rustc_codegen_ssa/

codegen_attrs.rs

use std::str::FromStr;

use rustc_abi::ExternAbi;
use rustc_ast::expand::autodiff_attrs::{AutoDiffAttrs, DiffActivity, DiffMode};
use rustc_ast::{MetaItem, MetaItemInner, attr};
use rustc_attr_parsing::ReprAttr::ReprAlign;
use rustc_attr_parsing::{AttributeKind, InlineAttr, InstructionSetAttr, OptimizeAttr};
use rustc_data_structures::fx::FxHashMap;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{DefId, LOCAL_CRATE, LocalDefId};
use rustc_hir::weak_lang_items::WEAK_LANG_ITEMS;
use rustc_hir::{self as hir, HirId, LangItem, lang_items};
use rustc_middle::middle::codegen_fn_attrs::{
    CodegenFnAttrFlags, CodegenFnAttrs, PatchableFunctionEntry,
};
use rustc_middle::mir::mono::Linkage;
use rustc_middle::query::Providers;
use rustc_middle::span_bug;
use rustc_middle::ty::{self as ty, TyCtxt};
use rustc_session::parse::feature_err;
use rustc_session::{Session, lint};
use rustc_span::{Ident, Span, sym};
use rustc_target::spec::SanitizerSet;
use tracing::debug;

use crate::errors;
use crate::target_features::{check_target_feature_trait_unsafe, from_target_feature_attr};

fn linkage_by_name(tcx: TyCtxt<'_>, def_id: LocalDefId, name: &str) -> Linkage {
    use rustc_middle::mir::mono::Linkage::*;

    // Use the names from src/llvm/docs/LangRef.rst here. Most types are only
    // applicable to variable declarations and may not really make sense for
    // Rust code in the first place but allow them anyway and trust that the
    // user knows what they're doing. Who knows, unanticipated use cases may pop
    // up in the future.
    //
    // ghost, dllimport, dllexport and linkonce_odr_autohide are not supported
    // and don't have to be, LLVM treats them as no-ops.
    match name {
        "available_externally" => AvailableExternally,
        "common" => Common,
        "extern_weak" => ExternalWeak,
        "external" => External,
        "internal" => Internal,
        "linkonce" => LinkOnceAny,
        "linkonce_odr" => LinkOnceODR,
        "weak" => WeakAny,
        "weak_odr" => WeakODR,
        _ => tcx.dcx().span_fatal(tcx.def_span(def_id), "invalid linkage specified"),
    }
}

fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
    if cfg!(debug_assertions) {
        let def_kind = tcx.def_kind(did);
        assert!(
            def_kind.has_codegen_attrs(),
            "unexpected `def_kind` in `codegen_fn_attrs`: {def_kind:?}",
        );
    }

    let attrs = tcx.hir_attrs(tcx.local_def_id_to_hir_id(did));
    let mut codegen_fn_attrs = CodegenFnAttrs::new();
    if tcx.should_inherit_track_caller(did) {
        codegen_fn_attrs.flags |= CodegenFnAttrFlags::TRACK_CALLER;
    }

    // If our rustc version supports autodiff/enzyme, then we call our handler
    // to check for any `#[rustc_autodiff(...)]` attributes.
    if cfg!(llvm_enzyme) {
        let ad = autodiff_attrs(tcx, did.into());
        codegen_fn_attrs.autodiff_item = ad;
    }

    // When `no_builtins` is applied at the crate level, we should add the
    // `no-builtins` attribute to each function to ensure it takes effect in LTO.
    let crate_attrs = tcx.hir_attrs(rustc_hir::CRATE_HIR_ID);
    let no_builtins = attr::contains_name(crate_attrs, sym::no_builtins);
    if no_builtins {
        codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_BUILTINS;
    }

    let rust_target_features = tcx.rust_target_features(LOCAL_CRATE);

    let mut inline_span = None;
    let mut link_ordinal_span = None;
    let mut no_sanitize_span = None;
    let mut mixed_export_name_no_mangle_lint_state = MixedExportNameAndNoMangleState::default();

    for attr in attrs.iter() {
        // In some cases, attribute are only valid on functions, but it's the `check_attr`
        // pass that check that they aren't used anywhere else, rather this module.
        // In these cases, we bail from performing further checks that are only meaningful for
        // functions (such as calling `fn_sig`, which ICEs if given a non-function). We also
        // report a delayed bug, just in case `check_attr` isn't doing its job.
        let fn_sig = || {
            use DefKind::*;

