Only allow immutable statics with #[linkage]

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -327,6 +327,10 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
                     } else {
                         codegen_fn_attrs.linkage = linkage;
                     }
+                    if tcx.is_mutable_static(did.into()) {
+                        tcx.dcx()
+                            .span_err(attr.span, "mutable statics are not allowed with #[linkage]");
+                    }
                 }
             }
             sym::link_section => {

tests/ui/linkage-attr/linkage-attr-mutable-static.rs

@@ -0,0 +1,15 @@
+//! The symbols are resolved by the linker. It doesn't make sense to change
+//! them at runtime, so deny mutable statics with #[linkage].
+
+#![feature(linkage)]
+
+fn main() {
+    extern "C" {
+        #[linkage = "weak"] //~ ERROR mutable statics are not allowed with #[linkage]
+        static mut ABC: *const u8;
+    }
+
+    unsafe {
+        assert_eq!(ABC as usize, 0);
+    }
+}

tests/ui/linkage-attr/linkage-attr-mutable-static.stderr

@@ -0,0 +1,8 @@
+error: mutable statics are not allowed with #[linkage]
+  --> $DIR/linkage-attr-mutable-static.rs:8:9
+   |
+LL |         #[linkage = "weak"]
+   |         ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+