use std::str::FromStr;

use rustc_abi::{Align, ExternAbi};
use rustc_ast::expand::autodiff_attrs::{AutoDiffAttrs, DiffActivity, DiffMode};
use rustc_ast::{LitKind, MetaItem, MetaItemInner, attr};
use rustc_hir::attrs::{
    AttributeKind, InlineAttr, InstructionSetAttr, Linkage, RtsanSetting, UsedBy,
};
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{DefId, LOCAL_CRATE, LocalDefId};
use rustc_hir::{self as hir, Attribute, LangItem, find_attr, lang_items};
use rustc_middle::middle::codegen_fn_attrs::{
    CodegenFnAttrFlags, CodegenFnAttrs, PatchableFunctionEntry, SanitizerFnAttrs,
};
use rustc_middle::mir::mono::Visibility;
use rustc_middle::query::Providers;
use rustc_middle::span_bug;
use rustc_middle::ty::{self as ty, Instance, TyCtxt};
use rustc_session::lint;
use rustc_session::parse::feature_err;
use rustc_span::{Ident, Span, Symbol, sym};
use rustc_target::spec::Os;

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

/// In some cases, attributes are only valid on functions, but it's the `check_attr`
/// pass that checks that they aren't used anywhere else, rather than 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.
fn try_fn_sig<'tcx>(
    tcx: TyCtxt<'tcx>,
    did: LocalDefId,
    attr_span: Span,
) -> Option<ty::EarlyBinder<'tcx, ty::PolyFnSig<'tcx>>> {
    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
    }
}

// FIXME(jdonszelmann): remove when instruction_set becomes a parsed attr
fn parse_instruction_set_attr(tcx: TyCtxt<'_>, attr: &Attribute) -> Option<InstructionSetAttr> {
    let list = attr.meta_item_list()?;

    match &list[..] {
        [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::UnsupportedInstructionSet { 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
        }
    }
}

// FIXME(jdonszelmann): remove when patchable_function_entry becomes a parsed attr
fn parse_patchable_function_entry(
    tcx: TyCtxt<'_>,
    attr: &Attribute,
) -> Option<PatchableFunctionEntry> {
    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() {
                Some(sym::prefix_nops) => &mut prefix,
                Some(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)))
    })
}

/// Spans that are collected when processing built-in attributes,
/// that are useful for emitting diagnostics later.
#[derive(Default)]
struct InterestingAttributeDiagnosticSpans {
    link_ordinal: Option<Span>,
    sanitize: Option<Span>,
    inline: Option<Span>,
    no_mangle: Option<Span>,
}

/// Process the builtin attrs ([`hir::Attribute`]) on the item.
/// Many of them directly translate to codegen attrs.
fn process_builtin_attrs(
    tcx: TyCtxt<'_>,
    did: LocalDefId,
    attrs: &[Attribute],
    codegen_fn_attrs: &mut CodegenFnAttrs,
) -> InterestingAttributeDiagnosticSpans {
    let mut interesting_spans = InterestingAttributeDiagnosticSpans::default();
    let rust_target_features = tcx.rust_target_features(LOCAL_CRATE);

    for attr in attrs.iter() {
        if let hir::Attribute::Parsed(p) = attr {
            match p {
                AttributeKind::Cold(_) => codegen_fn_attrs.flags |= CodegenFnAttrFlags::COLD,
                AttributeKind::ExportName { name, .. } => {
                    codegen_fn_attrs.symbol_name = Some(*name)
                }
                AttributeKind::Inline(inline, span) => {
                    codegen_fn_attrs.inline = *inline;
                    interesting_spans.inline = Some(*span);
                }
                AttributeKind::Naked(_) => codegen_fn_attrs.flags |= CodegenFnAttrFlags::NAKED,
                AttributeKind::Align { align, .. } => codegen_fn_attrs.alignment = Some(*align),
                AttributeKind::LinkName { name, .. } => {
                    // FIXME Remove check for foreign functions once #[link_name] on non-foreign
                    // functions is a hard error
                    if tcx.is_foreign_item(did) {
                        codegen_fn_attrs.symbol_name = Some(*name);
                    }
                }
                AttributeKind::LinkOrdinal { ordinal, span } => {
                    codegen_fn_attrs.link_ordinal = Some(*ordinal);
                    interesting_spans.link_ordinal = Some(*span);
                }
                AttributeKind::LinkSection { name, .. } => {
                    codegen_fn_attrs.link_section = Some(*name)
                }
                AttributeKind::NoMangle(attr_span) => {
                    interesting_spans.no_mangle = Some(*attr_span);
                    if tcx.opt_item_name(did.to_def_id()).is_some() {
                        codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_MANGLE;
                    } else {
                        tcx.dcx().span_delayed_bug(
                            *attr_span,
                            "no_mangle should be on a named function",
                        );
                    }
                }
                AttributeKind::Optimize(optimize, _) => codegen_fn_attrs.optimize = *optimize,
                AttributeKind::TargetFeature { features, attr_span, was_forced } => {
                    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 && !was_forced {
                        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,
                        did,
                        features,
                        *was_forced,
                        rust_target_features,
                        &mut codegen_fn_attrs.target_features,
                    );
                }
                AttributeKind::TrackCaller(attr_span) => {
                    let is_closure = tcx.is_closure_like(did.to_def_id());

