Improve warning

src/librustc/lint/builtin.rs

@@ -489,6 +489,7 @@ pub enum BuiltinLintDiagnostics {
     UnknownCrateTypes(Span, String, String),
     UnusedImports(String, Vec<(Span, String)>),
     NestedImplTrait { outer_impl_trait_span: Span, inner_impl_trait_span: Span },
+    RedundantImport(Vec<Span>, ast::Ident),
 }
 
 impl BuiltinLintDiagnostics {
@@ -585,6 +586,14 @@ impl BuiltinLintDiagnostics {
                 db.span_label(outer_impl_trait_span, "outer `impl Trait`");
                 db.span_label(inner_impl_trait_span, "nested `impl Trait` here");
             }
+            BuiltinLintDiagnostics::RedundantImport(spans, ident) => {
+                for span in spans {
+                    db.span_label(
+                        span,
+                        format!("the item `{}` was already imported here", ident)
+                    );
+                }
+            }
         }
     }
 }

src/librustc_resolve/resolve_imports.rs

@@ -1306,21 +1306,16 @@ impl<'a, 'b:'a> ImportResolver<'a, 'b> {
         if !is_redundant.is_empty() &&
             is_redundant.present_items().all(|is_redundant| is_redundant)
         {
-            self.session.buffer_lint(
+            self.session.buffer_lint_with_diagnostic(
                 REDUNDANT_IMPORT,
                 directive.id,
                 directive.span,
                 &format!("the item `{}` is imported redundantly", ident),
+                BuiltinLintDiagnostics::RedundantImport(
+                    redundant_span.present_items().collect(),
+                    ident,
+                ),
             );
-
-            for span in redundant_span.present_items() {
-                self.session.buffer_lint(
-                    REDUNDANT_IMPORT,
-                    directive.id,
-                    span,
-                    "another import"
-                );
-            }
         }
     }
 

src/test/ui/lint/use-redundant.stderr

@@ -1,14 +1,11 @@
 warning: the item `Bar` is imported redundantly
   --> $DIR/use-redundant.rs:14:9
    |
-LL |     use crate::foo::Bar; //~ WARNING redundant import
+LL | use crate::foo::Bar;
+   |     --------------- the item `Bar` was already imported here
+...
+LL |     use crate::foo::Bar;
    |         ^^^^^^^^^^^^^^^
    |
    = note: #[warn(redundant_import)] on by default
 
-warning: another import
-  --> $DIR/use-redundant.rs:3:5
-   |
-LL | use crate::foo::Bar; //~ WARNING first import
-   |     ^^^^^^^^^^^^^^^
-