_projections: &mut Vec<(ty::PolyProjectionPredicate<'tcx>, Span)>,
         speculative: bool)
-        -> ty::PolyTraitRef<'tcx>
+        -> (ty::PolyTraitRef<'tcx>, Option<Vec<Span>>)
     {
         let trait_def_id = self.trait_def_id(trait_ref);
 
@@ -728,11 +726,12 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
 
         self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1);
 
-        let (substs, assoc_bindings) =
-            self.create_substs_for_ast_trait_ref(trait_ref.path.span,
-                                                 trait_def_id,
-                                                 self_ty,
-                                                 trait_ref.path.segments.last().unwrap());
+        let (substs, assoc_bindings, potential_assoc_types) = self.create_substs_for_ast_trait_ref(
+            trait_ref.path.span,
+            trait_def_id,
+            self_ty,
+            trait_ref.path.segments.last().unwrap(),
+        );
         let poly_trait_ref = ty::Binder::bind(ty::TraitRef::new(trait_def_id, substs));
 
         let mut dup_bindings = FxHashMap::default();
@@ -747,14 +746,14 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
 
         debug!("instantiate_poly_trait_ref({:?}, projections={:?}) -> {:?}",
                trait_ref, poly_projections, poly_trait_ref);
-        poly_trait_ref
+        (poly_trait_ref, potential_assoc_types)
     }
 
     pub fn instantiate_poly_trait_ref(&self,
         poly_trait_ref: &hir::PolyTraitRef,
         self_ty: Ty<'tcx>,
         poly_projections: &mut Vec<(ty::PolyProjectionPredicate<'tcx>, Span)>)
-        -> ty::PolyTraitRef<'tcx>
+        -> (ty::PolyTraitRef<'tcx>, Option<Vec<Span>>)
     {
         self.instantiate_poly_trait_ref_inner(&poly_trait_ref.trait_ref, self_ty,
                                               poly_projections, false)
@@ -767,7 +766,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
                                   trait_segment: &hir::PathSegment)
                                   -> ty::TraitRef<'tcx>
     {
-        let (substs, assoc_bindings) =
+        let (substs, assoc_bindings, _) =
             self.create_substs_for_ast_trait_ref(span,
                                                  trait_def_id,
                                                  self_ty,
@@ -776,13 +775,13 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         ty::TraitRef::new(trait_def_id, substs)
     }
 
-    fn create_substs_for_ast_trait_ref(&self,
-                                       span: Span,
-                                       trait_def_id: DefId,
-                                       self_ty: Ty<'tcx>,
-                                       trait_segment: &hir::PathSegment)
-                                       -> (&'tcx Substs<'tcx>, Vec<ConvertedBinding<'tcx>>)
-    {
+    fn create_substs_for_ast_trait_ref(
+        &self,
+        span: Span,
+        trait_def_id: DefId,
+        self_ty: Ty<'tcx>,
+        trait_segment: &hir::PathSegment,
+    ) -> (&'tcx Substs<'tcx>, Vec<ConvertedBinding<'tcx>>, Option<Vec<Span>>) {
         debug!("create_substs_for_ast_trait_ref(trait_segment={:?})",
                trait_segment);
 
@@ -972,9 +971,11 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
 
         let mut projection_bounds = Vec::new();
         let dummy_self = tcx.mk_ty(TRAIT_OBJECT_DUMMY_SELF);
-        let principal = self.instantiate_poly_trait_ref(&trait_bounds[0],
-                                                        dummy_self,
-                                                        &mut projection_bounds);
+        let (principal, potential_assoc_types) = self.instantiate_poly_trait_ref(
+            &trait_bounds[0],
+            dummy_self,
+            &mut projection_bounds,
+        );
         debug!("principal: {:?}", principal);
 
         for trait_bound in trait_bounds[1..].iter() {
@@ -1047,16 +1048,47 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
                 if associated_types.len() == 1 { "" } else { "s" },
                 names,
             );
-            for item_def_id in associated_types {
-                let assoc_item = tcx.associated_item(item_def_id);
+            let mut suggest = false;
+            let mut potential_assoc_types_spans = vec![];
+            if let Some(potential_assoc_types) = potential_assoc_types {
+                if potential_assoc_types.len() == associated_types.len() {
+                    // Only suggest when the amount of missing associated types is equals to the
+                    // extra type arguments present, as that gives us a relatively high confidence
+                    // that the user forgot to give the associtated type's name. The canonical
+                    // example would be trying to use `Iterator<isize>` instead of
+                    // `Iterator<Item=isize>`.
+                    suggest = true;
+                    potential_assoc_types_spans = potential_assoc_types;
+                }
+            }
+            let mut suggestions = vec![];
+            for (i, item_def_id) in associated_types.iter().enumerate() {
+                let assoc_item = tcx.associated_item(*item_def_id);
                 err.span_label(
                     span,
                     format!("missing associated type `{}` value", assoc_item.ident),
                 );
                 err.span_label(
-                    tcx.def_span(item_def_id),
+                    tcx.def_span(*item_def_id),
                     format!("`{}` defined here", assoc_item.ident),
                 );
+                if suggest {
+                    if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(
+                        potential_assoc_types_spans[i],
+                    ) {
+                        suggestions.push((
+                            potential_assoc_types_spans[i],
+                            format!("{} = {}", assoc_item.ident, snippet),
+                        ));
+                    }
+                }
+            }
+            if !suggestions.is_empty() {
+                err.multipart_suggestion_with_applicability(
+                    "if you meant to assign the missing associated type, use the name",
+                    suggestions,
+                    Applicability::MaybeIncorrect,
+                );
             }
             err.emit();
         }

