frontend 0.4.0

/*!

# typeck: check phase

Within the check phase of type check, we check each item one at a time
(bodies of function expressions are checked as part of the containing
function). Inference is used to supply types wherever they are unknown.

By far the most complex case is checking the body of a function. This
can be broken down into several distinct phases:

- gather: creates type variables to represent the type of each local
  variable and pattern binding.

- main: the main pass does the lion's share of the work: it
  determines the types of all expressions, resolves
  methods, checks for most invalid conditions, and so forth. In
  some cases, where a type is unknown, it may create a type or region
  variable and use that as the type of an expression.

  In the process of checking, various constraints will be placed on
  these type variables through the subtyping relationships requested
  through the `demand` module. The `infer` module is in charge
  of resolving those constraints.

- regionck: after main is complete, the regionck pass goes over all
  types looking for regions and making sure that they did not escape
  into places where they are not in scope. This may also influence the
  final assignments of the various region variables if there is some
  flexibility.

- writeback: writes the final types within a function body, replacing
  type variables with their final inferred types. These final types
  are written into the `tcx.node_types` table, which should *never* contain
  any reference to a type variable.

## Intermediate types

While type checking a function, the intermediate types for the
expressions, blocks, and so forth contained within the function are
stored in `fcx.node_types` and `fcx.node_args`. These types
may contain unresolved type variables. After type checking is
complete, the functions in the writeback module are used to take the
types from this table, resolve them, and then write them into their
permanent home in the type context `tcx`.

This means that during inferencing you should use `fcx.write_ty()`
and `fcx.expr_ty()` / `fcx.node_ty()` to write/obtain the types of
nodes within the function.

The types of top-level items, which never contain unbound type
variables, are stored directly into the `tcx` typeck_results.

N.B., a type variable is not the same thing as a type parameter. A
type variable is an instance of a type parameter. That is,
given a generic function `fn foo<T>(t: T)`, while checking the
function `foo`, the type `ty_param(0)` refers to the type `T`, which
is treated in abstract. However, when `foo()` is called, `T` will be
instantiated with a fresh type variable `N`. This variable will
eventually be resolved to some concrete type (which might itself be
a type parameter).

*/

// `#![no_std]`: these arrive with the standard prelude and name no path, so a `std::`
// search cannot see them - and a `#[derive]` can use them without the name appearing
// in this file at all, which is why they are not trimmed by inspection.
use alloc::borrow::ToOwned;
use alloc::boxed::Box;
use alloc::format;
use alloc::string::{String, ToString};
use alloc::vec;
use alloc::vec::Vec;

pub mod always_applicable;
mod check;
mod compare_eii;
mod compare_impl_item;
mod entry;
pub mod intrinsic;
mod region;
pub mod wfcheck;

use alloc::borrow::Cow;
use core::num::NonZero;

pub use check::check_abi;
use crate::rustc_abi::VariantIdx;
use crate::rustc_data_structures::fx::{FxHashSet, FxIndexMap};
use crate::rustc_errors::{ErrorGuaranteed, pluralize, struct_span_code_err};
use crate::rustc_hir::attrs::lang_items::LangItem;
use crate::rustc_hir::def_id::{DefId, LocalDefId};
use crate::rustc_hir::intravisit::Visitor;
use crate::rustc_index::bit_set::DenseBitSet;
use crate::rustc_infer::infer::{self, TyCtxtInferExt as _};
use crate::rustc_infer::traits::{ObligationCause, TraitErrors};
use crate::rustc_middle::middle::stability::EvalResult;
use crate::rustc_middle::query::Providers;
use crate::rustc_middle::ty::error::{ExpectedFound, TypeError};
use crate::rustc_middle::ty::print::with_types_for_signature;
use crate::rustc_middle::ty::{
    self, GenericArgs, GenericArgsRef, OutlivesClause, Region, RegionExt, Ty, TyCtxt, TypingMode,
};
use crate::rustc_middle::{bug, span_bug};
use crate::rustc_session::diagnostics::feature_err;
use crate::rustc_span::def_id::CRATE_DEF_ID;
use crate::rustc_span::{BytePos, DUMMY_SP, Ident, Span, Symbol, kw};
use crate::rustc_trait_selection::error_reporting::InferCtxtErrorExt;
use crate::rustc_trait_selection::error_reporting::infer::ObligationCauseExt as _;
use crate::rustc_trait_selection::error_reporting::traits::suggestions::ReturnsVisitor;
use crate::rustc_trait_selection::traits::ObligationCtxt;
use tracing::debug;

