Add validation for link attribute position.

Author: ehuss

compiler/rustc_passes/src/check_attr.rs

@@ -126,6 +126,7 @@ impl CheckAttrVisitor<'_> {
             // lint-only checks
             match attr.name_or_empty() {
                 sym::cold => self.check_cold(hir_id, attr, span, target),
+                sym::link => self.check_link(hir_id, attr, span, target),
                 sym::link_name => self.check_link_name(hir_id, attr, span, target),
                 sym::link_section => self.check_link_section(hir_id, attr, span, target),
                 sym::no_mangle => self.check_no_mangle(hir_id, attr, span, target),
@@ -1140,6 +1141,26 @@ impl CheckAttrVisitor<'_> {
         }
     }
 
+    /// Checks if `#[link]` is applied to an item other than a foreign module.
+    fn check_link(&self, hir_id: HirId, attr: &Attribute, span: &Span, target: Target) {
+        match target {
+            Target::ForeignMod => {}
+            _ => {
+                self.tcx.struct_span_lint_hir(UNUSED_ATTRIBUTES, hir_id, attr.span, |lint| {
+                    let mut diag = lint.build("attribute should be applied to an `extern` block");
+                    diag.warn(
+                        "this was previously accepted by the compiler but is \
+                         being phased out; it will become a hard error in \
+                         a future release!",
+                    );
+
+                    diag.span_label(*span, "not an `extern` block");
+                    diag.emit();
+                });
+            }
+        }
+    }
+
     /// Checks if `#[link_name]` is applied to an item other than a foreign function or static.
     fn check_link_name(&self, hir_id: HirId, attr: &Attribute, span: &Span, target: Target) {
         match target {

src/test/ui/feature-gates/issue-43106-gating-of-builtin-attrs.rs

@@ -1,6 +1,7 @@
 //~ NOTE not a function
-//~^ NOTE not a foreign function or static
-//~^^ NOTE not a function or static
+//~| NOTE not a foreign function or static
+//~| NOTE not a function or static
+//~| NOTE not an `extern` block
 // This test enumerates as many compiler-builtin ungated attributes as
 // possible (that is, all the mutually compatible ones), and checks
 // that we get "expected" (*) warnings for each in the various weird
@@ -59,9 +60,9 @@
 #![proc_macro_derive()] //~ WARN `#[proc_macro_derive]` only has an effect
 #![doc = "2400"]
 #![cold] //~ WARN attribute should be applied to a function
-//~^ WARN
-// see issue-43106-gating-of-builtin-attrs-error.rs
-#![link()]
+//~^ WARN this was previously accepted
+#![link()] //~ WARN attribute should be applied to an `extern` block
+//~^ WARN this was previously accepted
 #![link_name = "1900"]
 //~^ WARN attribute should be applied to a foreign function
 //~^^ WARN this was previously accepted by the compiler
@@ -547,22 +548,38 @@ mod link_section {
 }
 
 
-// Note that this is a `check-pass` test, so it
-// will never invoke the linker. These are here nonetheless to point
-// out that we allow them at non-crate-level (though I do not know
-// whether they have the same effect here as at crate-level).
+// Note that this is a `check-pass` test, so it will never invoke the linker.
 
 #[link()]
+//~^ WARN attribute should be applied to an `extern` block
+//~| WARN this was previously accepted
 mod link {
+    //~^ NOTE not an `extern` block
+
     mod inner { #![link()] }
+    //~^ WARN attribute should be applied to an `extern` block
+    //~| WARN this was previously accepted
+    //~| NOTE not an `extern` block
 
     #[link()] fn f() { }
+    //~^ WARN attribute should be applied to an `extern` block
+    //~| WARN this was previously accepted
+    //~| NOTE not an `extern` block
 
     #[link()] struct S;
+    //~^ WARN attribute should be applied to an `extern` block
+    //~| WARN this was previously accepted
+    //~| NOTE not an `extern` block
 
     #[link()] type T = S;
+    //~^ WARN attribute should be applied to an `extern` block
+    //~| WARN this was previously accepted
+    //~| NOTE not an `extern` block
 
     #[link()] impl S { }
+    //~^ WARN attribute should be applied to an `extern` block
+    //~| WARN this was previously accepted
+    //~| NOTE not an `extern` block
 }
 
 struct StructForDeprecated;