Repository: rust-lang/rust
File: compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs (Rust)

Code before:
    cx: &ExtCtxt<'_>,
    span: Span,
    mitem: &MetaItem,
    item: &Annotatable,
    push: &mut dyn FnMut(Annotatable),
    is_const: bool,
) {
    let span = cx.with_def_site_ctxt(span);

    let trait_def = TraitDef {
        span,
        path: path_std!(cmp::Eq),
        skip_path_as_bound: false,
        needs_copy_as_bound_if_packed: true,
        additional_bounds: Vec::new(),
        supports_unions: true,
        methods: vec![MethodDef {
            name: sym::assert_fields_are_eq,
            generics: Bounds::empty(),
            explicit_self: true,
            nonself_args: vec![],
            ret_ty: Unit,
            attributes: thin_vec![
                cx.attr_word(sym::inline, span),
                cx.attr_nested_word(sym::doc, sym::hidden, span),
                cx.attr_nested_word(sym::coverage, sym::off, span),
            ],
            fieldless_variants_strategy: FieldlessVariantsStrategy::Unify,
            combine_substructure: combine_substructure(Box::new(|a, b, c| {
                cs_total_eq_assert(a, b, c)
            })),
        }],
        associated_types: Vec::new(),
        is_const,
        is_staged_api_crate: cx.ecfg.features.staged_api(),
        safety: Safety::Default,
        document: true,
    };
    trait_def.expand_ext(cx, mitem, item, push, true)
}

fn cs_total_eq_assert(
    cx: &ExtCtxt<'_>,
    trait_span: Span,
    substr: &Substructure<'_>,
) -> BlockOrExpr {
    let mut stmts = ThinVec::new();
    let mut seen_type_names = FxHashSet::default();
    let mut process_variant = |variant: &ast::VariantData| {
        for field in variant.fields() {
            // This basic redundancy checking only prevents duplication of

Review comment (suggestion):
A comment explaining why `inline` is present on a never-called function would be useful.

Code after:
    cx: &ExtCtxt<'_>,
    span: Span,
    mitem: &MetaItem,
    item: &Annotatable,
    push: &mut dyn FnMut(Annotatable),
    is_const: bool,
) {
    let span = cx.with_def_site_ctxt(span);

    let trait_def = TraitDef {
        span,
        path: path_std!(cmp::Eq),
        skip_path_as_bound: false,
        needs_copy_as_bound_if_packed: true,
        additional_bounds: Vec::new(),
        supports_unions: true,
        methods: vec![MethodDef {
            name: sym::assert_fields_are_eq,
            generics: Bounds::empty(),
            explicit_self: true,
            nonself_args: vec![],
            ret_ty: Unit,
            attributes: thin_vec![
                // This method will never be called, so doing codegen etc. for it is unnecessary.
                // We prevent this by adding `#[inline]`, which improves compile-time.
                cx.attr_word(sym::inline, span),
                cx.attr_nested_word(sym::doc, sym::hidden, span),
                cx.attr_nested_word(sym::coverage, sym::off, span),
            ],
            fieldless_variants_strategy: FieldlessVariantsStrategy::Unify,
            combine_substructure: combine_substructure(Box::new(|a, b, c| {
                cs_total_eq_assert(a, b, c)
            })),
        }],
        associated_types: Vec::new(),
        is_const,
        is_staged_api_crate: cx.ecfg.features.staged_api(),
        safety: Safety::Default,
        document: true,
    };
    trait_def.expand_ext(cx, mitem, item, push, true)
}

fn cs_total_eq_assert(
    cx: &ExtCtxt<'_>,
    trait_span: Span,
    substr: &Substructure<'_>,
) -> BlockOrExpr {
    let mut stmts = ThinVec::new();
    let mut seen_type_names = FxHashSet::default();
    let mut process_variant = |variant: &ast::VariantData| {