            let def_kind = tcx.def_kind(did);
            if let Fn | AssocFn | Variant | Ctor(..) = def_kind {
                Some(tcx.fn_sig(did))
            } else {
                tcx.dcx().span_delayed_bug(
                    attr.span(),
                    "this attribute can only be applied to functions",
                );
                None
            }
        };

        if let hir::Attribute::Parsed(p) = attr {
            match p {
                AttributeKind::Repr(reprs) => {
                    codegen_fn_attrs.alignment = reprs
                        .iter()
                        .find_map(|(r, _)| if let ReprAlign(x) = r { Some(*x) } else { None });
                }

                _ => {}
            }
        }

        let Some(Ident { name, .. }) = attr.ident() else {
            continue;
        };

        match name {
            sym::cold => codegen_fn_attrs.flags |= CodegenFnAttrFlags::COLD,
            sym::rustc_allocator => codegen_fn_attrs.flags |= CodegenFnAttrFlags::ALLOCATOR,
            sym::ffi_pure => codegen_fn_attrs.flags |= CodegenFnAttrFlags::FFI_PURE,
            sym::ffi_const => codegen_fn_attrs.flags |= CodegenFnAttrFlags::FFI_CONST,
            sym::rustc_nounwind => codegen_fn_attrs.flags |= CodegenFnAttrFlags::NEVER_UNWIND,
            sym::rustc_reallocator => codegen_fn_attrs.flags |= CodegenFnAttrFlags::REALLOCATOR,
            sym::rustc_deallocator => codegen_fn_attrs.flags |= CodegenFnAttrFlags::DEALLOCATOR,
            sym::rustc_allocator_zeroed => {
                codegen_fn_attrs.flags |= CodegenFnAttrFlags::ALLOCATOR_ZEROED
            }
            sym::naked => codegen_fn_attrs.flags |= CodegenFnAttrFlags::NAKED,
            sym::no_mangle => {
                if tcx.opt_item_name(did.to_def_id()).is_some() {
                    codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_MANGLE;
                    mixed_export_name_no_mangle_lint_state.track_no_mangle(
                        attr.span(),
                        tcx.local_def_id_to_hir_id(did),
                        attr,
                    );
                } else {
                    tcx.dcx()
                        .struct_span_err(
                            attr.span(),
                            format!(
                                "`#[no_mangle]` cannot be used on {} {} as it has no name",
                                tcx.def_descr_article(did.to_def_id()),
                                tcx.def_descr(did.to_def_id()),
                            ),
                        )
                        .emit();
                }
            }
            sym::rustc_std_internal_symbol => {
                codegen_fn_attrs.flags |= CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL
            }
            sym::used => {
                let inner = attr.meta_item_list();
                match inner.as_deref() {
                    Some([item]) if item.has_name(sym::linker) => {
                        if !tcx.features().used_with_arg() {
                            feature_err(
                                &tcx.sess,
                                sym::used_with_arg,
                                attr.span(),
                                "`#[used(linker)]` is currently unstable",
                            )
                            .emit();
                        }
                        codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED_LINKER;
                    }
                    Some([item]) if item.has_name(sym::compiler) => {
                        if !tcx.features().used_with_arg() {
                            feature_err(
                                &tcx.sess,
                                sym::used_with_arg,
                                attr.span(),
                                "`#[used(compiler)]` is currently unstable",
                            )
                            .emit();
                        }
                        codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED;
                    }
                    Some(_) => {
                        tcx.dcx().emit_err(errors::ExpectedUsedSymbol { span: attr.span() });
                    }
                    None => {
                        // Unfortunately, unconditionally using `llvm.used` causes
                        // issues in handling `.init_array` with the gold linker,
                        // but using `llvm.compiler.used` caused a nontrivial amount
                        // of unintentional ecosystem breakage -- particularly on
                        // Mach-O targets.
                        //
                        // As a result, we emit `llvm.compiler.used` only on ELF
                        // targets. This is somewhat ad-hoc, but actually follows
                        // our pre-LLVM 13 behavior (prior to the ecosystem
                        // breakage), and seems to match `clang`'s behavior as well
                        // (both before and after LLVM 13), possibly because they
                        // have similar compatibility concerns to us. See
                        // https://github.com/rust-lang/rust/issues/47384#issuecomment-1019080146
                        // and following comments for some discussion of this, as
                        // well as the comments in `rustc_codegen_llvm` where these
                        // flags are handled.
                        //
                        // Anyway, to be clear: this is still up in the air
                        // somewhat, and is subject to change in the future (which
                        // is a good thing, because this would ideally be a bit
                        // more firmed up).
                        let is_like_elf = !(tcx.sess.target.is_like_osx
                            || tcx.sess.target.is_like_windows
                            || tcx.sess.target.is_like_wasm);
                        codegen_fn_attrs.flags |= if is_like_elf {
                            CodegenFnAttrFlags::USED
                        } else {
                            CodegenFnAttrFlags::USED_LINKER
                        };
                    }
                }
            }
            sym::thread_local => codegen_fn_attrs.flags |= CodegenFnAttrFlags::THREAD_LOCAL,
            sym::track_caller => {
                let is_closure = tcx.is_closure_like(did.to_def_id());