use self::compare_impl_item::collect_return_position_impl_trait_in_trait_tys;
use self::region::region_scope_tree;
use crate::rustc_hir_analysis::diagnostics::{
    MissingTraitItemLabel, MissingTraitItemSuggestion, MissingTraitItemSuggestionNone,
    MissingTraitItemSuggestionUnstable,
};
use crate::rustc_hir_analysis::{check_c_variadic_abi, diagnostics};

/// Adds query implementations to the [Providers] vtable, see [`crate::rustc_middle::query`]
pub(super) fn provide(providers: &mut Providers) {
    *providers = Providers {
        adt_destructor,
        adt_async_destructor,
        region_scope_tree,
        collect_return_position_impl_trait_in_trait_tys,
        compare_impl_item: compare_impl_item::compare_impl_item,
        check_coroutine_obligations: check::check_coroutine_obligations,
        check_potentially_region_dependent_goals: check::check_potentially_region_dependent_goals,
        check_type_wf: wfcheck::check_type_wf,
        check_well_formed: wfcheck::check_well_formed,
        ..*providers
    };
}

fn adt_destructor(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ty::Destructor> {
    let dtor = tcx.calculate_dtor(def_id, always_applicable::check_drop_impl);
    if dtor.is_none() && tcx.features().async_drop() {
        if let Some(async_dtor) = adt_async_destructor(tcx, def_id) {
            // When type has AsyncDrop impl, but doesn't have Drop impl, generate error
            let span = tcx.def_span(async_dtor.impl_did);
            tcx.dcx().emit_err(diagnostics::AsyncDropWithoutSyncDrop { span });
        }
    }
    dtor
}

fn adt_async_destructor(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ty::AsyncDestructor> {
    let result = tcx.calculate_async_dtor(def_id, always_applicable::check_drop_impl);
    // Async drop in libstd/libcore would become insta-stable — catch that mistake.
    if result.is_some() && tcx.features().staged_api() {
        span_bug!(tcx.def_span(def_id), "don't use async drop in libstd, it becomes insta-stable");
    }
    result
}

/// Given a `DefId` for an opaque type in return position, find its parent item's return
/// expressions.
fn get_owner_return_paths(
    tcx: TyCtxt<'_>,
    def_id: LocalDefId,
) -> Option<(LocalDefId, ReturnsVisitor<'_>)> {
    let hir_id = tcx.local_def_id_to_hir_id(def_id);
    let parent_id = tcx.hir_get_parent_item(hir_id).def_id;
    tcx.hir_node_by_def_id(parent_id).body_id().map(|body_id| {
        let body = tcx.hir_body(body_id);
        let mut visitor = ReturnsVisitor::default();
        visitor.visit_body(body);
        (parent_id, visitor)
    })
}

pub(super) fn maybe_check_static_with_link_section(tcx: TyCtxt<'_>, id: LocalDefId) {
    // Only restricted on wasm target for now
    if !tcx.sess.target.is_like_wasm {
        return;
    }

    // If `#[link_section]` is missing, then nothing to verify
    let Some(link_section) = tcx.codegen_fn_attrs(id).link_section else {
        return;
    };

