rust-analyzer/crates/hir_ty/src/infer/closure.rs

//! Inference of closure parameter types based on the closure's expected type.

use chalk_ir::{fold::Shift, AliasTy, FnSubst, WhereClause};
use hir_def::HasModule;
use smallvec::SmallVec;

use crate::{
    to_chalk_trait_id, utils, ChalkTraitId, DynTy, FnPointer, FnSig, Interner, Substitution, Ty,
    TyKind,
};

use super::{Expectation, InferenceContext};

impl InferenceContext<'_> {
    pub(super) fn deduce_closure_type_from_expectations(
        &mut self,
        closure_ty: &Ty,
        sig_ty: &Ty,
        expectation: &Expectation,
    ) {
        let expected_ty = match expectation.to_option(&mut self.table) {
            Some(ty) => ty,
            None => return,
        };

        // Deduction from where-clauses in scope, as well as fn-pointer coercion are handled here.
        self.coerce(closure_ty, &expected_ty);

        // Deduction based on the expected `dyn Fn` is done separately.
        if let TyKind::Dyn(dyn_ty) = expected_ty.kind(&Interner) {
            if let Some(sig) = self.deduce_sig_from_dyn_ty(dyn_ty) {
                let expected_sig_ty = TyKind::Function(sig).intern(&Interner);

                self.unify(sig_ty, &expected_sig_ty);
            }
        }
    }

    fn deduce_sig_from_dyn_ty(&self, dyn_ty: &DynTy) -> Option<FnPointer> {
        // Search for predicates like `$self: FnX<Args>` and `<$self as FnOnce<...>>::Output == Ret`

        let fn_traits: SmallVec<[ChalkTraitId; 3]> =
            utils::fn_traits(self.db.upcast(), self.owner.module(self.db.upcast()).krate())
                .map(|tid| to_chalk_trait_id(tid))
                .collect();

        for bound in dyn_ty.bounds.map_ref(|b| b.iter(&Interner)) {
            let bound = bound.map(|b| b.clone()).fuse_binders(&Interner);
            match bound.skip_binders() {
                WhereClause::AliasEq(eq) => match &eq.alias {
                    AliasTy::Projection(projection) => {
                        let assoc_data = self.db.associated_ty_data(projection.associated_ty_id);
                        if !fn_traits.contains(&assoc_data.trait_id) {
                            return None;
                        }

                        // Skip `Self`, get the type argument.
                        let arg = projection.substitution.as_slice(&Interner).get(1)?;
                        match arg.ty(&Interner)?.kind(&Interner) {
                            TyKind::Tuple(_, subst) => {
                                let generic_args = subst.as_slice(&Interner);
                                let mut sig_tys = Vec::new();
                                for arg in generic_args {
                                    sig_tys.push(arg.ty(&Interner)?.clone());
                                }
                                sig_tys.push(eq.ty.clone());

                                cov_mark::hit!(dyn_fn_param_informs_call_site_closure_signature);
                                return Some(FnPointer {
                                    num_binders: 0,
                                    sig: FnSig {
                                        abi: (),
                                        safety: chalk_ir::Safety::Safe,
                                        variadic: false,
                                    },
                                    substitution: FnSubst(
                                        Substitution::from_iter(&Interner, sig_tys.clone())
                                            .shifted_in(&Interner),
                                    ),
                                });
                            }
                            _ => {}
                        }
                    }
                    AliasTy::Opaque(_) => {}
                },
                _ => {}
            }
        }

        None
    }
}