                if !is_closure
                    && let Some(fn_sig) = fn_sig()
                    && fn_sig.skip_binder().abi() != ExternAbi::Rust
                {
                    tcx.dcx().emit_err(errors::RequiresRustAbi { span: attr.span() });
                }
                if is_closure
                    && !tcx.features().closure_track_caller()
                    && !attr.span().allows_unstable(sym::closure_track_caller)
                {
                    feature_err(
                        &tcx.sess,
                        sym::closure_track_caller,
                        attr.span(),
                        "`#[track_caller]` on closures is currently unstable",
                    )
                    .emit();
                }
                codegen_fn_attrs.flags |= CodegenFnAttrFlags::TRACK_CALLER
            }
            sym::export_name => {
                if let Some(s) = attr.value_str() {
                    if s.as_str().contains('\0') {
                        // `#[export_name = ...]` will be converted to a null-terminated string,
                        // so it may not contain any null characters.
                        tcx.dcx().emit_err(errors::NullOnExport { span: attr.span() });
                    }
                    codegen_fn_attrs.export_name = Some(s);
                    mixed_export_name_no_mangle_lint_state.track_export_name(attr.span());
                }
            }
            sym::target_feature => {
                let Some(sig) = tcx.hir_node_by_def_id(did).fn_sig() else {
                    tcx.dcx().span_delayed_bug(attr.span(), "target_feature applied to non-fn");
                    continue;
                };
                let safe_target_features =
                    matches!(sig.header.safety, hir::HeaderSafety::SafeTargetFeatures);
                codegen_fn_attrs.safe_target_features = safe_target_features;
                if safe_target_features {
                    if tcx.sess.target.is_like_wasm || tcx.sess.opts.actually_rustdoc {
                        // The `#[target_feature]` attribute is allowed on
                        // WebAssembly targets on all functions. Prior to stabilizing
                        // the `target_feature_11` feature, `#[target_feature]` was
                        // only permitted on unsafe functions because on most targets
                        // execution of instructions that are not supported is
                        // considered undefined behavior. For WebAssembly which is a
                        // 100% safe target at execution time it's not possible to
                        // execute undefined instructions, and even if a future
                        // feature was added in some form for this it would be a
                        // deterministic trap. There is no undefined behavior when
                        // executing WebAssembly so `#[target_feature]` is allowed
                        // on safe functions (but again, only for WebAssembly)
                        //
                        // Note that this is also allowed if `actually_rustdoc` so
                        // if a target is documenting some wasm-specific code then
                        // it's not spuriously denied.
                        //
                        // Now that `#[target_feature]` is permitted on safe functions,
                        // this exception must still exist for allowing the attribute on
                        // `main`, `start`, and other functions that are not usually
                        // allowed.
                    } else {
                        check_target_feature_trait_unsafe(tcx, did, attr.span());
                    }
                }
                from_target_feature_attr(
                    tcx,
                    attr,
                    rust_target_features,
                    &mut codegen_fn_attrs.target_features,
                );
            }
            sym::linkage => {
                if let Some(val) = attr.value_str() {
                    let linkage = Some(linkage_by_name(tcx, did, val.as_str()));
                    if tcx.is_foreign_item(did) {
                        codegen_fn_attrs.import_linkage = linkage;

