#[used(linker)] attribute (dtolnay/linkme#41)

Author: cynecx

compiler/rustc_codegen_gcc/src/consts.rs

@@ -144,7 +144,7 @@ impl<'gcc, 'tcx> StaticMethods for CodegenCx<'gcc, 'tcx> {
             // TODO(antoyo): set link section.
         }
 
-        if attrs.flags.contains(CodegenFnAttrFlags::USED) {
+        if attrs.flags.contains(CodegenFnAttrFlags::USED) || attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER) {
             self.add_used_global(global.to_rvalue());
         }
     }

compiler/rustc_codegen_llvm/src/consts.rs

@@ -522,6 +522,9 @@ impl<'ll> StaticMethods for CodegenCx<'ll, '_> {
             }
 
             if attrs.flags.contains(CodegenFnAttrFlags::USED) {
+                // `USED` and `USED_LINKER` can't be used together.
+                assert!(!attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER));
+
                 // The semantics of #[used] in Rust only require the symbol to make it into the
                 // object file. It is explicitly allowed for the linker to strip the symbol if it
                 // is dead. As such, use llvm.compiler.used instead of llvm.used.
@@ -530,6 +533,12 @@ impl<'ll> StaticMethods for CodegenCx<'ll, '_> {
                 // in some versions of the gold linker.
                 self.add_compiler_used_global(g);
             }
+            if attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER) {
+                // `USED` and `USED_LINKER` can't be used together.
+                assert!(!attrs.flags.contains(CodegenFnAttrFlags::USED));
+
+                self.add_used_global(g);
+            }
         }
     }
 

compiler/rustc_feature/src/active.rs

@@ -533,6 +533,8 @@ declare_features! (
     ///
     /// NOTE: A limited form of `union U { ... }` was accepted in 1.19.0.
     (active, untagged_unions, "1.13.0", Some(55149), None),
+    /// Allows using the `#[used(retain)]` attribute.
+    (active, used_with_arg, "1.60.0", None, None),
     /// Allows `extern "wasm" fn`
     (active, wasm_abi, "1.53.0", Some(83788), None),
     // !!!!    !!!!    !!!!    !!!!   !!!!    !!!!    !!!!    !!!!    !!!!    !!!!    !!!!

compiler/rustc_feature/src/builtin_attrs.rs

@@ -324,7 +324,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ungated!(export_name, Normal, template!(NameValueStr: "name"), FutureWarnPreceding),
     ungated!(link_section, Normal, template!(NameValueStr: "name"), FutureWarnPreceding),
     ungated!(no_mangle, Normal, template!(Word), WarnFollowing),
-    ungated!(used, Normal, template!(Word), WarnFollowing),
+    ungated!(used, Normal, template!(Word, List: "used"), WarnFollowing),
 
     // Limits:
     ungated!(recursion_limit, CrateLevel, template!(NameValueStr: "N"), FutureWarnFollowing),

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -89,6 +89,8 @@ bitflags! {
         /// the MIR `InstrumentCoverage` pass and not added to the coverage map
         /// during codegen.
         const NO_COVERAGE               = 1 << 15;
+        /// `#[used(linker)]`: indicates that LLVM nor the linker can eliminate this function.
+        const USED_LINKER               = 1 << 16;
     }
 }
 

compiler/rustc_passes/src/dead.rs

@@ -466,7 +466,10 @@ fn has_allow_dead_code_or_lang_attr(tcx: TyCtxt<'_>, id: hir::HirId) -> bool {
 
     // #[used], #[no_mangle], #[export_name], etc also keeps the item alive
     // forcefully, e.g., for placing it in a specific section.
-    if cg_attrs.contains_extern_indicator() || cg_attrs.flags.contains(CodegenFnAttrFlags::USED) {
+    if cg_attrs.contains_extern_indicator()
+        || cg_attrs.flags.contains(CodegenFnAttrFlags::USED)
+        || cg_attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER)
+    {
         return true;
     }
 

compiler/rustc_span/src/symbol.rs

@@ -449,6 +449,7 @@ symbols! {
         compare_exchange_weak,
         compile_error,
         compile_error_macro,
+        compiler,
         compiler_builtins,
         compiler_fence,
         concat,
@@ -815,6 +816,7 @@ symbols! {
         link_ordinal,
         link_section,
         linkage,
+        linker,
         lint_reasons,
         literal,
         load,
@@ -1458,6 +1460,7 @@ symbols! {
         use_extern_macros,
         use_nested_groups,
         used,
+        used_with_arg,
         usize,
         v1,
         va_arg,

compiler/rustc_typeck/src/collect.rs

@@ -2849,7 +2849,42 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, id: DefId) -> CodegenFnAttrs {
         } else if attr.has_name(sym::rustc_std_internal_symbol) {
             codegen_fn_attrs.flags |= CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL;
         } else if attr.has_name(sym::used) {
-            codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED;
+            let inner = attr.meta_item_list();
+            match inner.as_deref() {
+                Some([item]) if item.has_name(sym::linker) => {
+                    if !tcx.features().used_with_arg {
+                        feature_err(
+                            &tcx.sess.parse_sess,
+                            sym::used_with_arg,
+                            attr.span,
+                            "`#[used(linker)]` is currently unstable",
+                        )
+                        .emit();
+                    }
+                    codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED_LINKER;
+                }
+                Some([item]) if item.has_name(sym::compiler) => {
+                    if !tcx.features().used_with_arg {
+                        feature_err(
+                            &tcx.sess.parse_sess,
+                            sym::used_with_arg,
+                            attr.span,
+                            "`#[used(compiler)]` is currently unstable",
+                        )
+                        .emit();
+                    }
+                    codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED;
+                }
+                Some(_) => {
+                    tcx.sess
+                        .struct_span_err(
+                            attr.span,
+                            "expected `used`, `used(compiler)` or `used(linker)`",
+                        )
+                        .emit();
+                }
+                None => codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED,
+            }
         } else if attr.has_name(sym::cmse_nonsecure_entry) {
             if !matches!(tcx.fn_sig(id).abi(), abi::Abi::C { .. }) {
                 struct_span_err!(