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//! Coercion logic. Coercions are certain type conversions that can implicitly
//! happen in certain places, e.g. weakening `&mut` to `&` or deref coercions
//! like going from `&Vec<T>` to `&[T]`.
//!
//! See: https://doc.rust-lang.org/nomicon/coercions.html
use chalk_ir::{cast::Cast, Mutability, TyVariableKind};
use hir_def::lang_item::LangItemTarget;
use crate::{
autoderef, to_chalk_trait_id, traits::Solution, Interner, Substitution, TraitRef, Ty, TyKind,
};
use super::{InEnvironment, InferenceContext};
impl<'a> InferenceContext<'a> {
/// Unify two types, but may coerce the first one to the second one
/// using "implicit coercion rules" if needed.
pub(super) fn coerce(&mut self, from_ty: &Ty, to_ty: &Ty) -> bool {
let from_ty = self.resolve_ty_shallow(from_ty).into_owned();
let to_ty = self.resolve_ty_shallow(to_ty);
self.coerce_inner(from_ty, &to_ty)
}
/// Merge two types from different branches, with possible coercion.
///
/// Mostly this means trying to coerce one to the other, but
/// - if we have two function types for different functions, we need to
/// coerce both to function pointers;
/// - if we were concerned with lifetime subtyping, we'd need to look for a
/// least upper bound.
pub(super) fn coerce_merge_branch(&mut self, ty1: &Ty, ty2: &Ty) -> Ty {
if self.coerce(ty1, ty2) {
ty2.clone()
} else if self.coerce(ty2, ty1) {
ty1.clone()
} else {
if let (TyKind::FnDef(..), TyKind::FnDef(..)) =
(ty1.kind(&Interner), ty2.kind(&Interner))
{
cov_mark::hit!(coerce_fn_reification);
// Special case: two function types. Try to coerce both to
// pointers to have a chance at getting a match. See
// https://github.com/rust-lang/rust/blob/7b805396bf46dce972692a6846ce2ad8481c5f85/src/librustc_typeck/check/coercion.rs#L877-L916
let sig1 = ty1.callable_sig(self.db).expect("FnDef without callable sig");
let sig2 = ty2.callable_sig(self.db).expect("FnDef without callable sig");
let ptr_ty1 = Ty::fn_ptr(sig1);
let ptr_ty2 = Ty::fn_ptr(sig2);
self.coerce_merge_branch(&ptr_ty1, &ptr_ty2)
} else {
cov_mark::hit!(coerce_merge_fail_fallback);
ty1.clone()
}
}
}
fn coerce_inner(&mut self, mut from_ty: Ty, to_ty: &Ty) -> bool {
match (from_ty.kind(&Interner), to_ty.kind(&Interner)) {
// Never type will make type variable to fallback to Never Type instead of Unknown.
(TyKind::Never, TyKind::InferenceVar(tv, TyVariableKind::General)) => {
self.table.type_variable_table.set_diverging(*tv, true);
return true;
}
(TyKind::Never, _) => return true,
// Trivial cases, this should go after `never` check to
// avoid infer result type to be never
_ => {
if self.table.unify_inner_trivial(&from_ty, &to_ty, 0) {
return true;
}
}
}
// Pointer weakening and function to pointer
match (from_ty.interned_mut(), to_ty.kind(&Interner)) {
// `*mut T` -> `*const T`
// `&mut T` -> `&T`
(TyKind::Raw(m1, ..), TyKind::Raw(m2 @ Mutability::Not, ..))
| (TyKind::Ref(m1, ..), TyKind::Ref(m2 @ Mutability::Not, ..)) => {
*m1 = *m2;
}
// `&T` -> `*const T`
// `&mut T` -> `*mut T`/`*const T`
(TyKind::Ref(.., substs), &TyKind::Raw(m2 @ Mutability::Not, ..))