                        if tcx.is_mutable_static(did.into()) {
                            let mut diag = tcx.dcx().struct_span_err(
                                attr.span(),
                                "extern mutable statics are not allowed with `#[linkage]`",
                            );
                            diag.note(
                                "marking the extern static mutable would allow changing which \
                                 symbol the static references rather than make the target of the \
                                 symbol mutable",
                            );
                            diag.emit();
                        }
                    } else {
                        codegen_fn_attrs.linkage = linkage;
                    }
                }
            }
            sym::link_section => {
                if let Some(val) = attr.value_str() {
                    if val.as_str().bytes().any(|b| b == 0) {
                        let msg = format!("illegal null byte in link_section value: `{val}`");
                        tcx.dcx().span_err(attr.span(), msg);
                    } else {
                        codegen_fn_attrs.link_section = Some(val);
                    }
                }
            }
            sym::link_name => codegen_fn_attrs.link_name = attr.value_str(),
            sym::link_ordinal => {
                link_ordinal_span = Some(attr.span());
                if let ordinal @ Some(_) = check_link_ordinal(tcx, attr) {
                    codegen_fn_attrs.link_ordinal = ordinal;
                }
            }
            sym::no_sanitize => {
                no_sanitize_span = Some(attr.span());
                if let Some(list) = attr.meta_item_list() {
                    for item in list.iter() {
                        match item.name_or_empty() {
                            sym::address => {
                                codegen_fn_attrs.no_sanitize |=
                                    SanitizerSet::ADDRESS | SanitizerSet::KERNELADDRESS
                            }
                            sym::cfi => codegen_fn_attrs.no_sanitize |= SanitizerSet::CFI,
                            sym::kcfi => codegen_fn_attrs.no_sanitize |= SanitizerSet::KCFI,
                            sym::memory => codegen_fn_attrs.no_sanitize |= SanitizerSet::MEMORY,
                            sym::memtag => codegen_fn_attrs.no_sanitize |= SanitizerSet::MEMTAG,
                            sym::shadow_call_stack => {
                                codegen_fn_attrs.no_sanitize |= SanitizerSet::SHADOWCALLSTACK
                            }
                            sym::thread => codegen_fn_attrs.no_sanitize |= SanitizerSet::THREAD,
                            sym::hwaddress => {
                                codegen_fn_attrs.no_sanitize |= SanitizerSet::HWADDRESS
                            }
                            _ => {
                                tcx.dcx().emit_err(errors::InvalidNoSanitize { span: item.span() });
                            }
                        }
                    }
                }
            }
            sym::instruction_set => {
                codegen_fn_attrs.instruction_set =
                    attr.meta_item_list().and_then(|l| match &l[..] {
                        [MetaItemInner::MetaItem(set)] => {
                            let segments =
                                set.path.segments.iter().map(|x| x.ident.name).collect::<Vec<_>>();
                            match segments.as_slice() {
                                [sym::arm, sym::a32 | sym::t32]
                                    if !tcx.sess.target.has_thumb_interworking =>
                                {
                                    tcx.dcx().emit_err(errors::UnsuportedInstructionSet {
                                        span: attr.span(),
                                    });
                                    None
                                }
                                [sym::arm, sym::a32] => Some(InstructionSetAttr::ArmA32),
                                [sym::arm, sym::t32] => Some(InstructionSetAttr::ArmT32),
                                _ => {
                                    tcx.dcx().emit_err(errors::InvalidInstructionSet {
                                        span: attr.span(),
                                    });
                                    None
                                }
                            }
                        }
                        [] => {
                            tcx.dcx().emit_err(errors::BareInstructionSet { span: attr.span() });
                            None
                        }
                        _ => {
                            tcx.dcx()
                                .emit_err(errors::MultipleInstructionSet { span: attr.span() });
                            None
                        }
                    })
            }
            sym::patchable_function_entry => {
                codegen_fn_attrs.patchable_function_entry = attr.meta_item_list().and_then(|l| {
                    let mut prefix = None;
                    let mut entry = None;
                    for item in l {
                        let Some(meta_item) = item.meta_item() else {
                            tcx.dcx().emit_err(errors::ExpectedNameValuePair { span: item.span() });
                            continue;
                        };

                        let Some(name_value_lit) = meta_item.name_value_literal() else {
                            tcx.dcx().emit_err(errors::ExpectedNameValuePair { span: item.span() });
                            continue;
                        };