    // For the wasm32 target statics with `#[link_section]` other than `.init_array`
    // are placed into custom sections of the final output file, but this isn't like
    // custom sections of other executable formats. Namely we can only embed a list
    // of bytes, nothing with provenance (pointers to anything else). If any
    // provenance show up, reject it here.
    // `#[link_section]` may contain arbitrary, or even undefined bytes, but it is
    // the consumer's responsibility to ensure all bytes that have been read
    // have defined values.
    //
    // The `.init_array` section is left to go through the normal custom section code path.
    // When dealing with `.init_array` wasm-ld currently has several limitations. This manifests
    // in workarounds in user-code.
    //
    //   * The linker fails to merge multiple items in a crate into the .init_array section.
    //     To work around this, a single array can be used placing multiple items in the array.
    //     #[link_section = ".init_array"]
    //     static FOO: [unsafe extern "C" fn(); 2] = [ctor, ctor];
    //   * Even symbols marked used get gc'd from dependant crates unless at least one symbol
    //     in the crate is marked with an `#[export_name]`
    //
    //  Once `.init_array` support in wasm-ld is complete, the user code workarounds should
    //  continue to work, but would no longer be necessary.

    if let Ok(alloc) = tcx.eval_static_initializer(id.to_def_id())
        && !alloc.inner().provenance().ptrs().is_empty()
        && !link_section.as_str().starts_with(".init_array")
    {
        let msg = "statics with a custom `#[link_section]` must be a \
                        simple list of bytes on the wasm target with no \
                        extra levels of indirection such as references";
        tcx.dcx().span_err(tcx.def_span(id), msg);
    }
}

fn impl_suggestion_span(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) -> Span {
    let full_impl_span = tcx.hir_span_with_body(tcx.local_def_id_to_hir_id(impl_def_id));
    if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(full_impl_span)
        && snippet.ends_with("}")
    {
        // `Span` before impl block closing brace.
        let hi = full_impl_span.hi() - BytePos(1);
        // Point at the place right before the closing brace of the relevant `impl` to suggest
        // adding the associated item at the end of its body.
        full_impl_span.with_lo(hi).with_hi(hi)
    } else {
        full_impl_span.shrink_to_hi()
    }
}

fn missing_items_suggestions(
    tcx: TyCtxt<'_>,
    impl_def_id: LocalDefId,
    missing_items: &[ty::AssocItem],
) -> (
    String,
    Vec<MissingTraitItemSuggestion>,
    Vec<MissingTraitItemSuggestionNone>,
    Vec<MissingTraitItemSuggestionUnstable>,
    Vec<MissingTraitItemLabel>,
) {
    let missing_items =
        missing_items.iter().filter(|trait_item| !trait_item.is_impl_trait_in_trait());

    let missing_items_msg = missing_items
        .clone()
        .map(|trait_item| trait_item.name().to_string())
        .collect::<Vec<_>>()
        .join("`, `");

    let sugg_sp = impl_suggestion_span(tcx, impl_def_id);

    // Obtain the level of indentation ending in `sugg_sp`.
    let padding = tcx.sess.source_map().indentation_before(sugg_sp).unwrap_or_else(String::new);
    let (
        mut missing_trait_item,
        mut missing_trait_item_none,
        mut missing_trait_item_unstable,
        mut missing_trait_item_label,
    ) = (Vec::new(), Vec::new(), Vec::new(), Vec::new());

    for &trait_item in missing_items {
        let snippet = with_types_for_signature!(suggestion_signature(
            tcx,
            trait_item,
            tcx.impl_trait_ref(impl_def_id).instantiate_identity().skip_norm_wip(),
        ));
        let code = format!("{padding}{snippet}\n{padding}");
        if let Some(span) = tcx.hir_span_if_local(trait_item.def_id) {
            missing_trait_item_label
                .push(diagnostics::MissingTraitItemLabel { span, item: trait_item.name() });
            missing_trait_item.push(diagnostics::MissingTraitItemSuggestion {
                span: sugg_sp,
                code,
                snippet,
            });
        } else {
            if let EvalResult::Deny { feature, .. } =
                tcx.eval_stability(trait_item.def_id, None, sugg_sp, None)
            {
                missing_trait_item_unstable.push(diagnostics::MissingTraitItemSuggestionUnstable {
                    span: sugg_sp,
                    code,
                    snippet,
                    feature,
                });
            } else {
                missing_trait_item_none.push(diagnostics::MissingTraitItemSuggestionNone {
                    span: sugg_sp,
                    code,
                    snippet,
                });
            }
        }
    }

