Rollup merge of #86376 - asquared31415:extern-no-mangle-84204, r=Mark-Simulacrum

Emit specific warning to clarify that `#[no_mangle]` should not be applied on foreign statics or functions

Foreign statics and foreign functions should not have `#[no_mangle]` applied, as it does nothing to the name and has some extra hidden behavior that is normally unwanted.  There was an existing warning for this, but it says the attribute is only allowed on "statics or functions", which to the user can be confusing.

This PR adds a specific version of the unused `#[no_mangle]` warning that explains that the target is a *foreign* static or function and that they do not need the attribute.

Fixes #78989

1 files changed, 30 insertions, 0 deletions

diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index d3dac35d2c9..fd438bdc900 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -1331,6 +1331,36 @@ impl CheckAttrVisitor<'tcx> {
             Target::Field | Target::Arm | Target::MacroDef => {
                 self.inline_attr_str_error_with_macro_def(hir_id, attr, "no_mangle");
             }
+            // FIXME: #[no_mangle] was previously allowed on non-functions/statics, this should be an error
+            // The error should specify that the item that is wrong is specifically a *foreign* fn/static
+            // otherwise the error seems odd
+            Target::ForeignFn | Target::ForeignStatic => {
+                let foreign_item_kind = match target {
+                    Target::ForeignFn => "function",
+                    Target::ForeignStatic => "static",
+                    _ => unreachable!(),
+                };
+                self.tcx.struct_span_lint_hir(UNUSED_ATTRIBUTES, hir_id, attr.span, |lint| {
+                    lint.build(&format!(
+                        "`#[no_mangle]` has no effect on a foreign {}",
+                        foreign_item_kind
+                    ))
+                    .warn(
+                        "this was previously accepted by the compiler but is \
+                            being phased out; it will become a hard error in \
+                            a future release!",
+                    )
+                    .span_label(*span, format!("foreign {}", foreign_item_kind))
+                    .note("symbol names in extern blocks are not mangled")
+                    .span_suggestion(
+                        attr.span,
+                        "remove this attribute",
+                        String::new(),
+                        Applicability::MachineApplicable,
+                    )
+                    .emit();
+                });
+            }
             _ => {
                 // FIXME: #[no_mangle] was previously allowed on non-functions/statics and some
                 // crates used this, so only emit a warning.