                    if !is_closure
                        && let Some(fn_sig) = try_fn_sig(tcx, did, *attr_span)
                        && 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
                }
                AttributeKind::Used { used_by, .. } => match used_by {
                    UsedBy::Compiler => codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED_COMPILER,
                    UsedBy::Linker => codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED_LINKER,
                    UsedBy::Default => {
                        let used_form = if tcx.sess.target.os == Os::Illumos {
                            // illumos' `ld` doesn't support a section header that would represent
                            // `#[used(linker)]`, see
                            // https://github.com/rust-lang/rust/issues/146169. For that target,
                            // downgrade as if `#[used(compiler)]` was requested and hope for the
                            // best.
                            CodegenFnAttrFlags::USED_COMPILER
                        } else {
                            CodegenFnAttrFlags::USED_LINKER
                        };
                        codegen_fn_attrs.flags |= used_form;
                    }
                },
                AttributeKind::FfiConst(_) => {
                    codegen_fn_attrs.flags |= CodegenFnAttrFlags::FFI_CONST
                }
                AttributeKind::FfiPure(_) => codegen_fn_attrs.flags |= CodegenFnAttrFlags::FFI_PURE,
                AttributeKind::StdInternalSymbol(_) => {
                    codegen_fn_attrs.flags |= CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL
                }
                AttributeKind::Linkage(linkage, span) => {
                    let linkage = Some(*linkage);

                    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(
                                *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;
                    }
                }
                AttributeKind::Sanitize { span, .. } => {
                    interesting_spans.sanitize = Some(*span);
                }
                AttributeKind::ObjcClass { classname, .. } => {
                    codegen_fn_attrs.objc_class = Some(*classname);
                }
                AttributeKind::ObjcSelector { methname, .. } => {
                    codegen_fn_attrs.objc_selector = Some(*methname);
                }
                AttributeKind::EiiExternItem => {
                    codegen_fn_attrs.flags |= CodegenFnAttrFlags::EXTERNALLY_IMPLEMENTABLE_ITEM;
                }
                AttributeKind::EiiImpls(impls) => {
                    for i in impls {
                        let extern_item = find_attr!(
                            tcx.get_all_attrs(i.eii_macro),
                            AttributeKind::EiiExternTarget(target) => target.eii_extern_target
                        )
                        .expect("eii should have declaration macro with extern target attribute");

                        let symbol_name = tcx.symbol_name(Instance::mono(tcx, extern_item));

                        // this is to prevent a bug where a single crate defines both the default and explicit implementation
                        // for an EII. In that case, both of them may be part of the same final object file. I'm not 100% sure
                        // what happens, either rustc deduplicates the symbol or llvm, or it's random/order-dependent.
                        // However, the fact that the default one of has weak linkage isn't considered and you sometimes get that
                        // the default implementation is used while an explicit implementation is given.
                        if
                        // if this is a default impl
                        i.is_default
                            // iterate over all implementations *in the current crate*
                            // (this is ok since we generate codegen fn attrs in the local crate)
                            // if any of them is *not default* then don't emit the alias.
                            && tcx.externally_implementable_items(LOCAL_CRATE).get(&i.eii_macro).expect("at least one").1.iter().any(|(_, imp)| !imp.is_default)
                        {
                            continue;
                        }

