warn on unused linker_messages warning attributes

Author: jyn514

compiler/rustc_passes/messages.ftl

@@ -817,6 +817,9 @@ passes_unused_duplicate =
 passes_unused_empty_lints_note =
     attribute `{$name}` with an empty list has no effect
 
+passes_unused_linker_warnings_note =
+    the `linker_warnings` lint can only be controlled at the root of a crate that needs to be linked
+
 passes_unused_multiple =
     multiple `{$name}` attributes
     .suggestion = remove this attribute

compiler/rustc_passes/src/check_attr.rs

@@ -25,6 +25,7 @@ use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::{self, TyCtxt, TypingMode};
 use rustc_middle::{bug, span_bug};
+use rustc_session::config::CrateType;
 use rustc_session::lint::builtin::{
     CONFLICTING_REPR_HINTS, INVALID_DOC_ATTRIBUTES, INVALID_MACRO_EXPORT_ARGUMENTS,
     UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES, UNUSED_ATTRIBUTES,
@@ -2328,6 +2329,33 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             && item.path == sym::reason
         {
             errors::UnusedNote::NoLints { name: attr.name_or_empty() }
+        } else if matches!(
+            attr.name_or_empty(),
+            sym::allow | sym::warn | sym::deny | sym::forbid | sym::expect
+        ) && let Some(meta) = attr.meta_item_list()
+            && meta.iter().any(|meta| {
+                meta.meta_item().map_or(false, |item| item.path == sym::linker_messages)
+            })
+        {
+            if hir_id != CRATE_HIR_ID {
+                let err = match attr.style {
+                    ast::AttrStyle::Outer => errors::OuterCrateLevelAttr,
+                    ast::AttrStyle::Inner => errors::OuterCrateLevelAttr,
+                };
+                self.tcx.emit_node_span_lint(UNUSED_ATTRIBUTES, hir_id, attr.span, err);
+                return;
+            } else {
+                let never_needs_link = self
+                    .tcx
+                    .crate_types()
+                    .iter()
+                    .all(|kind| matches!(kind, CrateType::Rlib | CrateType::Staticlib));
+                if never_needs_link {
+                    errors::UnusedNote::LinkerWarningsBinaryCrateOnly
+                } else {
+                    return;
+                }
+            }
         } else if attr.name_or_empty() == sym::default_method_body_is_const {
             errors::UnusedNote::DefaultMethodBodyConst
         } else {

compiler/rustc_passes/src/errors.rs

@@ -802,6 +802,8 @@ pub(crate) enum UnusedNote {
     NoLints { name: Symbol },
     #[note(passes_unused_default_method_body_const_note)]
     DefaultMethodBodyConst,
+    #[note(passes_unused_linker_warnings_note)]
+    LinkerWarningsBinaryCrateOnly,
 }
 
 #[derive(LintDiagnostic)]

compiler/rustc_span/src/symbol.rs

@@ -1189,6 +1189,7 @@ symbols! {
         link_section,
         linkage,
         linker,
+        linker_messages,
         lint_reasons,
         literal,
         load,

tests/ui/lint/linker-warning-bin.rs

@@ -0,0 +1,6 @@
+//@ build-pass
+#![crate_type = "bin"]
+#![warn(unused_attributes)]
+#![allow(linker_messages)]
+
+fn main() {}

tests/ui/lint/linker-warning.rs

@@ -0,0 +1,9 @@
+//@ check-pass
+#![crate_type = "lib"]
+#![warn(unused_attributes)]
+#![allow(linker_messages)]
+//~^ WARNING unused attribute
+
+#[allow(linker_messages)]
+//~^ WARNING should be an inner attribute
+fn foo() {}

tests/ui/lint/linker-warning.stderr

@@ -0,0 +1,22 @@
+warning: crate-level attribute should be an inner attribute: add an exclamation mark: `#![foo]`
+  --> $DIR/linker-warning.rs:7:1
+   |
+LL | #[allow(linker_messages)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/linker-warning.rs:3:9
+   |
+LL | #![warn(unused_attributes)]
+   |         ^^^^^^^^^^^^^^^^^
+
+warning: unused attribute
+  --> $DIR/linker-warning.rs:4:1
+   |
+LL | #![allow(linker_messages)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+   = note: the `linker_warnings` lint can only be controlled at the root of a crate that needs to be linked
+
+warning: 2 warnings emitted
+