Point at individual type args on arg count mismatch

src/librustc/hir/map/mod.rs

@@ -628,10 +628,6 @@ impl<'hir> Map<'hir> {
         })
     }
 
-    pub fn get_generics_span(&self, id: DefId) -> Option<Span> {
-        self.get_generics(id).map(|generics| generics.span).filter(|sp| *sp != DUMMY_SP)
-    }
-
     /// Retrieves the `Node` corresponding to `id`, returning `None` if cannot be found.
     pub fn find(&self, id: NodeId) -> Option<Node<'hir>> {
         let hir_id = self.node_to_hir_id(id);

src/librustc/hir/mod.rs

@@ -16,7 +16,7 @@ use crate::util::nodemap::{NodeMap, FxHashSet};
 use crate::mir::mono::Linkage;
 
 use errors::FatalError;
-use syntax_pos::{Span, DUMMY_SP, symbol::InternedString};
+use syntax_pos::{Span, DUMMY_SP, symbol::InternedString, MultiSpan};
 use syntax::source_map::Spanned;
 use rustc_target::spec::abi::Abi;
 use syntax::ast::{self, CrateSugar, Ident, Name, NodeId, AsmDialect};
@@ -624,6 +624,14 @@ impl Generics {
         }
         None
     }
+
+    pub fn spans(&self) -> MultiSpan {
+        if self.params.is_empty() {
+            self.span.into()
+        } else {
+            self.params.iter().map(|p| p.span).collect::<Vec<Span>>().into()
+        }
+    }
 }
 
 /// Synthetic type parameters are converted to another form during lowering; this allows

src/librustc_typeck/check/compare_method.rs

@@ -385,7 +385,7 @@ fn check_region_bounds_on_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     // the moment, give a kind of vague error message.
     if trait_params != impl_params {
         let def_span = tcx.sess.source_map().def_span(span);
-        let span = tcx.hir().get_generics_span(impl_m.def_id).unwrap_or(def_span);
+        let span = tcx.hir().get_generics(impl_m.def_id).map(|g| g.span).unwrap_or(def_span);
         let mut err = struct_span_err!(
             tcx.sess,
             span,
@@ -396,7 +396,7 @@ fn check_region_bounds_on_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         err.span_label(span, "lifetimes do not match method in trait");
         if let Some(sp) = tcx.hir().span_if_local(trait_m.def_id) {
             let def_sp = tcx.sess.source_map().def_span(sp);
-            let sp = tcx.hir().get_generics_span(trait_m.def_id).unwrap_or(def_sp);
+            let sp = tcx.hir().get_generics(trait_m.def_id).map(|g| g.span).unwrap_or(def_sp);
             err.span_label(sp, "lifetimes in impl do not match this method in trait");
         }
         err.emit();
@@ -583,7 +583,7 @@ fn compare_self_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 fn compare_number_of_generics<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     impl_: &ty::AssocItem,
-    impl_span: Span,
+    _impl_span: Span,
     trait_: &ty::AssocItem,
     trait_span: Option<Span>,
 ) -> Result<(), ErrorReported> {
@@ -600,17 +600,42 @@ fn compare_number_of_generics<'a, 'tcx>(
         if impl_count != trait_count {
             err_occurred = true;
 
-            let impl_hir_id = tcx.hir().as_local_hir_id(impl_.def_id).unwrap();
-            let impl_item = tcx.hir().expect_impl_item(impl_hir_id);
-            let span = if impl_item.generics.params.is_empty()
-                || impl_item.generics.span.is_dummy() { // argument position impl Trait (#55374)
-                impl_span
+            let (
+                trait_spans,
+                impl_trait_spans,
+            ) = if let Some(trait_hir_id) = tcx.hir().as_local_hir_id(trait_.def_id) {
+                let trait_item = tcx.hir().expect_trait_item(trait_hir_id);
+                if trait_item.generics.params.is_empty() {
+                    (Some(vec![trait_item.generics.span]), vec![])
+                } else {
+                    let arg_spans: Vec<Span> = trait_item.generics.params.iter()
+                        .map(|p| p.span)
+                        .collect();
+                    let impl_trait_spans: Vec<Span> = trait_item.generics.params.iter()
+                        .filter_map(|p| if !trait_item.generics.span.overlaps(p.span) {
+                            Some(p.span)
+                        } else {
+                            None
+                        }).collect();
+                    (Some(arg_spans), impl_trait_spans)
+                }
             } else {
-                impl_item.generics.span
+                (trait_span.map(|s| vec![s]), vec![])
             };
 