                        codegen_fn_attrs.foreign_item_symbol_aliases.push((
                            Symbol::intern(symbol_name.name),
                            if i.is_default { Linkage::LinkOnceAny } else { Linkage::External },
                            Visibility::Default,
                        ));
                        codegen_fn_attrs.flags |= CodegenFnAttrFlags::EXTERNALLY_IMPLEMENTABLE_ITEM;
                    }
                }
                _ => {}
            }
        }

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

        match name {
            sym::rustc_allocator => codegen_fn_attrs.flags |= CodegenFnAttrFlags::ALLOCATOR,
            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::thread_local => codegen_fn_attrs.flags |= CodegenFnAttrFlags::THREAD_LOCAL,
            sym::instruction_set => {
                codegen_fn_attrs.instruction_set = parse_instruction_set_attr(tcx, attr)
            }
            sym::patchable_function_entry => {
                codegen_fn_attrs.patchable_function_entry =
                    parse_patchable_function_entry(tcx, attr);
            }
            sym::rustc_offload_kernel => {
                codegen_fn_attrs.flags |= CodegenFnAttrFlags::OFFLOAD_KERNEL
            }
            _ => {}
        }
    }

    interesting_spans
}

/// Applies overrides for codegen fn attrs. These often have a specific reason why they're necessary.
/// Please comment why when adding a new one!
fn apply_overrides(tcx: TyCtxt<'_>, did: LocalDefId, codegen_fn_attrs: &mut CodegenFnAttrs) {
    // Apply the minimum function alignment here. This ensures that a function's alignment is
    // determined by the `-C` flags of the crate it is defined in, not the `-C` flags of the crate
    // it happens to be codegen'd (or const-eval'd) in.
    codegen_fn_attrs.alignment =
        Ord::max(codegen_fn_attrs.alignment, tcx.sess.opts.unstable_opts.min_function_alignment);

    // Passed in sanitizer settings are always the default.
    assert!(codegen_fn_attrs.sanitizers == SanitizerFnAttrs::default());
    // Replace with #[sanitize] value
    codegen_fn_attrs.sanitizers = tcx.sanitizer_settings_for(did);
    // On trait methods, inherit the `#[align]` of the trait's method prototype.
    codegen_fn_attrs.alignment = Ord::max(codegen_fn_attrs.alignment, tcx.inherited_align(did));

    // 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;
    }

    // #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());
        }
    }

    // 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;
    }

    // inherit track-caller properly
    if tcx.should_inherit_track_caller(did) {
        codegen_fn_attrs.flags |= CodegenFnAttrFlags::TRACK_CALLER;
    }

    // Foreign items by default use no mangling for their symbol name.
    if tcx.is_foreign_item(did) {
        codegen_fn_attrs.flags |= CodegenFnAttrFlags::FOREIGN_ITEM;

        // There's a few exceptions to this rule though:
        if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL) {
            // * `#[rustc_std_internal_symbol]` mangles the symbol name in a special way
            //   both for exports and imports through foreign items. This is handled further,
            //   during symbol mangling logic.
        } else if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::EXTERNALLY_IMPLEMENTABLE_ITEM)
        {
            // * externally implementable items keep their mangled symbol name.
            //   multiple EIIs can have the same name, so not mangling them would be a bug.
            //   Implementing an EII does the appropriate name resolution to make sure the implementations
            //   get the same symbol name as the *mangled* foreign item they refer to so that's all good.
        } else if codegen_fn_attrs.symbol_name.is_some() {
            // * This can be overridden with the `#[link_name]` attribute
        } else {
            // NOTE: there's one more exception that we cannot apply here. On wasm,
            // some items cannot be `no_mangle`.
            // However, we don't have enough information here to determine that.
            // As such, no_mangle foreign items on wasm that have the same defid as some
            // import will *still* be mangled despite this.
            //
            // if none of the exceptions apply; apply no_mangle
            codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_MANGLE;
        }
    }
}

fn check_result(
    tcx: TyCtxt<'_>,
    did: LocalDefId,
    interesting_spans: InterestingAttributeDiagnosticSpans,
    codegen_fn_attrs: &CodegenFnAttrs,
) {
    // 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)
        && !tcx.features().target_feature_inline_always()
        && let Some(span) = interesting_spans.inline
    {
        feature_err(
            tcx.sess,
            sym::target_feature_inline_always,
            span,
            "cannot use `#[inline(always)]` with `#[target_feature]`",
        )
        .emit();
    }