| (TyKind::Ref(Mutability::Mut, substs), &TyKind::Raw(m2, ..)) => {
from_ty = TyKind::Raw(m2, substs.clone()).intern(&Interner);
}
// Illegal mutability conversion
(TyKind::Raw(Mutability::Not, ..), TyKind::Raw(Mutability::Mut, ..))
| (TyKind::Ref(Mutability::Not, ..), TyKind::Ref(Mutability::Mut, ..)) => return false,
// `{function_type}` -> `fn()`
(TyKind::FnDef(..), TyKind::Function { .. }) => match from_ty.callable_sig(self.db) {
None => return false,
Some(sig) => {
from_ty = Ty::fn_ptr(sig);
}
},
(TyKind::Closure(.., substs), TyKind::Function { .. }) => {
from_ty = substs.at(&Interner, 0).assert_ty_ref(&Interner).clone();
}
_ => {}
}
if let Some(ret) = self.try_coerce_unsized(&from_ty, &to_ty) {
return ret;
}
// Auto Deref if cannot coerce
match (from_ty.kind(&Interner), to_ty.kind(&Interner)) {
// FIXME: DerefMut
(TyKind::Ref(_, st1), TyKind::Ref(_, st2)) => self.unify_autoderef_behind_ref(st1, st2),
// Otherwise, normal unify
_ => self.unify(&from_ty, to_ty),
}
}
/// Coerce a type using `from_ty: CoerceUnsized<ty_ty>`
///
/// See: https://doc.rust-lang.org/nightly/std/marker/trait.CoerceUnsized.html
fn try_coerce_unsized(&mut self, from_ty: &Ty, to_ty: &Ty) -> Option<bool> {
let krate = self.resolver.krate().unwrap();
let coerce_unsized_trait = match self.db.lang_item(krate, "coerce_unsized".into()) {
Some(LangItemTarget::TraitId(trait_)) => trait_,
_ => return None,
};
let generic_params = crate::utils::generics(self.db.upcast(), coerce_unsized_trait.into());
if generic_params.len() != 2 {
// The CoerceUnsized trait should have two generic params: Self and T.
return None;
}
let substs = Substitution::build_for_generics(&generic_params)
.push(from_ty.clone())
.push(to_ty.clone())
.build();
let trait_ref =
TraitRef { trait_id: to_chalk_trait_id(coerce_unsized_trait), substitution: substs };
let goal = InEnvironment::new(self.trait_env.env.clone(), trait_ref.cast(&Interner));
let canonicalizer = self.canonicalizer();
let canonicalized = canonicalizer.canonicalize_obligation(goal);
let solution = self.db.trait_solve(krate, canonicalized.value.clone())?;
match solution {
Solution::Unique(v) => {
canonicalized.apply_solution(self, v.0);
}
_ => return None,
};
Some(true)
}
/// Unify `from_ty` to `to_ty` with optional auto Deref
///
/// Note that the parameters are already stripped the outer reference.
fn unify_autoderef_behind_ref(&mut self, from_ty: &Ty, to_ty: &Ty) -> bool {
let canonicalized = self.canonicalizer().canonicalize_ty(from_ty.clone());
let to_ty = self.resolve_ty_shallow(&to_ty);
// FIXME: Auto DerefMut
for derefed_ty in autoderef::autoderef(
self.db,
self.resolver.krate(),
InEnvironment {
goal: canonicalized.value.clone(),
environment: self.trait_env.env.clone(),
},
) {
let derefed_ty = canonicalized.decanonicalize_ty(derefed_ty.value);
let from_ty = self.resolve_ty_shallow(&derefed_ty);
// Stop when constructor matches.
if from_ty.equals_ctor(&to_ty) {
// It will not recurse to `coerce`.
return self.table.unify(&from_ty, &to_ty);
} else if self.table.unify_inner_trivial(&derefed_ty, &to_ty, 0) {
return true;
}
}
false
}
}
|