+            let impl_hir_id = tcx.hir().as_local_hir_id(impl_.def_id).unwrap();
+            let impl_item = tcx.hir().expect_impl_item(impl_hir_id);
+            let impl_item_impl_trait_spans: Vec<Span> = impl_item.generics.params.iter()
+                .filter_map(|p| if !impl_item.generics.span.overlaps(p.span) {
+                    Some(p.span)
+                } else {
+                    None
+                }).collect();
+            let spans = impl_item.generics.spans();
+            let span = spans.primary_span();
+
             let mut err = tcx.sess.struct_span_err_with_code(
-                span,
+                spans,
                 &format!(
                     "method `{}` has {} {kind} parameter{} but its trait \
                      declaration has {} {kind} parameter{}",
@@ -626,22 +651,36 @@ fn compare_number_of_generics<'a, 'tcx>(
 
             let mut suffix = None;
 
-            if let Some(span) = trait_span {
-                err.span_label(
-                    span,
-                    format!("expected {} {} parameter{}", trait_count, kind,
-                        if trait_count != 1 { "s" } else { "" })
-                );
+            if let Some(spans) = trait_spans {
+                let mut spans = spans.iter();
+                if let Some(span) = spans.next() {
+                    err.span_label(*span, format!(
+                        "expected {} {} parameter{}",
+                        trait_count,
+                        kind,
+                        if trait_count != 1 { "s" } else { "" },
+                    ));
+                }
+                for span in spans {
+                    err.span_label(*span, "");
+                }
             } else {
                 suffix = Some(format!(", expected {}", trait_count));
             }
 
-            err.span_label(
-                span,
-                format!("found {} {} parameter{}{}", impl_count, kind,
+            if let Some(span) = span {
+                err.span_label(span, format!(
+                    "found {} {} parameter{}{}",
+                    impl_count,
+                    kind,
                     if impl_count != 1 { "s" } else { "" },
-                    suffix.unwrap_or_else(|| String::new())),
-            );
+                    suffix.unwrap_or_else(|| String::new()),
+                ));
+            }
+
+            for span in impl_trait_spans.iter().chain(impl_item_impl_trait_spans.iter()) {
+                err.span_label(*span, "`impl Trait` introduces an implicit type parameter");
+            }
 
             err.emit();
         }

src/libsyntax/parse/parser.rs

@@ -5050,21 +5050,22 @@ impl<'a> Parser<'a> {
     /// where   typaramseq = ( typaram ) | ( typaram , typaramseq )
     fn parse_generics(&mut self) -> PResult<'a, ast::Generics> {
         let span_lo = self.span;
-        if self.eat_lt() {
+        let (params, span) = if self.eat_lt() {
             let params = self.parse_generic_params()?;
             self.expect_gt()?;
-            Ok(ast::Generics {
-                params,
-                where_clause: WhereClause {
-                    id: ast::DUMMY_NODE_ID,
-                    predicates: Vec::new(),
-                    span: DUMMY_SP,
-                },
-                span: span_lo.to(self.prev_span),
-            })
+            (params, span_lo.to(self.prev_span))
         } else {
-            Ok(ast::Generics::default())
-        }
+            (vec![], self.prev_span.between(self.span))
+        };
+        Ok(ast::Generics {
+            params,
+            where_clause: WhereClause {
+                id: ast::DUMMY_NODE_ID,
+                predicates: Vec::new(),
+                span: DUMMY_SP,
+            },
+            span,
+        })
     }
 
     /// Parses generic args (within a path segment) with recovery for extra leading angle brackets.

