Introduce REDUNDANT_IMPORTS lint

compiler/rustc_lint/messages.ftl

@@ -700,10 +700,10 @@ lint_reason_must_be_string_literal = reason must be a string literal
 lint_reason_must_come_last = reason in lint attribute must come last
 
 lint_redundant_import = the item `{$ident}` is imported redundantly
-    .label_imported_here = the item `{ident}` is already imported here
-    .label_defined_here = the item `{ident}` is already defined here
-    .label_imported_prelude = the item `{ident}` is already imported by the extern prelude
-    .label_defined_prelude = the item `{ident}` is already defined by the extern prelude
+    .label_imported_here = the item `{$ident}` is already imported here
+    .label_defined_here = the item `{$ident}` is already defined here
+    .label_imported_prelude = the item `{$ident}` is already imported by the extern prelude
+    .label_defined_prelude = the item `{$ident}` is already defined by the extern prelude
 
 lint_redundant_import_visibility = glob import doesn't reexport anything with visibility `{$import_vis}` because no imported item is public enough
     .note = the most public imported item is `{$max_vis}`

compiler/rustc_lint_defs/src/builtin.rs

@@ -82,6 +82,7 @@ declare_lint_pass! {
         PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
         PTR_CAST_ADD_AUTO_TO_OBJECT,
         PUB_USE_OF_PRIVATE_EXTERN_CRATE,
+        REDUNDANT_IMPORTS,
         REDUNDANT_LIFETIMES,
         REFINING_IMPL_TRAIT_INTERNAL,
         REFINING_IMPL_TRAIT_REACHABLE,
@@ -426,6 +427,31 @@ declare_lint! {
     "imports that are never used"
 }
 
+declare_lint! {
+    /// The `redundant_imports` lint detects imports that are redundant due to being
+    /// imported already; either through a previous import, or being present in
+    /// the prelude.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(redundant_imports)]
+    /// use std::option::Option::None;
+    /// fn foo() -> Option<i32> { None }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Redundant imports are unnecessary and can be removed to simplify code.
+    /// If you intended to re-export the item to make it available outside of the
+    /// module, add a visibility modifier like `pub`.
+    pub REDUNDANT_IMPORTS,
+    Allow,
+    "imports that are redundant due to being imported already"
+}
+
 declare_lint! {
     /// The `must_not_suspend` lint guards against values that shouldn't be held across suspend points
     /// (`.await`)

compiler/rustc_resolve/src/imports.rs

@@ -14,7 +14,7 @@ use rustc_middle::metadata::{ModChild, Reexport};
 use rustc_middle::{span_bug, ty};
 use rustc_session::lint::builtin::{
     AMBIGUOUS_GLOB_REEXPORTS, HIDDEN_GLOB_REEXPORTS, PUB_USE_OF_PRIVATE_EXTERN_CRATE,
-    UNUSED_IMPORTS,
+    REDUNDANT_IMPORTS, UNUSED_IMPORTS,
 };
 use rustc_session::lint::BuiltinLintDiag;
 use rustc_span::edit_distance::find_best_match_for_name;
@@ -1387,14 +1387,12 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             let mut redundant_spans: Vec<_> = redundant_span.present_items().collect();
             redundant_spans.sort();
             redundant_spans.dedup();
-            /* FIXME(unused_imports): Add this back as a new lint
-            self.lint_buffer.buffer_lint_with_diagnostic(
-                UNUSED_IMPORTS,
+            self.lint_buffer.buffer_lint(
+                REDUNDANT_IMPORTS,
                 id,
                 import.span,
                 BuiltinLintDiag::RedundantImport(redundant_spans, source),
             );
-            */
             return true;
         }
 

compiler/rustc_resolve/src/lib.rs

@@ -178,8 +178,8 @@ enum ImplTraitContext {
 
 /// Used for tracking import use types which will be used for redundant import checking.
 /// ### Used::Scope Example
-///  ```rust,ignore (redundant_imports)
-/// #![deny(unused_imports)]
+///  ```rust,compile_fail
+/// #![deny(redundant_imports)]
 /// use std::mem::drop;
 /// fn main() {
 ///     let s = Box::new(32);