/
coercion.rs
1488 lines (1377 loc) · 62.1 KB
/
coercion.rs
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//! # Type Coercion
//!
//! Under certain circumstances we will coerce from one type to another,
//! for example by auto-borrowing. This occurs in situations where the
//! compiler has a firm 'expected type' that was supplied from the user,
//! and where the actual type is similar to that expected type in purpose
//! but not in representation (so actual subtyping is inappropriate).
//!
//! ## Reborrowing
//!
//! Note that if we are expecting a reference, we will *reborrow*
//! even if the argument provided was already a reference. This is
//! useful for freezing mut/const things (that is, when the expected is &T
//! but you have &const T or &mut T) and also for avoiding the linearity
//! of mut things (when the expected is &mut T and you have &mut T). See
//! the various `src/test/ui/coerce-reborrow-*.rs` tests for
//! examples of where this is useful.
//!
//! ## Subtle note
//!
//! When deciding what type coercions to consider, we do not attempt to
//! resolve any type variables we may encounter. This is because `b`
//! represents the expected type "as the user wrote it", meaning that if
//! the user defined a generic function like
//!
//! fn foo<A>(a: A, b: A) { ... }
//!
//! and then we wrote `foo(&1, @2)`, we will not auto-borrow
//! either argument. In older code we went to some lengths to
//! resolve the `b` variable, which could mean that we'd
//! auto-borrow later arguments but not earlier ones, which
//! seems very confusing.
//!
//! ## Subtler note
//!
//! However, right now, if the user manually specifies the
//! values for the type variables, as so:
//!
//! foo::<&int>(@1, @2)
//!
//! then we *will* auto-borrow, because we can't distinguish this from a
//! function that declared `&int`. This is inconsistent but it's easiest
//! at the moment. The right thing to do, I think, is to consider the
//! *unsubstituted* type when deciding whether to auto-borrow, but the
//! *substituted* type when considering the bounds and so forth. But most
//! of our methods don't give access to the unsubstituted type, and
//! rightly so because they'd be error-prone. So maybe the thing to do is
//! to actually determine the kind of coercions that should occur
//! separately and pass them in. Or maybe it's ok as is. Anyway, it's
//! sort of a minor point so I've opted to leave it for later -- after all,
//! we may want to adjust precisely when coercions occur.
use crate::astconv::AstConv;
use crate::check::{FnCtxt, Needs};
use rustc_errors::{struct_span_err, DiagnosticBuilder};
use rustc_hir as hir;
use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use rustc_infer::infer::{Coercion, InferOk, InferResult};
use rustc_middle::ty::adjustment::{
Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast,
};
use rustc_middle::ty::error::TypeError;
use rustc_middle::ty::fold::TypeFoldable;
use rustc_middle::ty::relate::RelateResult;
use rustc_middle::ty::subst::SubstsRef;
use rustc_middle::ty::{self, Ty, TypeAndMut};
use rustc_session::parse::feature_err;
use rustc_span::symbol::sym;
use rustc_span::{self, Span};
use rustc_target::spec::abi::Abi;
use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
use smallvec::{smallvec, SmallVec};
use std::ops::Deref;
struct Coerce<'a, 'tcx> {
fcx: &'a FnCtxt<'a, 'tcx>,
cause: ObligationCause<'tcx>,
use_lub: bool,
/// Determines whether or not allow_two_phase_borrow is set on any
/// autoref adjustments we create while coercing. We don't want to
/// allow deref coercions to create two-phase borrows, at least initially,
/// but we do need two-phase borrows for function argument reborrows.