    // warn that inline has no effect when no_sanitize is present
    if codegen_fn_attrs.sanitizers != SanitizerFnAttrs::default()
        && codegen_fn_attrs.inline.always()
        && let (Some(sanitize_span), Some(inline_span)) =
            (interesting_spans.sanitize, interesting_spans.inline)
    {
        let hir_id = tcx.local_def_id_to_hir_id(did);
        tcx.node_span_lint(lint::builtin::INLINE_NO_SANITIZE, hir_id, sanitize_span, |lint| {
            lint.primary_message("non-default `sanitize` will have no effect after inlining");
            lint.span_note(inline_span, "inlining requested here");
        })
    }

    // warn for nonblocking async functions, blocks and closures.
    // This doesn't behave as expected, because the executor can run blocking code without the sanitizer noticing.
    if codegen_fn_attrs.sanitizers.rtsan_setting == RtsanSetting::Nonblocking
        && let Some(sanitize_span) = interesting_spans.sanitize
        // async fn
        && (tcx.asyncness(did).is_async()
            // async block
            || tcx.is_coroutine(did.into())
            // async closure
            || (tcx.is_closure_like(did.into())
                && tcx.hir_node_by_def_id(did).expect_closure().kind
                    != rustc_hir::ClosureKind::Closure))
    {
        let hir_id = tcx.local_def_id_to_hir_id(did);
        tcx.node_span_lint(
            lint::builtin::RTSAN_NONBLOCKING_ASYNC,
            hir_id,
            sanitize_span,
            |lint| {
                lint.primary_message(r#"the async executor can run blocking code, without realtime sanitizer catching it"#);
            }
        );
    }

    // error when specifying link_name together with link_ordinal
    if let Some(_) = codegen_fn_attrs.symbol_name
        && let Some(_) = codegen_fn_attrs.link_ordinal
    {
        let msg = "cannot use `#[link_name]` with `#[link_ordinal]`";
        if let Some(span) = interesting_spans.link_ordinal {
            tcx.dcx().span_err(span, msg);
        } else {
            tcx.dcx().err(msg);
        }
    }

    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 =
            find_attr!(tcx.get_all_attrs(did), AttributeKind::TargetFeature{attr_span: span, ..} => *span)
                .unwrap_or_else(|| tcx.def_span(did));

        tcx.dcx()
            .create_err(errors::TargetFeatureDisableOrEnable {
                features,
                span: Some(span),
                missing_features: Some(errors::MissingFeatures),
            })
            .emit();
    }
}

fn handle_lang_items(
    tcx: TyCtxt<'_>,
    did: LocalDefId,
    interesting_spans: &InterestingAttributeDiagnosticSpans,
    attrs: &[Attribute],
    codegen_fn_attrs: &mut CodegenFnAttrs,
) {
    let lang_item = lang_items::extract(attrs).and_then(|(name, _)| LangItem::from_name(name));

    // 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(lang_item) = lang_item
        && let Some(link_name) = lang_item.link_name()
    {
        codegen_fn_attrs.flags |= CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL;
        codegen_fn_attrs.symbol_name = Some(link_name);
    }

    // error when using no_mangle on a lang item item
    if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL)
        && codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE)
    {
        let mut err = tcx
            .dcx()
            .struct_span_err(
                interesting_spans.no_mangle.unwrap_or_default(),
                "`#[no_mangle]` cannot be used on internal language items",
            )
            .with_note("Rustc requires this item to have a specific mangled name.")
            .with_span_label(tcx.def_span(did), "should be the internal language item");
        if let Some(lang_item) = lang_item
            && let Some(link_name) = lang_item.link_name()
        {
            err = err
                .with_note("If you are trying to prevent mangling to ease debugging, many")
                .with_note(format!("debuggers support a command such as `rbreak {link_name}` to"))
                .with_note(format!(
                    "match `.*{link_name}.*` instead of `break {link_name}` on a specific name"
                ))
        }
        err.emit();
    }
}

