Introduce REDUNDANT_IMPORTS lint

Author: compiler-errors

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);

tests/ui/imports/redundant-import-extern-prelude.rs

@@ -1,16 +1,17 @@
-//@ check-pass
 // Check that we detect imports that are redundant due to the extern prelude
 // and that we emit a reasonable diagnostic.
 // issue: rust-lang/rust#121915
+//~^^^ NOTE the item `aux_issue_121915` is already defined by the extern prelude
 
 // See also the discussion in <https://github.com/rust-lang/rust/pull/122954>.
 
 //@ compile-flags: --extern aux_issue_121915 --edition 2018
 //@ aux-build: aux-issue-121915.rs
 
-#[deny(unused_imports)]
+#[deny(redundant_imports)]
+//~^ NOTE the lint level is defined here
 fn main() {
     use aux_issue_121915;
-    //FIXME(unused_imports): ~^ ERROR the item `aux_issue_121915` is imported redundantly
+    //~^ ERROR the item `aux_issue_121915` is imported redundantly
     aux_issue_121915::item();
 }

tests/ui/imports/redundant-import-extern-prelude.stderr

@@ -0,0 +1,14 @@
+error: the item `aux_issue_121915` is imported redundantly
+  --> $DIR/redundant-import-extern-prelude.rs:14:9
+   |
+LL |     use aux_issue_121915;
+   |         ^^^^^^^^^^^^^^^^ the item `aux_issue_121915` is already defined by the extern prelude
+   |
+note: the lint level is defined here
+  --> $DIR/redundant-import-extern-prelude.rs:11:8
+   |
+LL | #[deny(redundant_imports)]
+   |        ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/imports/redundant-import-issue-121915-2015.rs

@@ -1,12 +1,11 @@
-//@ check-pass
 //@ compile-flags: --extern aux_issue_121915 --edition 2015
 //@ aux-build: aux-issue-121915.rs
 
 extern crate aux_issue_121915;
 
-#[deny(unused_imports)]
+#[deny(redundant_imports)]
 fn main() {
     use aux_issue_121915;
-    //FIXME(unused_imports): ~^ ERROR the item `aux_issue_121915` is imported redundantly
+    //~^ ERROR the item `aux_issue_121915` is imported redundantly
     aux_issue_121915::item();
 }

tests/ui/imports/redundant-import-issue-121915-2015.stderr

@@ -0,0 +1,17 @@
+error: the item `aux_issue_121915` is imported redundantly
+  --> $DIR/redundant-import-issue-121915-2015.rs:8:9
+   |
+LL | extern crate aux_issue_121915;
+   | ------------------------------ the item `aux_issue_121915` is already imported here
+...
+LL |     use aux_issue_121915;
+   |         ^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/redundant-import-issue-121915-2015.rs:6:8
+   |
+LL | #[deny(redundant_imports)]
+   |        ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/imports/redundant-import-lang-prelude-attr.rs

@@ -1,6 +1,6 @@
-//@ check-pass
 // Check that we detect imports (of built-in attributes) that are redundant due to
 // the language prelude and that we emit a reasonable diagnostic.
+//~^^ NOTE the item `allow` is already defined by the extern prelude
 
 // Note that we use the term "extern prelude" in the label even though "language prelude"
 // would be more correct. However, it's not worth special-casing this.
@@ -9,9 +9,10 @@
 
 //@ edition: 2018
 
-#![deny(unused_imports)]
+#![deny(redundant_imports)]
+//~^ NOTE the lint level is defined here
 
-use allow; //FIXME(unused_imports): ~ ERROR the item `allow` is imported redundantly
+use allow; //~ ERROR the item `allow` is imported redundantly
 
 #[allow(unused)]
 fn main() {}

tests/ui/imports/redundant-import-lang-prelude-attr.stderr

@@ -0,0 +1,14 @@
+error: the item `allow` is imported redundantly
+  --> $DIR/redundant-import-lang-prelude-attr.rs:15:5
+   |
+LL | use allow;
+   |     ^^^^^ the item `allow` is already defined by the extern prelude
+   |
+note: the lint level is defined here
+  --> $DIR/redundant-import-lang-prelude-attr.rs:12:9
+   |
+LL | #![deny(redundant_imports)]
+   |         ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+