/// See #47489 and #48598
/// See docs on the "AllowTwoPhase" type for a more detailed discussion
allow_two_phase: AllowTwoPhase,
}
impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
type Target = FnCtxt<'a, 'tcx>;
fn deref(&self) -> &Self::Target {
&self.fcx
}
}
type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
fn coerce_mutbls<'tcx>(
from_mutbl: hir::Mutability,
to_mutbl: hir::Mutability,
) -> RelateResult<'tcx, ()> {
match (from_mutbl, to_mutbl) {
(hir::Mutability::Mut, hir::Mutability::Mut)
| (hir::Mutability::Not, hir::Mutability::Not)
| (hir::Mutability::Mut, hir::Mutability::Not) => Ok(()),
(hir::Mutability::Not, hir::Mutability::Mut) => Err(TypeError::Mutability),
}
}
fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> {
vec![]
}
fn simple(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
move |target| vec![Adjustment { kind, target }]
}
fn success<'tcx>(
adj: Vec<Adjustment<'tcx>>,
target: Ty<'tcx>,
obligations: traits::PredicateObligations<'tcx>,
) -> CoerceResult<'tcx> {
Ok(InferOk { value: (adj, target), obligations })
}
impl<'f, 'tcx> Coerce<'f, 'tcx> {
fn new(
fcx: &'f FnCtxt<'f, 'tcx>,
cause: ObligationCause<'tcx>,
allow_two_phase: AllowTwoPhase,
) -> Self {
Coerce { fcx, cause, allow_two_phase, use_lub: false }
}
fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
self.commit_if_ok(|_| {
if self.use_lub {
self.at(&self.cause, self.fcx.param_env).lub(b, a)
} else {
self.at(&self.cause, self.fcx.param_env)
.sup(b, a)
.map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
}
})
}
/// Unify two types (using sub or lub) and produce a specific coercion.
fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
where
F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
{
self.unify(&a, &b)
.and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
}
fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
let a = self.shallow_resolve(a);
debug!("Coerce.tys({:?} => {:?})", a, b);
// Just ignore error types.
if a.references_error() || b.references_error() {
return success(vec![], self.fcx.tcx.types.err, vec![]);
}
if a.is_never() {
// Subtle: If we are coercing from `!` to `?T`, where `?T` is an unbound
// type variable, we want `?T` to fallback to `!` if not
// otherwise constrained. An example where this arises:
//
// let _: Option<?T> = Some({ return; });
//
// here, we would coerce from `!` to `?T`.
let b = self.shallow_resolve(b);
return if self.shallow_resolve(b).is_ty_var() {
// Micro-optimization: no need for this if `b` is
// already resolved in some way.
let diverging_ty = self.next_diverging_ty_var(TypeVariableOrigin {
kind: TypeVariableOriginKind::AdjustmentType,
span: self.cause.span,
});
self.unify_and(&b, &diverging_ty, simple(Adjust::NeverToAny))
} else {
success(simple(Adjust::NeverToAny)(b), b, vec![])
};
}
// Consider coercing the subtype to a DST
//
// NOTE: this is wrapped in a `commit_if_ok` because it creates
// a "spurious" type variable, and we don't want to have that
// type variable in memory if the coercion fails.
let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
match unsize {
Ok(_) => {
debug!("coerce: unsize successful");
return unsize;
}
Err(TypeError::ObjectUnsafeCoercion(did)) => {
debug!("coerce: unsize not object safe");
return Err(TypeError::ObjectUnsafeCoercion(did));
}
Err(_) => {}
}
debug!("coerce: unsize failed");
// Examine the supertype and consider auto-borrowing.
//
// Note: does not attempt to resolve type variables we encounter.
// See above for details.
match b.kind {
ty::RawPtr(mt_b) => {
return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
}
ty::Ref(r_b, ty, mutbl) => {
let mt_b = ty::TypeAndMut { ty, mutbl };
return self.coerce_borrowed_pointer(a, b, r_b, mt_b);
}
_ => {}
}
match a.kind {
ty::FnDef(..) => {
// Function items are coercible to any closure
// type; function pointers are not (that would
// require double indirection).
// Additionally, we permit coercion of function
// items to drop the unsafe qualifier.
self.coerce_from_fn_item(a, b)
}
ty::FnPtr(a_f) => {
// We permit coercion of fn pointers to drop the
// unsafe qualifier.
self.coerce_from_fn_pointer(a, a_f, b)
}
ty::Closure(_, substs_a) => {
// Non-capturing closures are coercible to
// function pointers or unsafe function pointers.
