Bonus fix for rust-lang#35280. Part of rust-lang#35233. Fixes rust-lang#36057. Adding expanded notes/context for what trait a parameter shadows as part of E0194 error messages.

Author: leikahing

src/librustc_typeck/check/wfcheck.rs

@@ -18,7 +18,7 @@ use rustc::ty::subst::{self, TypeSpace, FnSpace, ParamSpace, SelfSpace};
 use rustc::traits;
 use rustc::ty::{self, Ty, TyCtxt};
 
-use std::collections::HashSet;
+use std::collections::{HashSet, HashMap};
 use syntax::ast;
 use syntax::parse::token::keywords;
 use syntax_pos::Span;
@@ -548,11 +548,23 @@ impl<'ccx, 'gcx> CheckTypeWellFormedVisitor<'ccx, 'gcx> {
 
 fn reject_shadowing_type_parameters(tcx: TyCtxt, span: Span, generics: &ty::Generics) {
     let impl_params = generics.types.get_slice(subst::TypeSpace).iter()
-        .map(|tp| tp.name).collect::<HashSet<_>>();
+        .map(|tp| (tp.name, tp.def_id)).collect::<HashMap<_, _>>();
 
     for method_param in generics.types.get_slice(subst::FnSpace) {
-        if impl_params.contains(&method_param.name) {
-            error_194(tcx, span, method_param.name);
+        if impl_params.contains_key(&method_param.name) {
+            // Tighten up the span to focus on only the shadowing type
+            let shadow_node_id = tcx.map.as_local_node_id(method_param.def_id).unwrap();
+            let type_span = match tcx.map.opt_span(shadow_node_id) {
+                Some(osp) => osp,
+                None => span
+            };
+
+            // The expectation here is that the original trait declaration is
+            // local so it should be okay to just unwrap everything
+            let trait_def_id = impl_params.get(&method_param.name).unwrap();
+            let trait_node_id = tcx.map.as_local_node_id(*trait_def_id).unwrap();
+            let trait_decl_span = tcx.map.opt_span(trait_node_id).unwrap();
+            error_194(tcx, type_span, trait_decl_span, method_param.name);
         }
     }
 }
@@ -657,8 +669,11 @@ fn error_392<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>, span: Span, param_name: ast::N
                      "parameter `{}` is never used", param_name)
 }
 
-fn error_194(tcx: TyCtxt, span: Span, name: ast::Name) {
-    span_err!(tcx.sess, span, E0194,
+fn error_194(tcx: TyCtxt, span: Span, trait_decl_span: Span, name: ast::Name) {
+    struct_span_err!(tcx.sess, span, E0194,
               "type parameter `{}` shadows another type parameter of the same name",
-              name);
+              name)
+        .span_label(span, &format!("shadows another type parameter"))
+        .span_label(trait_decl_span, &format!("first `{}` declared here", name))
+        .emit();
 }

src/test/compile-fail/E0194.rs

@@ -8,9 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-trait Foo<T> {
+trait Foo<T> { //~ NOTE first `T` declared here
     fn do_something(&self) -> T;
-    fn do_something_else<T: Clone>(&self, bar: T); //~ ERROR E0194
+    fn do_something_else<T: Clone>(&self, bar: T);
+    //~^ ERROR E0194
+    //~| NOTE shadows another type parameter
 }
 
 fn main() {