Enable link-arg link kind inside of #[link] attribute

- Implement link-arg as an attribute
- Apply suggestions from review
  - Co-authored-by: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>
- Add unstable book entry

compiler/rustc_feature/src/unstable.rs

@@ -500,6 +500,9 @@ declare_features! (
     (incomplete, lazy_type_alias, "1.72.0", Some(112792), None),
     /// Allows `if/while p && let q = r && ...` chains.
     (unstable, let_chains, "1.37.0", Some(53667), None),
+    /// Allows using `#[link(kind = "link-arg", name = "...")]`
+    /// to pass custom arguments to the linker.
+    (unstable, link_arg_attribute, "CURRENT_RUSTC_VERSION", Some(99427), None),
     /// Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.
     (unstable, lint_reasons, "1.31.0", Some(54503), None),
     /// Give access to additional metadata about declarative macro meta-variables.

compiler/rustc_metadata/messages.ftl

@@ -269,7 +269,7 @@ metadata_unknown_import_name_type =
     unknown import name type `{$import_name_type}`, expected one of: decorated, noprefix, undecorated
 
 metadata_unknown_link_kind =
-    unknown link kind `{$kind}`, expected one of: static, dylib, framework, raw-dylib
+    unknown link kind `{$kind}`, expected one of: static, dylib, framework, raw-dylib, link-arg
     .label = unknown link kind
 
 metadata_unknown_link_modifier =

compiler/rustc_metadata/src/native_libs.rs

@@ -160,6 +160,18 @@ impl<'tcx> Collector<'tcx> {
                                 }
                                 NativeLibKind::RawDylib
                             }
+                            "link-arg" => {
+                                if !features.link_arg_attribute {
+                                    feature_err(
+                                        &sess.parse_sess,
+                                        sym::link_arg_attribute,
+                                        span,
+                                        "link kind `link-arg` is unstable",
+                                    )
+                                    .emit();
+                                }
+                                NativeLibKind::LinkArg
+                            }
                             kind => {
                                 sess.emit_err(errors::UnknownLinkKind { span, kind });
                                 continue;

compiler/rustc_span/src/symbol.rs

@@ -934,6 +934,7 @@ symbols! {
         likely,
         line,
         link,
+        link_arg_attribute,
         link_args,
         link_cfg,
         link_llvm_intrinsics,