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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 hir_def::{lang_item::LangItemTarget, type_ref::Mutability};
use test_utils::tested_by;
use crate::{autoderef, traits::Solution, Obligation, Substs, TraitRef, Ty, TypeCtor};
use super::{unify::TypeVarValue, InEnvironment, InferTy, 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 implicit coerce.
///
/// Note that it is only possible that one type are coerced to another.
/// Coercing both types to another least upper bound type is not possible in rustc,
/// which will simply result in "incompatible types" error.
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 {
tested_by!(coerce_merge_fail_fallback);
// For incompatible types, we use the latter one as result
// to be better recovery for `if` without `else`.
ty2.clone()
}
}
fn coerce_inner(&mut self, mut from_ty: Ty, to_ty: &Ty) -> bool {
match (&from_ty, to_ty) {
// Never type will make type variable to fallback to Never Type instead of Unknown.
(ty_app!(TypeCtor::Never), Ty::Infer(InferTy::TypeVar(tv))) => {
let var = self.table.new_maybe_never_type_var();
self.table.var_unification_table.union_value(*tv, TypeVarValue::Known(var));
return true;
}
(ty_app!(TypeCtor::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 (&mut from_ty, to_ty) {
// `*mut T`, `&mut T, `&T`` -> `*const T`
// `&mut T` -> `&T`
// `&mut T` -> `*mut T`
(ty_app!(c1@TypeCtor::RawPtr(_)), ty_app!(c2@TypeCtor::RawPtr(Mutability::Shared)))
| (ty_app!(c1@TypeCtor::Ref(_)), ty_app!(c2@TypeCtor::RawPtr(Mutability::Shared)))
| (ty_app!(c1@TypeCtor::Ref(_)), ty_app!(c2@TypeCtor::Ref(Mutability::Shared)))
| (ty_app!(c1@TypeCtor::Ref(Mutability::Mut)), ty_app!(c2@TypeCtor::RawPtr(_))) => {
*c1 = *c2;
}
// Illegal mutablity conversion
(
ty_app!(TypeCtor::RawPtr(Mutability::Shared)),
ty_app!(TypeCtor::RawPtr(Mutability::Mut)),
)
| (
ty_app!(TypeCtor::Ref(Mutability::Shared)),
ty_app!(TypeCtor::Ref(Mutability::Mut)),
) => return false,
// `{function_type}` -> `fn()`
(ty_app!(TypeCtor::FnDef(_)), ty_app!(TypeCtor::FnPtr { .. })) => {
match from_ty.callable_sig(self.db) {
None => return false,
Some(sig) => {
let num_args = sig.params_and_return.len() as u16 - 1;
from_ty =
Ty::apply(TypeCtor::FnPtr { num_args }, Substs(sig.params_and_return));
}
}
}
(ty_app!(TypeCtor::Closure { .. }, params), ty_app!(TypeCtor::FnPtr { .. })) => {
from_ty = params[0].clone();
}
_ => {}
}
if let Some(ret) = self.try_coerce_unsized(&from_ty, &to_ty) {
return ret;
}
// Auto Deref if cannot coerce
match (&from_ty, to_ty) {
// FIXME: DerefMut
(ty_app!(TypeCtor::Ref(_), st1), ty_app!(TypeCtor::Ref(_), st2)) => {
self.unify_autoderef_behind_ref(&st1[0], &st2[0])
}
// 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 = Substs::build_for_generics(&generic_params)
.push(from_ty.clone())
.push(to_ty.clone())
.build();
let trait_ref = TraitRef { trait_: coerce_unsized_trait, substs };
let goal = InEnvironment::new(self.trait_env.clone(), Obligation::Trait(trait_ref));
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 {
value: canonicalized.value.clone(),
environment: self.trait_env.clone(),
},
) {
let derefed_ty = canonicalized.decanonicalize_ty(derefed_ty.value);
match (&*self.resolve_ty_shallow(&derefed_ty), &*to_ty) {
// Stop when constructor matches.
(ty_app!(from_ctor, st1), ty_app!(to_ctor, st2)) if from_ctor == to_ctor => {
// It will not recurse to `coerce`.
return self.table.unify_substs(st1, st2, 0);
}
_ => {
if self.table.unify_inner_trivial(&derefed_ty, &to_ty, 0) {
return true;
}
}
}
}
false
}
}
|