                        let attrib_to_write = match meta_item.name_or_empty() {
                            sym::prefix_nops => &mut prefix,
                            sym::entry_nops => &mut entry,
                            _ => {
                                tcx.dcx().emit_err(errors::UnexpectedParameterName {
                                    span: item.span(),
                                    prefix_nops: sym::prefix_nops,
                                    entry_nops: sym::entry_nops,
                                });
                                continue;
                            }
                        };

                        let rustc_ast::LitKind::Int(val, _) = name_value_lit.kind else {
                            tcx.dcx().emit_err(errors::InvalidLiteralValue {
                                span: name_value_lit.span,
                            });
                            continue;
                        };

                        let Ok(val) = val.get().try_into() else {
                            tcx.dcx()
                                .emit_err(errors::OutOfRangeInteger { span: name_value_lit.span });
                            continue;
                        };

                        *attrib_to_write = Some(val);
                    }

                    if let (None, None) = (prefix, entry) {
                        tcx.dcx().span_err(attr.span(), "must specify at least one parameter");
                    }

                    Some(PatchableFunctionEntry::from_prefix_and_entry(
                        prefix.unwrap_or(0),
                        entry.unwrap_or(0),
                    ))
                })
            }
            _ => {}
        }
    }

    mixed_export_name_no_mangle_lint_state.lint_if_mixed(tcx);

    codegen_fn_attrs.inline = attrs.iter().fold(InlineAttr::None, |ia, attr| {
        if !attr.has_name(sym::inline) {
            return ia;
        }

        if attr.is_word() {
            return InlineAttr::Hint;
        }
        let Some(ref items) = attr.meta_item_list() else {
            return ia;
        };
        inline_span = Some(attr.span());

        let [item] = &items[..] else {
            tcx.dcx().emit_err(errors::ExpectedOneArgument { span: attr.span() });
            return InlineAttr::None;
        };

        if item.has_name(sym::always) {
            InlineAttr::Always
        } else if item.has_name(sym::never) {
            InlineAttr::Never
        } else {
            tcx.dcx().emit_err(errors::InvalidArgument { span: items[0].span() });

            InlineAttr::None
        }
    });
    codegen_fn_attrs.inline = attrs.iter().fold(codegen_fn_attrs.inline, |ia, attr| {
        if !attr.has_name(sym::rustc_force_inline) || !tcx.features().rustc_attrs() {
            return ia;
        }

        if attr.is_word() {
            InlineAttr::Force { attr_span: attr.span(), reason: None }
        } else if let Some(val) = attr.value_str() {
            InlineAttr::Force { attr_span: attr.span(), reason: Some(val) }
        } else {
            debug!("`rustc_force_inline` not checked by attribute validation");
            ia
        }
    });

    // naked function MUST NOT be inlined! This attribute is required for the rust compiler itself,
    // but not for the code generation backend because at that point the naked function will just be
    // a declaration, with a definition provided in global assembly.
    if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
        codegen_fn_attrs.inline = InlineAttr::Never;
    }

    codegen_fn_attrs.optimize = attrs.iter().fold(OptimizeAttr::Default, |ia, attr| {
        if !attr.has_name(sym::optimize) {
            return ia;
        }
        if attr.is_word() {
            tcx.dcx().emit_err(errors::ExpectedOneArgumentOptimize { span: attr.span() });
            return ia;
        }
        let Some(ref items) = attr.meta_item_list() else {
            return OptimizeAttr::Default;
        };

        inline_span = Some(attr.span());
        let [item] = &items[..] else {
            tcx.dcx().emit_err(errors::ExpectedOneArgumentOptimize { span: attr.span() });
            return OptimizeAttr::Default;
        };
        if item.has_name(sym::size) {
            OptimizeAttr::Size
        } else if item.has_name(sym::speed) {
            OptimizeAttr::Speed
        } else if item.has_name(sym::none) {
            OptimizeAttr::DoNotOptimize
        } else {
            tcx.dcx().emit_err(errors::InvalidArgumentOptimize { span: item.span() });
            OptimizeAttr::Default
        }
    });