// It cannot convert closures that require unsafe.
self.coerce_closure_to_fn(a, substs_a, b)
}
_ => {
// Otherwise, just use unification rules.
self.unify_and(a, b, identity)
}
}
}
/// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
/// To match `A` with `B`, autoderef will be performed,
/// calling `deref`/`deref_mut` where necessary.
fn coerce_borrowed_pointer(
&self,
a: Ty<'tcx>,
b: Ty<'tcx>,
r_b: ty::Region<'tcx>,
mt_b: TypeAndMut<'tcx>,
) -> CoerceResult<'tcx> {
debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
// If we have a parameter of type `&M T_a` and the value
// provided is `expr`, we will be adding an implicit borrow,
// meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
// to type check, we will construct the type that `&M*expr` would
// yield.
let (r_a, mt_a) = match a.kind {
ty::Ref(r_a, ty, mutbl) => {
let mt_a = ty::TypeAndMut { ty, mutbl };
coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
(r_a, mt_a)
}
_ => return self.unify_and(a, b, identity),
};
let span = self.cause.span;
let mut first_error = None;
let mut r_borrow_var = None;
let mut autoderef = self.autoderef(span, a);
let mut found = None;
for (referent_ty, autoderefs) in autoderef.by_ref() {
if autoderefs == 0 {
// Don't let this pass, otherwise it would cause
// &T to autoref to &&T.
continue;
}
// At this point, we have deref'd `a` to `referent_ty`. So
// imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
// In the autoderef loop for `&'a mut Vec<T>`, we would get
// three callbacks:
//
// - `&'a mut Vec<T>` -- 0 derefs, just ignore it
// - `Vec<T>` -- 1 deref
// - `[T]` -- 2 deref
//
// At each point after the first callback, we want to
// check to see whether this would match out target type
// (`&'b mut [T]`) if we autoref'd it. We can't just
// compare the referent types, though, because we still
// have to consider the mutability. E.g., in the case
// we've been considering, we have an `&mut` reference, so
// the `T` in `[T]` needs to be unified with equality.
//
// Therefore, we construct reference types reflecting what
// the types will be after we do the final auto-ref and
// compare those. Note that this means we use the target
// mutability [1], since it may be that we are coercing
// from `&mut T` to `&U`.
//
// One fine point concerns the region that we use. We
// choose the region such that the region of the final
// type that results from `unify` will be the region we
// want for the autoref:
//
// - if in sub mode, that means we want to use `'b` (the
// region from the target reference) for both
// pointers [2]. This is because sub mode (somewhat
// arbitrarily) returns the subtype region. In the case
// where we are coercing to a target type, we know we
// want to use that target type region (`'b`) because --
// for the program to type-check -- it must be the
// smaller of the two.
// - One fine point. It may be surprising that we can
// use `'b` without relating `'a` and `'b`. The reason
// that this is ok is that what we produce is
// effectively a `&'b *x` expression (if you could
// annotate the region of a borrow), and regionck has
// code that adds edges from the region of a borrow
// (`'b`, here) into the regions in the borrowed
// expression (`*x`, here). (Search for "link".)
// - if in lub mode, things can get fairly complicated. The
// easiest thing is just to make a fresh
// region variable [4], which effectively means we defer
// the decision to region inference (and regionck, which will add
// some more edges to this variable). However, this can wind up
// creating a crippling number of variables in some cases --
// e.g., #32278 -- so we optimize one particular case [3].
// Let me try to explain with some examples:
// - The "running example" above represents the simple case,
// where we have one `&` reference at the outer level and
// ownership all the rest of the way down. In this case,
// we want `LUB('a, 'b)` as the resulting region.
// - However, if there are nested borrows, that region is
// too strong. Consider a coercion from `&'a &'x Rc<T>` to
// `&'b T`. In this case, `'a` is actually irrelevant.
// The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
// we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
// (The errors actually show up in borrowck, typically, because
// this extra edge causes the region `'a` to be inferred to something
// too big, which then results in borrowck errors.)