    (
        missing_items_msg,
        missing_trait_item,
        missing_trait_item_none,
        missing_trait_item_unstable,
        missing_trait_item_label,
    )
}

fn missing_items_err(tcx: TyCtxt<'_>, impl_def_id: LocalDefId, missing_items: &[ty::AssocItem]) {
    let (
        missing_items_msg,
        missing_trait_item,
        missing_trait_item_none,
        missing_trait_item_unstable,
        missing_trait_item_label,
    ) = missing_items_suggestions(tcx, impl_def_id, missing_items);

    tcx.dcx().emit_err(diagnostics::MissingTraitItem {
        span: tcx.span_of_impl(impl_def_id.to_def_id()).unwrap(),
        missing_items_msg,
        missing_trait_item_label,
        missing_trait_item,
        missing_trait_item_none,
        missing_trait_item_unstable,
    });
}

fn missing_items_must_implement_one_of_err(
    tcx: TyCtxt<'_>,
    impl_def_id: LocalDefId,
    missing_items: impl Iterator<Item = Symbol>,
    annotation_span: Option<Span>,
) -> ErrorGuaranteed {
    // Look up the associated items so we can use them to emit better errors.
    let trait_def_id = tcx.impl_trait_id(impl_def_id);
    let assoc_items = tcx.associated_items(trait_def_id);
    let missing_items = missing_items
        .flat_map(|s| assoc_items.filter_by_name_unhygienic_and_kind(s, ty::AssocTag::Fn))
        .cloned()
        .collect::<Vec<_>>();

    let (
        missing_items_msg,
        missing_trait_item,
        missing_trait_item_none,
        missing_trait_item_unstable,
        missing_trait_item_label,
    ) = missing_items_suggestions(tcx, impl_def_id, &missing_items);

    tcx.dcx().emit_err(diagnostics::MissingOneOfTraitItem {
        span: tcx.def_span(impl_def_id),
        note: annotation_span,
        missing_items_msg,
        missing_trait_item_label,
        missing_trait_item,
        missing_trait_item_unstable,
        missing_trait_item_none,
    })
}

fn default_body_is_unstable(
    tcx: TyCtxt<'_>,
    impl_span: Span,
    item_did: DefId,
    feature: Symbol,
    reason: Option<Symbol>,
    issue: Option<NonZero<u32>>,
) {
    let missing_item_name = tcx.item_ident(item_did);
    let (mut some_note, mut none_note, mut reason_str) = (false, false, String::new());
    match reason {
        Some(r) => {
            some_note = true;
            reason_str = r.to_string();
        }
        None => none_note = true,
    };

    let mut err = tcx.dcx().create_err(diagnostics::MissingTraitItemUnstable {
        span: impl_span,
        some_note,
        none_note,
        missing_item_name,
        feature,
        reason: reason_str,
    });

    let inject_span = item_did.is_local().then(|| tcx.crate_level_attribute_injection_span());
    crate::rustc_session::diagnostics::add_feature_diagnostics_for_issue(
        &mut err,
        &tcx.sess,
        feature,
        crate::rustc_feature::GateIssue::Library(issue),
        false,
        inject_span,
    );