/// Generate the [`CodegenFnAttrs`] for an item (identified by the [`LocalDefId`]).
///
/// This happens in 4 stages:
/// - apply built-in attributes that directly translate to codegen attributes.
/// - handle lang items. These have special codegen attrs applied to them.
/// - apply overrides, like minimum requirements for alignment and other settings that don't rely directly the built-in attrs on the item.
///   overrides come after applying built-in attributes since they may only apply when certain attributes were already set in the stage before.
/// - check that the result is valid. There's various ways in which this may not be the case, such as certain combinations of attrs.
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 mut codegen_fn_attrs = CodegenFnAttrs::new();
    let attrs = tcx.hir_attrs(tcx.local_def_id_to_hir_id(did));

    let interesting_spans = process_builtin_attrs(tcx, did, attrs, &mut codegen_fn_attrs);
    handle_lang_items(tcx, did, &interesting_spans, attrs, &mut codegen_fn_attrs);
    apply_overrides(tcx, did, &mut codegen_fn_attrs);
    check_result(tcx, did, interesting_spans, &codegen_fn_attrs);

    codegen_fn_attrs
}

fn sanitizer_settings_for(tcx: TyCtxt<'_>, did: LocalDefId) -> SanitizerFnAttrs {
    // Backtrack to the crate root.
    let mut settings = match tcx.opt_local_parent(did) {
        // Check the parent (recursively).
        Some(parent) => tcx.sanitizer_settings_for(parent),
        // We reached the crate root without seeing an attribute, so
        // there is no sanitizers to exclude.
        None => SanitizerFnAttrs::default(),
    };

    // Check for a sanitize annotation directly on this def.
    if let Some((on_set, off_set, rtsan)) = find_attr!(tcx.get_all_attrs(did), AttributeKind::Sanitize {on_set, off_set, rtsan, ..} => (on_set, off_set, rtsan))
    {
        // the on set is the set of sanitizers explicitly enabled.
        // we mask those out since we want the set of disabled sanitizers here
        settings.disabled &= !*on_set;
        // the off set is the set of sanitizers explicitly disabled.
        // we or those in here.
        settings.disabled |= *off_set;
        // the on set and off set are distjoint since there's a third option: unset.
        // a node may not set the sanitizer setting in which case it inherits from parents.
        // the code above in this function does this backtracking

        // if rtsan was specified here override the parent
        if let Some(rtsan) = rtsan {
            settings.rtsan_setting = *rtsan;
        }
    }
    settings
}

/// 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 {
    tcx.trait_item_of(def_id).is_some_and(|id| {
        tcx.codegen_fn_attrs(id).flags.intersects(CodegenFnAttrFlags::TRACK_CALLER)
    })
}

/// If the provided DefId is a method in a trait impl, return the value of the `#[align]`
/// attribute on the method prototype (if any).
fn inherited_align<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> Option<Align> {
    tcx.codegen_fn_attrs(tcx.trait_item_of(def_id)?).alignment
}

/// 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.
//FIXME(jdonszelmann): put in the main loop. No need to have two..... :/ Let's do that when we make autodiff parsed.
pub fn autodiff_attrs(tcx: TyCtxt<'_>, id: DefId) -> Option<AutoDiffAttrs> {
    let attrs = tcx.get_attrs(id, sym::rustc_autodiff);

    let attrs = attrs.filter(|attr| attr.has_name(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, width_meta, input_activities @ .., ret_activity] = &list[..] else {
        span_bug!(attr.span(), "rustc_autodiff attribute must contain mode, width 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");
        }
    };

    let width: u32 = match width_meta {
        MetaItemInner::MetaItem(MetaItem { path: p1, .. }) => {
            let w = p1.segments.first().unwrap().ident;
            match w.as_str().parse() {
                Ok(val) => val,
                Err(_) => {
                    span_bug!(w.span, "rustc_autodiff width should fit u32");
                }
            }
        }
        MetaItemInner::Lit(lit) => {
            if let LitKind::Int(val, _) = lit.kind {
                match val.get().try_into() {
                    Ok(val) => val,
                    Err(_) => {
                        span_bug!(lit.span, "rustc_autodiff width should fit u32");
                    }
                }
            } else {
                span_bug!(lit.span, "rustc_autodiff width should be an integer");
            }
        }
    };

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

    // 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, width, ret_activity, input_activity: arg_activities })
}

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