// - We could track the innermost shared reference, but there is already
// code in regionck that has the job of creating links between
// the region of a borrow and the regions in the thing being
// borrowed (here, `'a` and `'x`), and it knows how to handle
// all the various cases. So instead we just make a region variable
// and let regionck figure it out.
let r = if !self.use_lub {
r_b // [2] above
} else if autoderefs == 1 {
r_a // [3] above
} else {
if r_borrow_var.is_none() {
// create var lazilly, at most once
let coercion = Coercion(span);
let r = self.next_region_var(coercion);
r_borrow_var = Some(r); // [4] above
}
r_borrow_var.unwrap()
};
let derefd_ty_a = self.tcx.mk_ref(
r,
TypeAndMut {
ty: referent_ty,
mutbl: mt_b.mutbl, // [1] above
},
);
match self.unify(derefd_ty_a, b) {
Ok(ok) => {
found = Some(ok);
break;
}
Err(err) => {
if first_error.is_none() {
first_error = Some(err);
}
}
}
}
// Extract type or return an error. We return the first error
// we got, which should be from relating the "base" type
// (e.g., in example above, the failure from relating `Vec<T>`
// to the target type), since that should be the least
// confusing.
let InferOk { value: ty, mut obligations } = match found {
Some(d) => d,
None => {
let err = first_error.expect("coerce_borrowed_pointer had no error");
debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
return Err(err);
}
};
if ty == a && mt_a.mutbl == hir::Mutability::Not && autoderef.step_count() == 1 {
// As a special case, if we would produce `&'a *x`, that's
// a total no-op. We end up with the type `&'a T` just as
// we started with. In that case, just skip it
// altogether. This is just an optimization.
//
// Note that for `&mut`, we DO want to reborrow --
// otherwise, this would be a move, which might be an
// error. For example `foo(self.x)` where `self` and
// `self.x` both have `&mut `type would be a move of
// `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
// which is a borrow.
assert_eq!(mt_b.mutbl, hir::Mutability::Not); // can only coerce &T -> &U
return success(vec![], ty, obligations);
}
let needs = Needs::maybe_mut_place(mt_b.mutbl);
let InferOk { value: mut adjustments, obligations: o } =
autoderef.adjust_steps_as_infer_ok(self, needs);
obligations.extend(o);
obligations.extend(autoderef.into_obligations());
// Now apply the autoref. We have to extract the region out of
// the final ref type we got.
let r_borrow = match ty.kind {
ty::Ref(r_borrow, _, _) => r_borrow,
_ => span_bug!(span, "expected a ref type, got {:?}", ty),
};
let mutbl = match mt_b.mutbl {
hir::Mutability::Not => AutoBorrowMutability::Not,
hir::Mutability::Mut => {
AutoBorrowMutability::Mut { allow_two_phase_borrow: self.allow_two_phase }
}
};
adjustments.push(Adjustment {
kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
target: ty,
});
debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
success(adjustments, ty, obligations)
}
// &[T; n] or &mut [T; n] -> &[T]
// or &mut [T; n] -> &mut [T]
// or &Concrete -> &Trait, etc.
fn coerce_unsized(&self, source: Ty<'tcx>, target: Ty<'tcx>) -> CoerceResult<'tcx> {
debug!("coerce_unsized(source={:?}, target={:?})", source, target);
let traits =
(self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
let (unsize_did, coerce_unsized_did) = if let (Some(u), Some(cu)) = traits {
(u, cu)
} else {
debug!("missing Unsize or CoerceUnsized traits");
return Err(TypeError::Mismatch);
};
// Note, we want to avoid unnecessary unsizing. We don't want to coerce to
// a DST unless we have to. This currently comes out in the wash since
// we can't unify [T] with U. But to properly support DST, we need to allow
// that, at which point we will need extra checks on the target here.
// Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
let reborrow = match (&source.kind, &target.kind) {
(&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
coerce_mutbls(mutbl_a, mutbl_b)?;
let coercion = Coercion(self.cause.span);
let r_borrow = self.next_region_var(coercion);
let mutbl = match mutbl_b {
hir::Mutability::Not => AutoBorrowMutability::Not,
hir::Mutability::Mut => AutoBorrowMutability::Mut {
// We don't allow two-phase borrows here, at least for initial
// implementation. If it happens that this coercion is a function argument,
// the reborrow in coerce_borrowed_ptr will pick it up.
allow_two_phase_borrow: AllowTwoPhase::No,
},
};
Some((
Adjustment { kind: Adjust::Deref(None), target: ty_a },
Adjustment {
kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
target: self
.tcx
.mk_ref(r_borrow, ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a }),
},
))
}
(&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
coerce_mutbls(mt_a, mt_b)?;
Some((
Adjustment { kind: Adjust::Deref(None), target: ty_a },
Adjustment {
kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
target: self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mt_b, ty: ty_a }),
},
))
}
_ => None,
};
let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
// Setup either a subtyping or a LUB relationship between
// the `CoerceUnsized` target type and the expected type.
// We only have the latter, so we use an inference variable
// for the former and let type inference do the rest.
let origin = TypeVariableOrigin {
kind: TypeVariableOriginKind::MiscVariable,
span: self.cause.span,
};
let coerce_target = self.next_ty_var(origin);
let mut coercion = self.unify_and(coerce_target, target, |target| {
let unsize = Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target };
match reborrow {
None => vec![unsize],
Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
}
})?;
let mut selcx = traits::SelectionContext::new(self);
// Create an obligation for `Source: CoerceUnsized<Target>`.
let cause = ObligationCause::new(
self.cause.span,
self.body_id,
ObligationCauseCode::Coercion { source, target },
);
// Use a FIFO queue for this custom fulfillment procedure.
//
// A Vec (or SmallVec) is not a natural choice for a queue. However,
// this code path is hot, and this queue usually has a max length of 1
// and almost never more than 3. By using a SmallVec we avoid an
// allocation, at the (very small) cost of (occasionally) having to
// shift subsequent elements down when removing the front element.
let mut queue: SmallVec<[_; 4]> = smallvec![traits::predicate_for_trait_def(
self.tcx,
self.fcx.param_env,
cause,
coerce_unsized_did,
0,
coerce_source,
&[coerce_target.into()]
)];
let mut has_unsized_tuple_coercion = false;
// Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
// emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
// inference might unify those two inner type variables later.
let traits = [coerce_unsized_did, unsize_did];
while !queue.is_empty() {
let obligation = queue.remove(0);
debug!("coerce_unsized resolve step: {:?}", obligation);
let trait_pred = match obligation.predicate {
ty::Predicate::Trait(trait_pred, _) if traits.contains(&trait_pred.def_id()) => {
if unsize_did == trait_pred.def_id() {
let unsize_ty = trait_pred.skip_binder().trait_ref.substs[1].expect_ty();
if let ty::Tuple(..) = unsize_ty.kind {
debug!("coerce_unsized: found unsized tuple coercion");
has_unsized_tuple_coercion = true;
}
}
trait_pred
}
_ => {
coercion.obligations.push(obligation);
continue;
}
};
match selcx.select(&obligation.with(trait_pred)) {
// Uncertain or unimplemented.
Ok(None) => {
if trait_pred.def_id() == unsize_did {
let trait_pred = self.resolve_vars_if_possible(&trait_pred);
let self_ty = trait_pred.skip_binder().self_ty();
let unsize_ty = trait_pred.skip_binder().trait_ref.substs[1].expect_ty();
debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
match (&self_ty.kind, &unsize_ty.kind) {
(ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
if self.type_var_is_sized(*v) =>
{
debug!("coerce_unsized: have sized infer {:?}", v);
coercion.obligations.push(obligation);
// `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
// for unsizing.