src/librustc_typeck/collect.rs

@@ -1892,7 +1892,7 @@ fn explicit_predicates_of<'a, 'tcx>(
                         &hir::GenericBound::Trait(ref poly_trait_ref, _) => {
                             let mut projections = Vec::new();
 
-                            let trait_ref = AstConv::instantiate_poly_trait_ref(
+                            let (trait_ref, _) = AstConv::instantiate_poly_trait_ref(
                                 &icx,
                                 poly_trait_ref,
                                 ty,
@@ -2016,7 +2016,12 @@ pub fn compute_bounds<'gcx: 'tcx, 'tcx>(
     let mut projection_bounds = Vec::new();
 
     let mut trait_bounds: Vec<_> = trait_bounds.iter().map(|&bound| {
-        (astconv.instantiate_poly_trait_ref(bound, param_ty, &mut projection_bounds), bound.span)
+        let (poly_trait_ref, _) = astconv.instantiate_poly_trait_ref(
+            bound,
+            param_ty,
+            &mut projection_bounds,
+        );
+        (poly_trait_ref, bound.span)
     }).collect();
 
     let region_bounds = region_bounds
@@ -2057,7 +2062,7 @@ fn predicates_from_bound<'tcx>(
     match *bound {
         hir::GenericBound::Trait(ref tr, hir::TraitBoundModifier::None) => {
             let mut projections = Vec::new();
-            let pred = astconv.instantiate_poly_trait_ref(tr, param_ty, &mut projections);
+            let (pred, _) = astconv.instantiate_poly_trait_ref(tr, param_ty, &mut projections);
             iter::once((pred.to_predicate(), tr.span)).chain(
                 projections
                     .into_iter()

src/librustc_typeck/lib.rs

@@ -389,7 +389,7 @@ pub fn hir_trait_to_predicates<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, hir_trait:
     let env_def_id = tcx.hir.local_def_id(env_node_id);
     let item_cx = self::collect::ItemCtxt::new(tcx, env_def_id);
     let mut projections = Vec::new();
-    let principal = astconv::AstConv::instantiate_poly_trait_ref_inner(
+    let (principal, _) = astconv::AstConv::instantiate_poly_trait_ref_inner(
         &item_cx, hir_trait, tcx.types.err, &mut projections, true
     );
 

src/test/ui/suggestions/use-type-argument-instead-of-assoc-type.stderr

@@ -1,5 +1,5 @@
 error[E0107]: wrong number of type arguments: expected 2, found 4
-  --> $DIR/E0107-b.rs:6:42
+  --> $DIR/use-type-argument-instead-of-assoc-type.rs:6:42
    |
 LL |  pub struct Foo { i: Box<T<usize, usize, usize, usize, B=usize>> }
    |                                          ^^^^^  ^^^^^ unexpected type argument
@@ -7,7 +7,7 @@ LL |  pub struct Foo { i: Box<T<usize, usize, usize, usize, B=usize>> }
    |                                          unexpected type argument
 
 error[E0191]: the value of the associated types `A` (from the trait `T`), `C` (from the trait `T`) must be specified
-  --> $DIR/E0107-b.rs:6:26
+  --> $DIR/use-type-argument-instead-of-assoc-type.rs:6:26
    |
 LL |     type A;
    |     ------- `A` defined here
@@ -20,6 +20,10 @@ LL |  pub struct Foo { i: Box<T<usize, usize, usize, usize, B=usize>> }
    |                          |
    |                          missing associated type `A` value
    |                          missing associated type `C` value
+help: if you meant to assign the missing associated type, use the name
+   |
+LL |  pub struct Foo { i: Box<T<usize, usize, A = usize, C = usize, B=usize>> }
+   |                                          ^^^^^^^^^  ^^^^^^^^^
 
 error: aborting due to 2 previous errors