fix: #47446

- Add test for issue 47446
- Implement the new lint lint_builtin_mixed_export_name_and_no_mangle
- Add suggestion how to fix it

Author: sassman

Diff for: compiler/rustc_lint/messages.ftl

@@ -117,6 +117,11 @@ lint_builtin_missing_debug_impl =
 
 lint_builtin_missing_doc = missing documentation for {$article} {$desc}
 
+lint_builtin_mixed_export_name_and_no_mangle = the attribute `export_name` may not be used in combination with `no_mangle`
+    .label = `export_name` takes precedence
+    .note = the `no_mangle` attribute is ignored
+    .suggestion = remove the `no_mangle` attribute
+
 lint_builtin_mutable_transmutes =
     transmuting &T to &mut T is undefined behavior, even if the reference is unused, consider instead using an UnsafeCell
 

Diff for: compiler/rustc_lint/src/builtin.rs

@@ -3035,3 +3035,67 @@ impl EarlyLintPass for SpecialModuleName {
         }
     }
 }
+
+declare_lint! {
+    /// The `mixed_export_name_and_no_mangle` lint detects mixed usage of `export_name` and `no_mangle`
+    /// where `no_mangle` is not used by the compiler.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #[no_mangle]
+    /// #[export_name = "foo"]
+    /// pub fn bar() {}
+    ///
+    /// fn main() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// The compiler will not use the `no_mangle` attribute when generating the symbol name for the function,
+    /// as the `export_name` attribute is used instead. This can lead to confusion and is unnecessary.
+    ///
+    MIXED_EXPORT_NAME_AND_NO_MANGLE,
+    Warn,
+    "mixed usage of export_name and no_mangle, where no_mangle is not used by the compiler"
+}
+
+declare_lint_pass!(MixedExportNameAndNoMangle => [MIXED_EXPORT_NAME_AND_NO_MANGLE]);
+
+impl MixedExportNameAndNoMangle {
+    fn report_mixed_export_name_and_no_mangle(
+        &self,
+        cx: &EarlyContext<'_>,
+        span_export_name: Span,
+        span_no_mangle: Span,
+    ) {
+        let decorate = crate::lints::BuiltinMixedExportNameAndNoMangle {
+            export_name: span_export_name,
+            no_mangle: span_no_mangle.clone(),
+            removal_span: span_no_mangle,
+        };
+        cx.emit_span_lint(MIXED_EXPORT_NAME_AND_NO_MANGLE, span_export_name, decorate);
+    }
+}
+
+impl EarlyLintPass for MixedExportNameAndNoMangle {
+    fn check_item(&mut self, cx: &EarlyContext<'_>, it: &ast::Item) {
+        match it.kind {
+            ast::ItemKind::Fn(..) | ast::ItemKind::Static(..) => {
+                let no_mangle = attr::find_by_name(&it.attrs, sym::no_mangle);
+                let export_name = attr::find_by_name(&it.attrs, sym::export_name);
+                if let (Some(no_mangle_attr), Some(export_name_attr)) = (no_mangle, export_name) {
+                    self.report_mixed_export_name_and_no_mangle(
+                        cx,
+                        export_name_attr.span,
+                        no_mangle_attr.span,
+                    );
+                }
+            }
+
+            _ => {}
+        }
+    }
+}

Diff for: compiler/rustc_lint/src/lib.rs

@@ -187,6 +187,7 @@ early_lint_methods!(
             UnusedDocComment: UnusedDocComment,
             Expr2024: Expr2024,
             Precedence: Precedence,
+            MixedExportNameAndNoMangle: MixedExportNameAndNoMangle,
         ]
     ]
 );

Diff for: compiler/rustc_lint/src/lints.rs

@@ -3053,3 +3053,16 @@ pub(crate) enum MutRefSugg {
 #[derive(LintDiagnostic)]
 #[diag(lint_unqualified_local_imports)]
 pub(crate) struct UnqualifiedLocalImportsDiag {}
+
+#[derive(LintDiagnostic)]
+#[diag(lint_builtin_mixed_export_name_and_no_mangle)]
+pub(crate) struct BuiltinMixedExportNameAndNoMangle {
+    #[label]
+    pub export_name: Span,
+
+    #[note]
+    pub no_mangle: Span,
+
+    #[suggestion(style = "verbose", code = "", applicability = "machine-applicable")]
+    pub removal_span: Span,
+}

Diff for: tests/ui/attributes/mixed_export_name_and_no_mangle.rs

@@ -0,0 +1,7 @@
+// issue: rust-lang/rust#47446
+
+#[no_mangle]
+#[export_name = "foo"]
+pub fn bar() {}
+
+fn main() {}

Diff for: tests/ui/attributes/mixed_export_name_and_no_mangle.stderr

@@ -0,0 +1,17 @@
+warning: the attribute `export_name` may not be used in combination with `no_mangle`
+  --> $DIR/E47446.rs:2:1
+   |
+LL | #[export_name = "foo"]
+   | ^^^^^^^^^^^^^^^^^^^^^^ `export_name` takes precedence
+   |
+   = note: when `export_name` is used `no_mangle` is ignored
+note: the `no_mangle` attribute is ignored
+  --> $DIR/E47446.rs:1:1
+   |
+LL | #[no_mangle]
+   | ^^^^^^^^^^^^
+   = note: `#[warn(mixed_export_name_and_no_mangle)]` on by default
+help: remove the `no_mangle` attribute
+   |
+LL - #[no_mangle]
+   |