Fix redundant import errors for preload extern crate

Author: chenyukang

compiler/rustc_lint/src/context/diagnostics.rs

@@ -114,6 +114,9 @@ pub(super) fn builtin(sess: &Session, diagnostic: BuiltinLintDiagnostics, diag:
                 diag.span_label(span, format!("the item `{ident}` is already {introduced} here"));
             }
         }
+        BuiltinLintDiagnostics::RedundantImportRemove(span) => {
+            diag.span_suggestion(span, "remove this import", "", Applicability::MachineApplicable);
+        }
         BuiltinLintDiagnostics::DeprecatedMacro(suggestion, span) => {
             stability::deprecation_suggestion(diag, "macro", suggestion, span)
         }

compiler/rustc_lint_defs/src/lib.rs

@@ -578,6 +578,7 @@ pub enum BuiltinLintDiagnostics {
     UnknownCrateTypes(Span, String, String),
     UnusedImports(String, Vec<(Span, String)>, Option<Span>),
     RedundantImport(Vec<(Span, bool)>, Ident),
+    RedundantImportRemove(Span),
     DeprecatedMacro(Option<Symbol>, Span),
     MissingAbi(Span, Abi),
     UnusedDocComment(Span),

compiler/rustc_resolve/src/imports.rs

@@ -1336,7 +1336,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         let mut is_redundant = true;
 
         let mut redundant_span = PerNS { value_ns: None, type_ns: None, macro_ns: None };
-
         self.per_ns(|this, ns| {
             if is_redundant && let Ok(binding) = source_bindings[ns].get() {
                 if binding.res() == Res::Err {
@@ -1368,12 +1367,24 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             let mut redundant_spans: Vec<_> = redundant_span.present_items().collect();
             redundant_spans.sort();
             redundant_spans.dedup();
+            let is_preload = self.extern_prelude_get(target, false).is_some();
+            let (msg, diag) = if is_preload {
+                (
+                    "already exists in the extern prelude",
+                    BuiltinLintDiagnostics::RedundantImportRemove(import.use_span),
+                )
+            } else {
+                (
+                    "imported redundantly",
+                    BuiltinLintDiagnostics::RedundantImport(redundant_spans, source),
+                )
+            };
             self.lint_buffer.buffer_lint_with_diagnostic(
                 UNUSED_IMPORTS,
                 id,
                 import.span,
-                format!("the item `{source}` is imported redundantly"),
-                BuiltinLintDiagnostics::RedundantImport(redundant_spans, source),
+                format!("the item `{source}` is {msg}"),
+                diag,
             );
         }
     }

tests/ui/imports/auxiliary/aux-issue-121915.rs

@@ -0,0 +1 @@
+pub fn item() {}

tests/ui/imports/redundant-import-issue-121915-2015.edition2015.stderr

@@ -0,0 +1,14 @@
+error: the item `aux_issue_121915` is already exists in the extern prelude
+  --> $DIR/redundant-import-issue-121915-2015.rs:10:9
+   |
+LL |     use aux_issue_121915;
+   |         ^^^^^^^^^^^^^^^^ help: remove this import
+   |
+note: the lint level is defined here
+  --> $DIR/redundant-import-issue-121915-2015.rs:8:8
+   |
+LL | #[deny(unused_imports)]
+   |        ^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

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

@@ -0,0 +1,11 @@
+//@ compile-flags: --extern aux_issue_121915 --edition 2015
+//@ aux-build: aux-issue-121915.rs
+
+extern crate aux_issue_121915;
+
+#[deny(unused_imports)]
+fn main() {
+    use aux_issue_121915;
+    //~^ ERROR the item `aux_issue_121915` is already exists in the extern prelude
+    aux_issue_121915::item();
+}

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

@@ -0,0 +1,14 @@
+error: the item `aux_issue_121915` is already exists in the extern prelude
+  --> $DIR/redundant-import-issue-121915-2015.rs:8:9
+   |
+LL |     use aux_issue_121915;
+   |     ----^^^^^^^^^^^^^^^^- help: remove this import
+   |
+note: the lint level is defined here
+  --> $DIR/redundant-import-issue-121915-2015.rs:6:8
+   |
+LL | #[deny(unused_imports)]
+   |        ^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/imports/redundant-import-issue-121915.edition2015.stderr

@@ -0,0 +1,9 @@
+error[E0432]: unresolved import `aux_issue_121915`
+  --> $DIR/redundant-import-issue-121915.rs:10:9
+   |
+LL |     use aux_issue_121915;
+   |         ^^^^^^^^^^^^^^^^ no `aux_issue_121915` in the root
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0432`.

tests/ui/imports/redundant-import-issue-121915.edition2018.stderr

@@ -0,0 +1,14 @@
+error: the item `aux_issue_121915` is already exists in the extern prelude
+  --> $DIR/redundant-import-issue-121915.rs:9:9
+   |
+LL |     use aux_issue_121915;
+   |     ----^^^^^^^^^^^^^^^^- help: remove this import
+   |
+note: the lint level is defined here
+  --> $DIR/redundant-import-issue-121915.rs:7:8
+   |
+LL | #[deny(unused_imports)]
+   |        ^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/imports/redundant-import-issue-121915.edition2021.stderr

@@ -0,0 +1,14 @@
+error: the item `aux_issue_121915` is already exists in the extern prelude
+  --> $DIR/redundant-import-issue-121915.rs:9:9
+   |
+LL |     use aux_issue_121915;
+   |     ----^^^^^^^^^^^^^^^^- help: remove this import
+   |
+note: the lint level is defined here
+  --> $DIR/redundant-import-issue-121915.rs:7:8
+   |
+LL | #[deny(unused_imports)]
+   |        ^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

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

@@ -0,0 +1,12 @@
+//@ revisions: edition2018 edition2021
+//@ [edition2018] edition:2018
+//@ [edition2021] edition:2021
+//@ compile-flags: --extern aux_issue_121915
+//@ aux-build: aux-issue-121915.rs
+
+#[deny(unused_imports)]
+fn main() {
+    use aux_issue_121915;
+    //~^ ERROR the item `aux_issue_121915` is already exists in the extern prelude
+    aux_issue_121915::item();
+}