Introduce REDUNDANT_IMPORTS lint

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`)