    // #73631: closures inherit `#[target_feature]` annotations
    //
    // If this closure is marked `#[inline(always)]`, simply skip adding `#[target_feature]`.
    //
    // At this point, `unsafe` has already been checked and `#[target_feature]` only affects codegen.
    // Due to LLVM limitations, emitting both `#[inline(always)]` and `#[target_feature]` is *unsound*:
    // the function may be inlined into a caller with fewer target features. Also see
    // <https://github.com/rust-lang/rust/issues/116573>.
    //
    // Using `#[inline(always)]` implies that this closure will most likely be inlined into
    // its parent function, which effectively inherits the features anyway. Boxing this closure
    // would result in this closure being compiled without the inherited target features, but this
    // is probably a poor usage of `#[inline(always)]` and easily avoided by not using the attribute.
    if tcx.is_closure_like(did.to_def_id()) && codegen_fn_attrs.inline != InlineAttr::Always {
        let owner_id = tcx.parent(did.to_def_id());
        if tcx.def_kind(owner_id).has_codegen_attrs() {
            codegen_fn_attrs
                .target_features
                .extend(tcx.codegen_fn_attrs(owner_id).target_features.iter().copied());
        }
    }

    // If a function uses `#[target_feature]` it can't be inlined into general
    // purpose functions as they wouldn't have the right target features
    // enabled. For that reason we also forbid `#[inline(always)]` as it can't be
    // respected.
    //
    // `#[rustc_force_inline]` doesn't need to be prohibited here, only
    // `#[inline(always)]`, as forced inlining is implemented entirely within
    // rustc (and so the MIR inliner can do any necessary checks for compatible target
    // features).
    //
    // This sidesteps the LLVM blockers in enabling `target_features` +
    // `inline(always)` to be used together (see rust-lang/rust#116573 and
    // llvm/llvm-project#70563).
    if !codegen_fn_attrs.target_features.is_empty()
        && matches!(codegen_fn_attrs.inline, InlineAttr::Always)
        && let Some(span) = inline_span
    {
        tcx.dcx().span_err(span, "cannot use `#[inline(always)]` with `#[target_feature]`");
    }

    if !codegen_fn_attrs.no_sanitize.is_empty()
        && codegen_fn_attrs.inline.always()
        && let (Some(no_sanitize_span), Some(inline_span)) = (no_sanitize_span, inline_span)
    {
        let hir_id = tcx.local_def_id_to_hir_id(did);
        tcx.node_span_lint(lint::builtin::INLINE_NO_SANITIZE, hir_id, no_sanitize_span, |lint| {
            lint.primary_message("`no_sanitize` will have no effect after inlining");
            lint.span_note(inline_span, "inlining requested here");
        })
    }

    // Weak lang items have the same semantics as "std internal" symbols in the
    // sense that they're preserved through all our LTO passes and only
    // strippable by the linker.
    //
    // Additionally weak lang items have predetermined symbol names.
    if let Some((name, _)) = lang_items::extract(attrs)
        && let Some(lang_item) = LangItem::from_name(name)
    {
        if WEAK_LANG_ITEMS.contains(&lang_item) {
            codegen_fn_attrs.flags |= CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL;
        }
        if let Some(link_name) = lang_item.link_name() {
            codegen_fn_attrs.export_name = Some(link_name);
            codegen_fn_attrs.link_name = Some(link_name);
        }
    }
    check_link_name_xor_ordinal(tcx, &codegen_fn_attrs, link_ordinal_span);

    // Any linkage to LLVM intrinsics for now forcibly marks them all as never
    // unwinds since LLVM sometimes can't handle codegen which `invoke`s
    // intrinsic functions.
    if let Some(name) = &codegen_fn_attrs.link_name
        && name.as_str().starts_with("llvm.")
    {
        codegen_fn_attrs.flags |= CodegenFnAttrFlags::NEVER_UNWIND;
    }

    if let Some(features) = check_tied_features(
        tcx.sess,
        &codegen_fn_attrs
            .target_features
            .iter()
            .map(|features| (features.name.as_str(), true))
            .collect(),
    ) {
        let span = tcx
            .get_attrs(did, sym::target_feature)
            .next()
            .map_or_else(|| tcx.def_span(did), |a| a.span());
        tcx.dcx()
            .create_err(errors::TargetFeatureDisableOrEnable {
                features,
                span: Some(span),
                missing_features: Some(errors::MissingFeatures),
            })
            .emit();
    }

    codegen_fn_attrs
}

/// Given a map from target_features to whether they are enabled or disabled, ensure only valid
/// combinations are allowed.
pub fn check_tied_features(
    sess: &Session,
    features: &FxHashMap<&str, bool>,
) -> Option<&'static [&'static str]> {
    if !features.is_empty() {
        for tied in sess.target.tied_target_features() {
            // Tied features must be set to the same value, or not set at all
            let mut tied_iter = tied.iter();
            let enabled = features.get(tied_iter.next().unwrap());
            if tied_iter.any(|f| enabled != features.get(f)) {
                return Some(tied);
            }
        }
    }
    None
}

