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warn on align on fields to avoid breaking changes #143868

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4 changes: 4 additions & 0 deletions compiler/rustc_passes/messages.ftl
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down
28 changes: 22 additions & 6 deletions compiler/rustc_passes/src/check_attr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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)]
Expand Down Expand Up @@ -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)
Expand Down Expand Up @@ -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 },
);
}
Comment on lines +1947 to +1954
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Question: do we already have test coverage for this case?

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We have about 20 of these cases and to the best of my knowledge no test coverage for any :ferris clueless:

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Don't really have time to atm, am on holiday. Just knew how to fix this issue well

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Yes, we may wish to land this and maybe open an issue about getting test coverage.

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feel free to assign me or @JonathanBrouwer (who I think was interested in this sort of thing)

Comment on lines +1947 to +1954
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@jieyouxu jieyouxu Jul 14, 2025 •
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Question: wait hold on, what does this PR intend to do? If the intention was to not error on #[align] in various positions where we previously did not do, then this PR doesn't do that, because #142507 is a breaking change even if #[align] is an unstable built-in attribute, because unstable built-in attributes still participate in name resolution, and thus will still cause the ambiguity. This PR additionally produces another warning (i.e. unused_attributes) but does not prevent the name resolution ambiguity error from happening, cf. #134963.

Counter-example (I tried this test which fails on master, and also the commit from this PR, ignoring the irrelevant unstable fn_align feature gate warnings):

// tests/ui/whatever_subdir/test.rs

//@ proc-macro: my_derive.rs
//@ edition: 2024

use my_derive::MyDerive;

#[derive(MyDerive)]
#[align]
//~^ ERROR `align` is ambiguous
pub struct Foo;

fn main() {}
// tests/ui/whatever_subdir/auxiliary/my_derive.rs

//@ edition: 2024

extern crate proc_macro;
use proc_macro::{Span, TokenStream};

#[proc_macro_derive(
    MyDerive,
    attributes(
        align
    )
)]
pub fn derive_custom(_item: TokenStream) -> TokenStream {
    TokenStream::new()
}

I think if we wanted to prevent that ambiguity error from happening entirely, we'd need to pick one of the following options (or more) or some alternative solution:

  1. Have to fix how built-in attributes are resolved versus user attributes
  2. Revert use #[align] attribute for fn_align #142507
  3. Rename #[align] to something less common.

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(That is, I think this PR itself is correct, but doesn't address #143834.)

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.
Expand Down
8 changes: 8 additions & 0 deletions compiler/rustc_passes/src/errors.rs
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Expand Up @@ -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 {
Expand Down
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