Be more specific when flagging imports that are redundant due to the extern prelude

Author: fmease

compiler/rustc_lint/src/context/diagnostics.rs

@@ -105,7 +105,7 @@ pub(super) fn builtin(sess: &Session, diagnostic: BuiltinLintDiag, diag: &mut Di
         BuiltinLintDiag::RedundantImport(spans, ident) => {
             for (span, is_imported) in spans {
                 let introduced = if is_imported { "imported" } else { "defined" };
-                let span_msg = if span.is_dummy() { "by prelude" } else { "here" };
+                let span_msg = if span.is_dummy() { "by the extern prelude" } else { "here" };
                 diag.span_label(
                     span,
                     format!("the item `{ident}` is already {introduced} {span_msg}"),

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

@@ -2,7 +2,7 @@ error: the item `aux_issue_121915` is imported redundantly
   --> $DIR/redundant-import-issue-121915.rs:6:9
    |
 LL |     use aux_issue_121915;
-   |         ^^^^^^^^^^^^^^^^ the item `aux_issue_121915` is already defined by prelude
+   |         ^^^^^^^^^^^^^^^^ the item `aux_issue_121915` is already defined by the extern prelude
    |
 note: the lint level is defined here
   --> $DIR/redundant-import-issue-121915.rs:4:8