    err.emit();
}

/// Re-sugar `ty::GenericClauses` in a way suitable to be used in structured suggestions.
fn bounds_from_generic_clauses<'tcx>(
    tcx: TyCtxt<'tcx>,
    clauses: impl IntoIterator<Item = (ty::Clause<'tcx>, Span)>,
    assoc: ty::AssocItem,
) -> (String, String) {
    let mut types: FxIndexMap<Ty<'tcx>, Vec<DefId>> = FxIndexMap::default();
    let mut regions: FxIndexMap<Region<'tcx>, Vec<Region<'tcx>>> = FxIndexMap::default();
    let mut projections = vec![];
    for (clause, _) in clauses {
        debug!("clause {:?}", clause);
        let bound_clause = clause.kind();
        match bound_clause.skip_binder() {
            ty::ClauseKind::Trait(trait_predicate) => {
                let entry = types.entry(trait_predicate.self_ty()).or_default();
                let def_id = trait_predicate.def_id();
                if !tcx.is_default_trait(def_id) && !tcx.is_lang_item(def_id, LangItem::Sized) {
                    // Do not add that restriction to the list if it is a positive requirement.
                    entry.push(trait_predicate.def_id());
                }
            }
            ty::ClauseKind::Projection(projection_pred) => {
                projections.push(bound_clause.rebind(projection_pred));
            }
            ty::ClauseKind::RegionOutlives(OutlivesClause(a, b)) => {
                regions.entry(a).or_default().push(b);
            }
            _ => {}
        }
    }

    let mut where_clauses = vec![];
    let generics = tcx.generics_of(assoc.def_id);
    let params = generics
        .own_params
        .iter()
        .filter(|p| !p.kind.is_synthetic())
        .map(|p| match tcx.mk_param_from_def(p).kind() {
            ty::GenericArgKind::Type(ty) => {
                let bounds =
                    types.get(&ty).map(Cow::Borrowed).unwrap_or_else(|| Cow::Owned(Vec::new()));
                let mut bounds_str = vec![];
                for bound in bounds.iter().copied() {
                    let mut projections_str = vec![];
                    for projection in &projections {
                        let p = projection.skip_binder();
                        if bound == p.projection_term.trait_def_id(tcx)
                            && p.projection_term.self_ty() == ty
                        {
                            let name = tcx.item_name(p.projection_term.expect_projection_def_id());
                            projections_str.push(format!("{} = {}", name, p.term));
                        }
                    }
                    let bound_def_path = if tcx.is_lang_item(bound, LangItem::MetaSized) {
                        String::from("?Sized")
                    } else {
                        tcx.def_path_str(bound)
                    };
                    if projections_str.is_empty() {
                        where_clauses.push(format!("{}: {}", ty, bound_def_path));
                    } else {
                        bounds_str.push(format!(
                            "{}<{}>",
                            bound_def_path,
                            projections_str.join(", ")
                        ));
                    }
                }
                if bounds_str.is_empty() {
                    ty.to_string()
                } else {
                    format!("{}: {}", ty, bounds_str.join(" + "))
                }
            }
            ty::GenericArgKind::Const(ct) => {
                format!("const {ct}: {}", tcx.type_of(p.def_id).skip_binder())
            }
            ty::GenericArgKind::Lifetime(region) => {
                if let Some(v) = regions.get(&region)
                    && !v.is_empty()
                {
                    format!(
                        "{region}: {}",
                        v.into_iter().map(Region::to_string).collect::<Vec<_>>().join(" + ")
                    )
                } else {
                    region.to_string()
                }
            }
        })
        .collect::<Vec<_>>();
    for (ty, bounds) in types.into_iter() {
        if !matches!(ty.kind(), ty::Param(_)) {
            // Avoid suggesting the following:
            // fn foo<T, <T as Trait>::Bar>(_: T) where T: Trait, <T as Trait>::Bar: Other {}
            where_clauses.extend(
                bounds.into_iter().map(|bound| format!("{}: {}", ty, tcx.def_path_str(bound))),
            );
        }
    }

    let generics =
        if params.is_empty() { "".to_string() } else { format!("<{}>", params.join(", ")) };

    let where_clauses = if where_clauses.is_empty() {
        "".to_string()
    } else {
        format!(" where {}", where_clauses.join(", "))
    };