}
_ => {
// Some other case for `$0: Unsize<Something>`. Note that we
// hit this case even if `Something` is a sized type, so just
// don't do the coercion.
debug!("coerce_unsized: ambiguous unsize");
return Err(TypeError::Mismatch);
}
}
} else {
debug!("coerce_unsized: early return - ambiguous");
return Err(TypeError::Mismatch);
}
}
Err(traits::Unimplemented) => {
debug!("coerce_unsized: early return - can't prove obligation");
return Err(TypeError::Mismatch);
}
// Object safety violations or miscellaneous.
Err(err) => {
self.report_selection_error(&obligation, &err, false, false);
// Treat this like an obligation and follow through
// with the unsizing - the lack of a coercion should
// be silent, as it causes a type mismatch later.
}
Ok(Some(vtable)) => queue.extend(vtable.nested_obligations()),
}
}
if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
feature_err(
&self.tcx.sess.parse_sess,
sym::unsized_tuple_coercion,
self.cause.span,
"unsized tuple coercion is not stable enough for use and is subject to change",
)
.emit();
}
Ok(coercion)
}
fn coerce_from_safe_fn<F, G>(
&self,
a: Ty<'tcx>,
fn_ty_a: ty::PolyFnSig<'tcx>,
b: Ty<'tcx>,
to_unsafe: F,
normal: G,
) -> CoerceResult<'tcx>
where
F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
{
if let ty::FnPtr(fn_ty_b) = b.kind {
if let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) =
(fn_ty_a.unsafety(), fn_ty_b.unsafety())
{
let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
return self.unify_and(unsafe_a, b, to_unsafe);
}
}
self.unify_and(a, b, normal)
}
fn coerce_from_fn_pointer(
&self,
a: Ty<'tcx>,
fn_ty_a: ty::PolyFnSig<'tcx>,
b: Ty<'tcx>,
) -> CoerceResult<'tcx> {
//! Attempts to coerce from the type of a Rust function item
//! into a closure or a `proc`.
//!
let b = self.shallow_resolve(b);
debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
self.coerce_from_safe_fn(
a,
fn_ty_a,
b,
simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)),
identity,
)
}
fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
//! Attempts to coerce from the type of a Rust function item
//! into a closure or a `proc`.
let b = self.shallow_resolve(b);
debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
match b.kind {
ty::FnPtr(_) => {
let a_sig = a.fn_sig(self.tcx);
// Intrinsics are not coercible to function pointers
if a_sig.abi() == Abi::RustIntrinsic || a_sig.abi() == Abi::PlatformIntrinsic {
return Err(TypeError::IntrinsicCast);
}
let InferOk { value: a_sig, mut obligations } =
self.normalize_associated_types_in_as_infer_ok(self.cause.span, &a_sig);
let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
a_fn_pointer,
a_sig,
b,
|unsafe_ty| {
vec![
Adjustment {
kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
target: a_fn_pointer,
},
Adjustment {
kind: Adjust::Pointer(PointerCast::UnsafeFnPointer),
target: unsafe_ty,
},
]
},
simple(Adjust::Pointer(PointerCast::ReifyFnPointer)),
)?;
obligations.extend(o2);
Ok(InferOk { value, obligations })
}
_ => self.unify_and(a, b, identity),
}
}
fn coerce_closure_to_fn(
&self,
a: Ty<'tcx>,
substs_a: SubstsRef<'tcx>,
b: Ty<'tcx>,
) -> CoerceResult<'tcx> {
//! Attempts to coerce from the type of a non-capturing closure
//! into a function pointer.
//!
let b = self.shallow_resolve(b);
match b.kind {
ty::FnPtr(fn_ty) if substs_a.as_closure().upvar_tys().next().is_none() => {
// We coerce the closure, which has fn type
// `extern "rust-call" fn((arg0,arg1,...)) -> _`
// to
// `fn(arg0,arg1,...) -> _`
// or
// `unsafe fn(arg0,arg1,...) -> _`
let closure_sig = substs_a.as_closure().sig();
let unsafety = fn_ty.unsafety();
let pointer_ty = self.tcx.coerce_closure_fn_ty(closure_sig, unsafety);
debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
self.unify_and(
pointer_ty,
b,
simple(Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))),
)
}
_ => self.unify_and(a, b, identity),
}
}
fn coerce_unsafe_ptr(
&self,
a: Ty<'tcx>,
b: Ty<'tcx>,
mutbl_b: hir::Mutability,
) -> CoerceResult<'tcx> {
debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
let (is_ref, mt_a) = match a.kind {
ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
ty::RawPtr(mt) => (false, mt),
_ => return self.unify_and(a, b, identity),
};
// Check that the types which they point at are compatible.
