warn on align on fields to avoid breaking changes

Author: jdonszelmann

compiler/rustc_passes/messages.ftl

@@ -17,6 +17,10 @@ passes_align_attr_application =
     `#[align(...)]` should be applied to a function item
     .label = not a function item
 
+passes_align_on_fields =
+    attribute should be applied to a function or method
+    .warn = {-passes_previously_accepted}
+
 passes_align_should_be_repr_align =
     `#[align(...)]` is not supported on {$item} items
     .suggestion = use `#[repr(align(...))]` instead

compiler/rustc_passes/src/check_attr.rs

@@ -43,6 +43,7 @@ use rustc_trait_selection::infer::{TyCtxtInferExt, ValuePairs};
 use rustc_trait_selection::traits::ObligationCtxt;
 use tracing::debug;
 
+use crate::errors::AlignOnFields;
 use crate::{errors, fluent_generated as fluent};
 
 #[derive(LintDiagnostic)]
@@ -197,8 +198,8 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 Attribute::Parsed(AttributeKind::ExportName { span: attr_span, .. }) => {
                     self.check_export_name(hir_id, *attr_span, span, target)
                 }
-                Attribute::Parsed(AttributeKind::Align { align, span: repr_span }) => {
-                    self.check_align(span, target, *align, *repr_span)
+                Attribute::Parsed(AttributeKind::Align { align, span: attr_span }) => {
+                    self.check_align(span, hir_id, target, *align, *attr_span)
                 }
                 Attribute::Parsed(AttributeKind::LinkSection { span: attr_span, .. }) => {
                     self.check_link_section(hir_id, *attr_span, span, target)
@@ -1933,22 +1934,37 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
     }
 
     /// Checks if the `#[align]` attributes on `item` are valid.
-    fn check_align(&self, span: Span, target: Target, align: Align, repr_span: Span) {
+    fn check_align(
+        &self,
+        span: Span,
+        hir_id: HirId,
+        target: Target,
+        align: Align,
+        attr_span: Span,
+    ) {
         match target {
             Target::Fn | Target::Method(_) | Target::ForeignFn => {}
+            Target::Field => {
+                self.tcx.emit_node_span_lint(
+                    UNUSED_ATTRIBUTES,
+                    hir_id,
+                    attr_span,
+                    AlignOnFields { span },
+                );
+            }
             Target::Struct | Target::Union | Target::Enum => {
                 self.dcx().emit_err(errors::AlignShouldBeReprAlign {
-                    span: repr_span,
+                    span: attr_span,
                     item: target.name(),
                     align_bytes: align.bytes(),
                 });
             }
             _ => {
-                self.dcx().emit_err(errors::AlignAttrApplication { hint_span: repr_span, span });
+                self.dcx().emit_err(errors::AlignAttrApplication { hint_span: attr_span, span });
             }
         }
 
-        self.check_align_value(align, repr_span);
+        self.check_align_value(align, attr_span);
     }
 
     /// Checks if the `#[repr]` attributes on `item` are valid.

compiler/rustc_passes/src/errors.rs

@@ -604,6 +604,14 @@ pub(crate) struct NoMangle {
     pub span: Span,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(passes_align_on_fields)]
+#[warning]
+pub(crate) struct AlignOnFields {
+    #[label]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_repr_conflicting, code = E0566)]
 pub(crate) struct ReprConflicting {