Make #[link="dl"] an FCW rather than an error

compiler/rustc_attr_parsing/src/attributes/link_attrs.rs

@@ -65,10 +65,22 @@ impl<S: Stage> CombineAttributeParser<S> for LinkParser {
         cx: &'c mut AcceptContext<'_, '_, S>,
         args: &'c ArgParser<'_>,
     ) -> impl IntoIterator<Item = Self::Item> + 'c {
-        let mut result = None;
-        let Some(items) = args.list() else {
-            cx.expected_list(cx.attr_span);
-            return result;
+        let items = match args {
+            ArgParser::List(list) => list,
+            // This is an edgecase added because making this a hard error would break too many crates
+            // Specifically `#[link = "dl"]` is accepted with a FCW
+            // For more information, see https://github.com/rust-lang/rust/pull/143193
+            ArgParser::NameValue(nv) if nv.value_as_str().is_some_and(|v| v == sym::dl) => {
+                let suggestions = <Self as CombineAttributeParser<S>>::TEMPLATE
+                    .suggestions(cx.attr_style, "link");
+                let span = cx.attr_span;
+                cx.emit_lint(AttributeLintKind::IllFormedAttributeInput { suggestions }, span);
+                return None;
+            }
+            _ => {
+                cx.expected_list(cx.attr_span);
+                return None;
+            }
         };
 
         let sess = cx.sess();
@@ -113,7 +125,7 @@ impl<S: Stage> CombineAttributeParser<S> for LinkParser {
                 }
             };
             if !cont {
-                return result;
+                return None;
             }
         }
 
@@ -202,7 +214,7 @@ impl<S: Stage> CombineAttributeParser<S> for LinkParser {
         }
         let Some((name, name_span)) = name else {
             cx.emit_err(LinkRequiresName { span: cx.attr_span });
-            return result;
+            return None;
         };
 
         // Do this outside of the loop so that `import_name_type` can be specified before `kind`.
@@ -218,15 +230,14 @@ impl<S: Stage> CombineAttributeParser<S> for LinkParser {
             cx.emit_err(RawDylibNoNul { span: name_span });
         }
 
-        result = Some(LinkEntry {
+        Some(LinkEntry {
             span: cx.attr_span,
             kind: kind.unwrap_or(NativeLibKind::Unspecified),
             name,
             cfg,
             verbatim,
             import_name_type,
-        });
-        result
+        })
     }
 }
 

compiler/rustc_span/src/symbol.rs

@@ -873,6 +873,7 @@ symbols! {
         div,
         div_assign,
         diverging_block_default,
+        dl,
         do_not_recommend,
         doc,
         doc_alias,

tests/ui/attributes/link-dl.allowed.stderr

@@ -0,0 +1,10 @@
+Future incompatibility report: Future breakage diagnostic:
+warning: valid forms for the attribute are `#[link(name = "...")]`, `#[link(name = "...", import_name_type = "decorated|noprefix|undecorated")]`, `#[link(name = "...", kind = "dylib|static|...")]`, `#[link(name = "...", kind = "dylib|static|...", wasm_import_module = "...", import_name_type = "decorated|noprefix|undecorated")]`, and `#[link(name = "...", wasm_import_module = "...")]`
+  --> $DIR/link-dl.rs:14:1
+   |
+LL | #[link="dl"]
+   | ^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #57571 <https://github.com/rust-lang/rust/issues/57571>
+

tests/ui/attributes/link-dl.default_fcw.stderr

@@ -0,0 +1,23 @@
+error: valid forms for the attribute are `#[link(name = "...")]`, `#[link(name = "...", import_name_type = "decorated|noprefix|undecorated")]`, `#[link(name = "...", kind = "dylib|static|...")]`, `#[link(name = "...", kind = "dylib|static|...", wasm_import_module = "...", import_name_type = "decorated|noprefix|undecorated")]`, and `#[link(name = "...", wasm_import_module = "...")]`
+  --> $DIR/link-dl.rs:14:1
+   |
+LL | #[link="dl"]
+   | ^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #57571 <https://github.com/rust-lang/rust/issues/57571>
+   = note: `#[deny(ill_formed_attribute_input)]` (part of `#[deny(future_incompatible)]`) on by default
+
+error: aborting due to 1 previous error
+
+Future incompatibility report: Future breakage diagnostic:
+error: valid forms for the attribute are `#[link(name = "...")]`, `#[link(name = "...", import_name_type = "decorated|noprefix|undecorated")]`, `#[link(name = "...", kind = "dylib|static|...")]`, `#[link(name = "...", kind = "dylib|static|...", wasm_import_module = "...", import_name_type = "decorated|noprefix|undecorated")]`, and `#[link(name = "...", wasm_import_module = "...")]`
+  --> $DIR/link-dl.rs:14:1
+   |
+LL | #[link="dl"]
+   | ^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #57571 <https://github.com/rust-lang/rust/issues/57571>
+   = note: `#[deny(ill_formed_attribute_input)]` (part of `#[deny(future_incompatible)]`) on by default
+

tests/ui/attributes/link-dl.rs

@@ -0,0 +1,19 @@
+// Regression test for an issue discovered in https://github.com/rust-lang/rust/pull/143193/files and rediscovered in https://github.com/rust-lang/rust/issues/147254#event-20049906781
+// Malformed #[link] attribute was supposed to be deny-by-default report-in-deps FCW,
+// but accidentally was landed as a hard error.
+//
+// revision `default_fcw` tests that with `ill_formed_attribute_input` (the default) denied,
+// the attribute produces an FCW
+// revision `allowed` tests that with `ill_formed_attribute_input` allowed the test passes
+
+//@ revisions: default_fcw allowed
+//@[allowed] check-pass
+
+#[cfg_attr(allowed, allow(ill_formed_attribute_input))]
+
+#[link="dl"]
+//[default_fcw]~^ ERROR valid forms for the attribute are
+//[default_fcw]~| WARN previously accepted
+extern "C" { }
+
+fn main() {}