let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty });
coerce_mutbls(mt_a.mutbl, mutbl_b)?;
// Although references and unsafe ptrs have the same
// representation, we still register an Adjust::DerefRef so that
// regionck knows that the region for `a` must be valid here.
if is_ref {
self.unify_and(a_unsafe, b, |target| {
vec![
Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
]
})
} else if mt_a.mutbl != mutbl_b {
self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer)))
} else {
self.unify_and(a_unsafe, b, identity)
}
}
}
impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
/// Attempt to coerce an expression to a type, and return the
/// adjusted type of the expression, if successful.
/// Adjustments are only recorded if the coercion succeeded.
/// The expressions *must not* have any pre-existing adjustments.
pub fn try_coerce(
&self,
expr: &hir::Expr<'_>,
expr_ty: Ty<'tcx>,
target: Ty<'tcx>,
allow_two_phase: AllowTwoPhase,
) -> RelateResult<'tcx, Ty<'tcx>> {
let source = self.resolve_vars_with_obligations(expr_ty);
debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
let cause = self.cause(expr.span, ObligationCauseCode::ExprAssignable);
let coerce = Coerce::new(self, cause, allow_two_phase);
let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
let (adjustments, _) = self.register_infer_ok_obligations(ok);
self.apply_adjustments(expr, adjustments);
Ok(if expr_ty.references_error() { self.tcx.types.err } else { target })
}
/// Same as `try_coerce()`, but without side-effects.
pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
let source = self.resolve_vars_with_obligations(expr_ty);
debug!("coercion::can({:?} -> {:?})", source, target);
let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
// We don't ever need two-phase here since we throw out the result of the coercion
let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
self.probe(|_| coerce.coerce(source, target)).is_ok()
}
/// Given some expressions, their known unified type and another expression,
/// tries to unify the types, potentially inserting coercions on any of the
/// provided expressions and returns their LUB (aka "common supertype").
///
/// This is really an internal helper. From outside the coercion
/// module, you should instantiate a `CoerceMany` instance.
fn try_find_coercion_lub<E>(
&self,
cause: &ObligationCause<'tcx>,
exprs: &[E],
prev_ty: Ty<'tcx>,
new: &hir::Expr<'_>,
new_ty: Ty<'tcx>,
) -> RelateResult<'tcx, Ty<'tcx>>
where
E: AsCoercionSite,
{
let prev_ty = self.resolve_vars_with_obligations(prev_ty);
let new_ty = self.resolve_vars_with_obligations(new_ty);
debug!("coercion::try_find_coercion_lub({:?}, {:?})", prev_ty, new_ty);
// Special-case that coercion alone cannot handle:
// Two function item types of differing IDs or InternalSubsts.
if let (&ty::FnDef(..), &ty::FnDef(..)) = (&prev_ty.kind, &new_ty.kind) {
// Don't reify if the function types have a LUB, i.e., they
// are the same function and their parameters have a LUB.
let lub_ty = self
.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
.map(|ok| self.register_infer_ok_obligations(ok));
if lub_ty.is_ok() {
// We have a LUB of prev_ty and new_ty, just return it.
return lub_ty;
}
// The signature must match.
let a_sig = prev_ty.fn_sig(self.tcx);
let a_sig = self.normalize_associated_types_in(new.span, &a_sig);
let b_sig = new_ty.fn_sig(self.tcx);
let b_sig = self.normalize_associated_types_in(new.span, &b_sig);
let sig = self
.at(cause, self.param_env)
.trace(prev_ty, new_ty)
.lub(&a_sig, &b_sig)
.map(|ok| self.register_infer_ok_obligations(ok))?;
// Reify both sides and return the reified fn pointer type.