/// Checks if the provided DefId is a method in a trait impl for a trait which has track_caller
/// applied to the method prototype.
fn should_inherit_track_caller(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
    if let Some(impl_item) = tcx.opt_associated_item(def_id)
        && let ty::AssocItemContainer::Impl = impl_item.container
        && let Some(trait_item) = impl_item.trait_item_def_id
    {
        return tcx.codegen_fn_attrs(trait_item).flags.intersects(CodegenFnAttrFlags::TRACK_CALLER);
    }

    false
}

fn check_link_ordinal(tcx: TyCtxt<'_>, attr: &hir::Attribute) -> Option<u16> {
    use rustc_ast::{LitIntType, LitKind, MetaItemLit};
    let meta_item_list = attr.meta_item_list()?;
    let [sole_meta_list] = &meta_item_list[..] else {
        tcx.dcx().emit_err(errors::InvalidLinkOrdinalNargs { span: attr.span() });
        return None;
    };
    if let Some(MetaItemLit { kind: LitKind::Int(ordinal, LitIntType::Unsuffixed), .. }) =
        sole_meta_list.lit()
    {
        // According to the table at
        // https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#import-header, the
        // ordinal must fit into 16 bits. Similarly, the Ordinal field in COFFShortExport (defined
        // in llvm/include/llvm/Object/COFFImportFile.h), which we use to communicate import
        // information to LLVM for `#[link(kind = "raw-dylib"_])`, is also defined to be uint16_t.
        //
        // FIXME: should we allow an ordinal of 0?  The MSVC toolchain has inconsistent support for
        // this: both LINK.EXE and LIB.EXE signal errors and abort when given a .DEF file that
        // specifies a zero ordinal. However, llvm-dlltool is perfectly happy to generate an import
        // library for such a .DEF file, and MSVC's LINK.EXE is also perfectly happy to consume an
        // import library produced by LLVM with an ordinal of 0, and it generates an .EXE.  (I
        // don't know yet if the resulting EXE runs, as I haven't yet built the necessary DLL --
        // see earlier comment about LINK.EXE failing.)
        if *ordinal <= u16::MAX as u128 {
            Some(ordinal.get() as u16)
        } else {
            let msg = format!("ordinal value in `link_ordinal` is too large: `{ordinal}`");
            tcx.dcx()
                .struct_span_err(attr.span(), msg)
                .with_note("the value may not exceed `u16::MAX`")
                .emit();
            None
        }
    } else {
        tcx.dcx().emit_err(errors::InvalidLinkOrdinalFormat { span: attr.span() });
        None
    }
}

fn check_link_name_xor_ordinal(
    tcx: TyCtxt<'_>,
    codegen_fn_attrs: &CodegenFnAttrs,
    inline_span: Option<Span>,
) {
    if codegen_fn_attrs.link_name.is_none() || codegen_fn_attrs.link_ordinal.is_none() {
        return;
    }
    let msg = "cannot use `#[link_name]` with `#[link_ordinal]`";
    if let Some(span) = inline_span {
        tcx.dcx().span_err(span, msg);
    } else {
        tcx.dcx().err(msg);
    }
}

#[derive(Default)]
struct MixedExportNameAndNoMangleState<'a> {
    export_name: Option<Span>,
    hir_id: Option<HirId>,
    no_mangle: Option<Span>,
    no_mangle_attr: Option<&'a hir::Attribute>,
}

impl<'a> MixedExportNameAndNoMangleState<'a> {
    fn track_export_name(&mut self, span: Span) {
        self.export_name = Some(span);
    }

    fn track_no_mangle(&mut self, span: Span, hir_id: HirId, attr_name: &'a hir::Attribute) {
        self.no_mangle = Some(span);
        self.hir_id = Some(hir_id);
        self.no_mangle_attr = Some(attr_name);
    }

