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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,
resolver::{HasResolver, Resolver},
AdtId,
};
use rustc_hash::FxHashMap;
use test_utils::tested_by;
use crate::{
db::HirDatabase,
ty::{autoderef, Substs, TraitRef, Ty, TypeCtor, TypeWalk},
Mutability,
};
use super::{InEnvironment, InferTy, InferenceContext, TypeVarValue};
impl<'a, D: HirDatabase> InferenceContext<'a, D> {
/// 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<'t>(&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()
}
}
pub(super) fn init_coerce_unsized_map(
db: &'a D,
resolver: &Resolver,
) -> FxHashMap<(TypeCtor, TypeCtor), usize> {
let krate = resolver.krate().unwrap();
let impls = match db.lang_item(krate.into(), "coerce_unsized".into()) {
Some(LangItemTarget::TraitId(trait_)) => {
db.impls_for_trait(krate.into(), trait_.into())
}
_ => return FxHashMap::default(),
};
impls
.iter()
.filter_map(|&impl_id| {
let impl_data = db.impl_data(impl_id);
let resolver = impl_id.resolver(db);
let target_ty = Ty::from_hir(db, &resolver, &impl_data.target_type);
// `CoerseUnsized` has one generic parameter for the target type.
let trait_ref = TraitRef::from_hir(
db,
&resolver,
impl_data.target_trait.as_ref()?,
Some(target_ty),
)?;
let cur_from_ty = trait_ref.substs.0.get(0)?;
let cur_to_ty = trait_ref.substs.0.get(1)?;
match (&cur_from_ty, cur_to_ty) {
(ty_app!(ctor1, st1), ty_app!(ctor2, st2)) => {
// FIXME: We return the first non-equal bound as the type parameter to coerce to unsized type.
// This works for smart-pointer-like coercion, which covers all impls from std.
st1.iter().zip(st2.iter()).enumerate().find_map(|(i, (ty1, ty2))| {
match (ty1, ty2) {
(Ty::Param { idx: p1, .. }, Ty::Param { idx: p2, .. })
if p1 != p2 =>
{
Some(((*ctor1, *ctor2), i))
}
_ => None,
}
})
}
_ => None,
}
})
.collect()
}
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.new_maybe_never_type_var();
self.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.unify_inner_trivial(&from_ty, &to_ty) {
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));
}
}
}
_ => {}
}
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 (ctor1, st1, ctor2, st2) = match (from_ty, to_ty) {
(ty_app!(ctor1, st1), ty_app!(ctor2, st2)) => (ctor1, st1, ctor2, st2),
_ => return None,
};
let coerce_generic_index = *self.coerce_unsized_map.get(&(*ctor1, *ctor2))?;
// Check `Unsize` first
match self.check_unsize_and_coerce(
st1.0.get(coerce_generic_index)?,
st2.0.get(coerce_generic_index)?,
0,
) {
Some(true) => {}
ret => return ret,
}
let ret = st1
.iter()
.zip(st2.iter())
.enumerate()
.filter(|&(idx, _)| idx != coerce_generic_index)
.all(|(_, (ty1, ty2))| self.unify(ty1, ty2));
Some(ret)
}
/// Check if `from_ty: Unsize<to_ty>`, and coerce to `to_ty` if it holds.
///
/// It should not be directly called. It is only used by `try_coerce_unsized`.
///
/// See: https://doc.rust-lang.org/nightly/std/marker/trait.Unsize.html
fn check_unsize_and_coerce(&mut self, from_ty: &Ty, to_ty: &Ty, depth: usize) -> Option<bool> {
if depth > 1000 {
panic!("Infinite recursion in coercion");
}
match (&from_ty, &to_ty) {
// `[T; N]` -> `[T]`
(ty_app!(TypeCtor::Array, st1), ty_app!(TypeCtor::Slice, st2)) => {
Some(self.unify(&st1[0], &st2[0]))
}
// `T` -> `dyn Trait` when `T: Trait`
(_, Ty::Dyn(_)) => {
// FIXME: Check predicates
Some(true)
}
// `(..., T)` -> `(..., U)` when `T: Unsize<U>`
(
ty_app!(TypeCtor::Tuple { cardinality: len1 }, st1),
ty_app!(TypeCtor::Tuple { cardinality: len2 }, st2),
) => {
if len1 != len2 || *len1 == 0 {
return None;
}
match self.check_unsize_and_coerce(
st1.last().unwrap(),
st2.last().unwrap(),
depth + 1,
) {
Some(true) => {}
ret => return ret,
}
let ret = st1[..st1.len() - 1]
.iter()
.zip(&st2[..st2.len() - 1])
.all(|(ty1, ty2)| self.unify(ty1, ty2));
Some(ret)
}
// Foo<..., T, ...> is Unsize<Foo<..., U, ...>> if:
// - T: Unsize<U>
// - Foo is a struct
// - Only the last field of Foo has a type involving T
// - T is not part of the type of any other fields
// - Bar<T>: Unsize<Bar<U>>, if the last field of Foo has type Bar<T>
(
ty_app!(TypeCtor::Adt(AdtId::StructId(struct1)), st1),
ty_app!(TypeCtor::Adt(AdtId::StructId(struct2)), st2),
) if struct1 == struct2 => {
let field_tys = self.db.field_types((*struct1).into());
let struct_data = self.db.struct_data(*struct1);
let mut fields = struct_data.variant_data.fields().iter();
let (last_field_id, _data) = fields.next_back()?;
// Get the generic parameter involved in the last field.
let unsize_generic_index = {
let mut index = None;
let mut multiple_param = false;
field_tys[last_field_id].walk(&mut |ty| match ty {
&Ty::Param { idx, .. } => {
if index.is_none() {
index = Some(idx);
} else if Some(idx) != index {
multiple_param = true;
}
}
_ => {}
});
if multiple_param {
return None;
}
index?
};
// Check other fields do not involve it.
let mut multiple_used = false;
fields.for_each(|(field_id, _data)| {
field_tys[field_id].walk(&mut |ty| match ty {
&Ty::Param { idx, .. } if idx == unsize_generic_index => {
multiple_used = true
}
_ => {}
})
});
if multiple_used {
return None;
}
let unsize_generic_index = unsize_generic_index as usize;
// Check `Unsize` first
match self.check_unsize_and_coerce(
st1.get(unsize_generic_index)?,
st2.get(unsize_generic_index)?,
depth + 1,
) {
Some(true) => {}
ret => return ret,
}
// Then unify other parameters
let ret = st1
.iter()
.zip(st2.iter())
.enumerate()
.filter(|&(idx, _)| idx != unsize_generic_index)
.all(|(_, (ty1, ty2))| self.unify(ty1, ty2));
Some(ret)
}
_ => None,
}
}
/// 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.unify_substs(st1, st2, 0);
}
_ => {}
}
}
false
}
}
|