let fn_ptr = self.tcx.mk_fn_ptr(sig);
for expr in exprs.iter().map(|e| e.as_coercion_site()).chain(Some(new)) {
// The only adjustment that can produce an fn item is
// `NeverToAny`, so this should always be valid.
self.apply_adjustments(
expr,
vec![Adjustment {
kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
target: fn_ptr,
}],
);
}
return Ok(fn_ptr);
}
// Configure a Coerce instance to compute the LUB.
// We don't allow two-phase borrows on any autorefs this creates since we
// probably aren't processing function arguments here and even if we were,
// they're going to get autorefed again anyway and we can apply 2-phase borrows
// at that time.
let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
coerce.use_lub = true;
// First try to coerce the new expression to the type of the previous ones,
// but only if the new expression has no coercion already applied to it.
let mut first_error = None;
if !self.tables.borrow().adjustments().contains_key(new.hir_id) {
let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
match result {
Ok(ok) => {
let (adjustments, target) = self.register_infer_ok_obligations(ok);
self.apply_adjustments(new, adjustments);
return Ok(target);
}
Err(e) => first_error = Some(e),
}
}
// Then try to coerce the previous expressions to the type of the new one.
// This requires ensuring there are no coercions applied to *any* of the
// previous expressions, other than noop reborrows (ignoring lifetimes).
for expr in exprs {
let expr = expr.as_coercion_site();
let noop = match self.tables.borrow().expr_adjustments(expr) {
&[Adjustment { kind: Adjust::Deref(_), .. }, Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. }] =>
{
match self.node_ty(expr.hir_id).kind {
ty::Ref(_, _, mt_orig) => {
let mutbl_adj: hir::Mutability = mutbl_adj.into();
// Reborrow that we can safely ignore, because
// the next adjustment can only be a Deref
// which will be merged into it.
mutbl_adj == mt_orig
}
_ => false,
}
}
&[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
_ => false,
};
if !noop {
return self
.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
.map(|ok| self.register_infer_ok_obligations(ok));
}
}
match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
Err(_) => {
// Avoid giving strange errors on failed attempts.
if let Some(e) = first_error {
Err(e)
} else {
self.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
.map(|ok| self.register_infer_ok_obligations(ok))
}
}
Ok(ok) => {
let (adjustments, target) = self.register_infer_ok_obligations(ok);
for expr in exprs {
let expr = expr.as_coercion_site();
self.apply_adjustments(expr, adjustments.clone());
}
Ok(target)
}
}
}
}
/// CoerceMany encapsulates the pattern you should use when you have
/// many expressions that are all getting coerced to a common
/// type. This arises, for example, when you have a match (the result
/// of each arm is coerced to a common type). It also arises in less
/// obvious places, such as when you have many `break foo` expressions
/// that target the same loop, or the various `return` expressions in
/// a function.
///
/// The basic protocol is as follows:
///
/// - Instantiate the `CoerceMany` with an initial `expected_ty`.
/// This will also serve as the "starting LUB". The expectation is
/// that this type is something which all of the expressions *must*
/// be coercible to. Use a fresh type variable if needed.
/// - For each expression whose result is to be coerced, invoke `coerce()` with.
/// - In some cases we wish to coerce "non-expressions" whose types are implicitly
/// unit. This happens for example if you have a `break` with no expression,
/// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
/// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
/// from you so that you don't have to worry your pretty head about it.
/// But if an error is reported, the final type will be `err`.
/// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
/// previously coerced expressions.
/// - When all done, invoke `complete()`. This will return the LUB of
/// all your expressions.
/// - WARNING: I don't believe this final type is guaranteed to be
/// related to your initial `expected_ty` in any particular way,
/// although it will typically be a subtype, so you should check it.
/// - Invoking `complete()` may cause us to go and adjust the "adjustments" on