src/test/ui/borrowck/regions-bound-missing-bound-in-impl.stderr

@@ -36,13 +36,13 @@ LL |     fn wrong_bound1<'b,'c,'d:'a+'c>(self, b: Inv<'b>, c: Inv<'c>, d: Inv<'d
    |                        ^^
 
 error[E0195]: lifetime parameters or bounds on method `wrong_bound2` do not match the trait declaration
-  --> $DIR/regions-bound-missing-bound-in-impl.rs:41:5
+  --> $DIR/regions-bound-missing-bound-in-impl.rs:41:20
    |
 LL |     fn wrong_bound2<'b,'c,'d:'a+'b>(self, b: Inv<'b>, c: Inv<'c>, d: Inv<'d>);
    |                    ---------------- lifetimes in impl do not match this method in trait
 ...
 LL |     fn wrong_bound2(self, b: Inv, c: Inv, d: Inv) {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetimes do not match method in trait
+   |                    ^ lifetimes do not match method in trait
 
 error[E0276]: impl has stricter requirements than trait
   --> $DIR/regions-bound-missing-bound-in-impl.rs:48:5

src/test/ui/error-codes/E0049.rs

@@ -8,5 +8,15 @@ impl Foo for Bar {
     fn foo(x: bool) -> Self { Bar } //~ ERROR E0049
 }
 
+trait Fuzz {
+    fn fuzz<A: Default, B>(x: A, y: B) -> Self;
+}
+
+struct Baz;
+
+impl Fuzz for Baz {
+    fn fuzz(x: bool, y: bool) -> Self { Baz } //~ ERROR E0049
+}
+
 fn main() {
 }

src/test/ui/error-codes/E0049.stderr

@@ -1,12 +1,23 @@
 error[E0049]: method `foo` has 0 type parameters but its trait declaration has 1 type parameter
-  --> $DIR/E0049.rs:8:5
+  --> $DIR/E0049.rs:8:11
    |
 LL |     fn foo<T: Default>(x: T) -> Self;
-   |     --------------------------------- expected 1 type parameter
+   |            - expected 1 type parameter
 ...
 LL |     fn foo(x: bool) -> Self { Bar }
-   |     ^^^^^^^^^^^^^^^^^^^^^^^ found 0 type parameters
+   |           ^ found 0 type parameters
 
-error: aborting due to previous error
+error[E0049]: method `fuzz` has 0 type parameters but its trait declaration has 2 type parameters
+  --> $DIR/E0049.rs:18:12
+   |
+LL |     fn fuzz<A: Default, B>(x: A, y: B) -> Self;
+   |             -           -
+   |             |
+   |             expected 2 type parameters
+...
+LL |     fn fuzz(x: bool, y: bool) -> Self { Baz }
+   |            ^ found 0 type parameters
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0049`.

src/test/ui/issues/issue-36708.stderr

@@ -1,8 +1,8 @@
 error[E0049]: method `foo` has 1 type parameter but its trait declaration has 0 type parameters
-  --> $DIR/issue-36708.rs:8:11
+  --> $DIR/issue-36708.rs:8:12
    |
 LL |     fn foo<T>() {}
-   |           ^^^ found 1 type parameter, expected 0
+   |            ^ found 1 type parameter, expected 0
 
 error: aborting due to previous error
 

src/test/ui/issues/type-arg-mismatch-due-to-impl-trait.rs

@@ -10,6 +10,7 @@ impl Foo for u32 {
     fn foo(&self, t: impl Clone) {}
 //~^ ERROR method `foo` has 1 type parameter but its trait declaration has 0 type parameters
 //~| NOTE found 1 type parameter
+//~| NOTE `impl Trait` introduces an implicit type parameter
 }
 
 fn main() {}

src/test/ui/issues/type-arg-mismatch-due-to-impl-trait.stderr

@@ -1,11 +1,14 @@
 error[E0049]: method `foo` has 1 type parameter but its trait declaration has 0 type parameters
-  --> $DIR/type-arg-mismatch-due-to-impl-trait.rs:10:5
+  --> $DIR/type-arg-mismatch-due-to-impl-trait.rs:10:22
    |
 LL |     fn foo(&self, t: Self::T);
-   |     -------------------------- expected 0 type parameters
+   |           - expected 0 type parameters
 ...
 LL |     fn foo(&self, t: impl Clone) {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found 1 type parameter
+   |                      ^^^^^^^^^^
+   |                      |
+   |                      found 1 type parameter
+   |                      `impl Trait` introduces an implicit type parameter
 
 error: aborting due to previous error