    /// Emit diagnostics if the lint condition is met.
    fn lint_if_mixed(self, tcx: TyCtxt<'_>) {
        if let Self {
            export_name: Some(export_name),
            no_mangle: Some(no_mangle),
            hir_id: Some(hir_id),
            no_mangle_attr: Some(_),
        } = self
        {
            tcx.emit_node_span_lint(
                lint::builtin::UNUSED_ATTRIBUTES,
                hir_id,
                no_mangle,
                errors::MixedExportNameAndNoMangle {
                    no_mangle,
                    no_mangle_attr: "#[unsafe(no_mangle)]".to_string(),
                    export_name,
                    removal_span: no_mangle,
                },
            );
        }
    }
}

/// We now check the #\[rustc_autodiff\] attributes which we generated from the #[autodiff(...)]
/// macros. There are two forms. The pure one without args to mark primal functions (the functions
/// being differentiated). The other form is #[rustc_autodiff(Mode, ActivityList)] on top of the
/// placeholder functions. We wrote the rustc_autodiff attributes ourself, so this should never
/// panic, unless we introduced a bug when parsing the autodiff macro.
fn autodiff_attrs(tcx: TyCtxt<'_>, id: DefId) -> Option<AutoDiffAttrs> {
    let attrs = tcx.get_attrs(id, sym::rustc_autodiff);

    let attrs =
        attrs.filter(|attr| attr.name_or_empty() == sym::rustc_autodiff).collect::<Vec<_>>();

    // check for exactly one autodiff attribute on placeholder functions.
    // There should only be one, since we generate a new placeholder per ad macro.
    let attr = match &attrs[..] {
        [] => return None,
        [attr] => attr,
        _ => {
            span_bug!(attrs[1].span(), "cg_ssa: rustc_autodiff should only exist once per source");
        }
    };

    let list = attr.meta_item_list().unwrap_or_default();

    // empty autodiff attribute macros (i.e. `#[autodiff]`) are used to mark source functions
    if list.is_empty() {
        return Some(AutoDiffAttrs::source());
    }

    let [mode, input_activities @ .., ret_activity] = &list[..] else {
        span_bug!(attr.span(), "rustc_autodiff attribute must contain mode and activities");
    };
    let mode = if let MetaItemInner::MetaItem(MetaItem { path: p1, .. }) = mode {
        p1.segments.first().unwrap().ident
    } else {
        span_bug!(attr.span(), "rustc_autodiff attribute must contain mode");
    };

    // parse mode
    let mode = match mode.as_str() {
        "Forward" => DiffMode::Forward,
        "Reverse" => DiffMode::Reverse,
        _ => {
            span_bug!(mode.span, "rustc_autodiff attribute contains invalid mode");
        }
    };

    // First read the ret symbol from the attribute
    let ret_symbol = if let MetaItemInner::MetaItem(MetaItem { path: p1, .. }) = ret_activity {
        p1.segments.first().unwrap().ident
    } else {
        span_bug!(attr.span(), "rustc_autodiff attribute must contain the return activity");
    };

    // Then parse it into an actual DiffActivity
    let Ok(ret_activity) = DiffActivity::from_str(ret_symbol.as_str()) else {
        span_bug!(ret_symbol.span, "invalid return activity");
    };

    // Now parse all the intermediate (input) activities
    let mut arg_activities: Vec<DiffActivity> = vec![];
    for arg in input_activities {
        let arg_symbol = if let MetaItemInner::MetaItem(MetaItem { path: p2, .. }) = arg {
            match p2.segments.first() {
                Some(x) => x.ident,
                None => {
                    span_bug!(
                        arg.span(),
                        "rustc_autodiff attribute must contain the input activity"
                    );
                }
            }
        } else {
            span_bug!(arg.span(), "rustc_autodiff attribute must contain the input activity");
        };

        match DiffActivity::from_str(arg_symbol.as_str()) {
            Ok(arg_activity) => arg_activities.push(arg_activity),
            Err(_) => {
                span_bug!(arg_symbol.span, "invalid input activity");
            }
        }
    }

    Some(AutoDiffAttrs { mode, ret_activity, input_activity: arg_activities })
}

pub(crate) fn provide(providers: &mut Providers) {
    *providers = Providers { codegen_fn_attrs, should_inherit_track_caller, ..*providers };
}