    (generics, where_clauses)
}

/// Return placeholder code for the given function.
fn fn_sig_suggestion<'tcx>(
    tcx: TyCtxt<'tcx>,
    sig: ty::FnSig<'tcx>,
    ident: Ident,
    clauses: impl IntoIterator<Item = (ty::Clause<'tcx>, Span)>,
    assoc: ty::AssocItem,
) -> String {
    let splatted_arg_index = sig.splatted().map(usize::from);
    let args = sig
        .inputs()
        .iter()
        .enumerate()
        .map(|(i, ty)| {
            let splat = if splatted_arg_index == Some(i) { "#[rustc_splat] " } else { "" };
            let arg_ty = match ty.kind() {
                ty::Param(_) if assoc.is_method() && i == 0 => "self".to_string(),
                ty::Ref(reg, ref_ty, mutability) if i == 0 => {
                    let reg = format!("{reg} ");
                    let reg = match &reg[..] {
                        "'_ " | " " => "",
                        reg => reg,
                    };
                    if assoc.is_method() {
                        match ref_ty.kind() {
                            ty::Param(param) if param.name == kw::SelfUpper => {
                                format!("&{}{}self", reg, mutability.prefix_str())
                            }

                            _ => format!("self: {ty}"),
                        }
                    } else {
                        format!("_: {ty}")
                    }
                }
                _ => {
                    if assoc.is_method() && i == 0 {
                        format!("self: {ty}")
                    } else {
                        format!("_: {ty}")
                    }
                }
            };
            format!("{splat}{arg_ty}")
        })
        .chain(if sig.c_variadic() { Some("...".to_string()) } else { None })
        .collect::<Vec<String>>()
        .join(", ");
    let mut output = sig.output();

    let asyncness = if tcx.asyncness(assoc.def_id).is_async() {
        output = tcx.get_impl_future_output_ty(output).unwrap_or_else(|| {
            span_bug!(
                ident.span,
                "expected async fn to have `impl Future` output, but it returns {output}"
            )
        });
        "async "
    } else {
        ""
    };

    let output = if !output.is_unit() { format!(" -> {output}") } else { String::new() };

    let safety = sig.safety().prefix_str();
    let (generics, where_clauses) = bounds_from_generic_clauses(tcx, clauses, assoc);

    // FIXME: this is not entirely correct, as the lifetimes from borrowed params will
    // not be present in the `fn` definition, nor will we account for renamed
    // lifetimes between the `impl` and the `trait`, but this should be good enough to
    // fill in a significant portion of the missing code, and other subsequent
    // suggestions can help the user fix the code.
    // ignore-tidy-todo
    format!("{safety}{asyncness}fn {ident}{generics}({args}){output}{where_clauses} {{ todo!() }}")
}

/// Return placeholder code for the given associated item.
/// Similar to `ty::AssocItem::suggestion`, but appropriate for use as the code snippet of a
/// structured suggestion.
fn suggestion_signature<'tcx>(
    tcx: TyCtxt<'tcx>,
    assoc: ty::AssocItem,
    impl_trait_ref: ty::TraitRef<'tcx>,
) -> String {
    let args = ty::GenericArgs::identity_for_item(tcx, assoc.def_id).rebase_onto(
        tcx,
        assoc.container_id(tcx),
        impl_trait_ref.with_replaced_self_ty(tcx, tcx.types.self_param).args,
    );

    match assoc.kind {
        ty::AssocKind::Fn { .. } => fn_sig_suggestion(
            tcx,
            tcx.liberate_late_bound_regions(
                assoc.def_id,
                tcx.fn_sig(assoc.def_id).instantiate(tcx, args).skip_norm_wip(),
            ),
            assoc.ident(tcx),
            tcx.clauses_of(assoc.def_id)
                .instantiate_own(tcx, args)
                .map(|(c, s)| (c.skip_norm_wip(), s)),
            assoc,
        ),
        ty::AssocKind::Type { .. } => {
            let (generics, where_clauses) = bounds_from_generic_clauses(
                tcx,
                tcx.clauses_of(assoc.def_id)
                    .instantiate_own(tcx, args)
                    .map(|(c, s)| (c.skip_norm_wip(), s)),
                assoc,
            );
            format!("type {}{generics} = /* Type */{where_clauses};", assoc.name())
        }
        ty::AssocKind::Const { name, .. } => {
            let ty = tcx.type_of(assoc.def_id).instantiate_identity().skip_norm_wip();
            let val = tcx
                .infer_ctxt()
                .build(TypingMode::non_body_analysis())
                .err_ctxt()
                .ty_kind_suggestion(tcx.param_env(assoc.def_id), ty)
                .unwrap_or_else(|| "value".to_string());
            format!("const {}: {} = {};", name, ty, val)
        }
    }
}

/// Emit an error when encountering two or more variants in a transparent enum.
fn bad_variant_count<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>, sp: Span, did: DefId) {
    let variant_spans: Vec<_> = adt
        .variants()
        .iter()
        .map(|variant| tcx.hir_span_if_local(variant.def_id).unwrap())
        .collect();
    let (mut spans, mut many) = (Vec::new(), None);
    if let [start @ .., end] = &*variant_spans {
        spans = start.to_vec();
        many = Some(*end);
    }
    tcx.dcx().emit_err(diagnostics::TransparentEnumVariant {
        span: sp,
        spans,
        many,
        number: adt.variants().len(),
        path: tcx.def_path_str(did),
    });
}

// FIXME: Consider moving this method to a more fitting place.
pub fn potentially_plural_count(count: usize, word: &str) -> String {
    format!("{} {}{}", count, word, pluralize!(count))
}

pub fn check_function_signature<'tcx>(
    tcx: TyCtxt<'tcx>,
    mut cause: ObligationCause<'tcx>,
    fn_id: DefId,
    expected_sig: ty::PolyFnSig<'tcx>,
) -> Result<(), ErrorGuaranteed> {
    fn extract_span_for_error_reporting<'tcx>(
        tcx: TyCtxt<'tcx>,
        err: TypeError<'_>,
        cause: &ObligationCause<'tcx>,
        fn_id: LocalDefId,
    ) -> crate::rustc_span::Span {
        let mut args = {
            let node = tcx.expect_hir_owner_node(fn_id);
            let decl = node.fn_decl().unwrap_or_else(|| bug!("expected fn decl, found {:?}", node));
            decl.inputs.iter().map(|t| t.span).chain(core::iter::once(decl.output.span()))
        };

        match err {
            TypeError::ArgumentMutability(i)
            | TypeError::ArgumentSorts(ExpectedFound { .. }, i) => args.nth(i).unwrap(),
            _ => cause.span,
        }
    }

    let local_id = fn_id.as_local().unwrap_or(CRATE_DEF_ID);

    let param_env = ty::ParamEnv::empty();

    let infcx = &tcx.infer_ctxt().build(TypingMode::non_body_analysis());
    let ocx = ObligationCtxt::new_with_diagnostics(infcx);

    let actual_sig = tcx.fn_sig(fn_id).instantiate_identity();

    let norm_cause = ObligationCause::misc(cause.span, local_id);
    let actual_sig = ocx.normalize(&norm_cause, param_env, actual_sig);

    match ocx.eq(&cause, param_env, expected_sig, actual_sig) {
        Ok(()) => {
            let errors = ocx.evaluate_obligations_error_on_ambiguity();
            if let TraitErrors::HasErrors(errors) = errors {
                return Err(infcx.err_ctxt().report_fulfillment_errors(errors));
            }
        }
        Err(err) => {
            let err_ctxt = infcx.err_ctxt();
            if fn_id.is_local() {
                cause.span = extract_span_for_error_reporting(tcx, err, &cause, local_id);
            }
            let failure_code = cause.as_failure_code_diag(err, cause.span, vec![]);
            let mut diag = tcx.dcx().create_err(failure_code);
            err_ctxt.note_type_err(
                &mut diag,
                &cause,
                None,
                Some(param_env.and(infer::ValuePairs::PolySigs(ExpectedFound {
                    expected: expected_sig,
                    found: actual_sig,
                }))),
                err,
                false,
                None,
            );
            return Err(diag.emit());
        }
    }

    if let Err(e) = ocx.resolve_regions_and_report_errors(local_id, param_env, []) {
        return Err(e);
    }

    Ok(())
}