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shallow resolve target type in coercion

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We used to avoid doing this because we didn't want to make coercion depend on
the state of inference. For better or worse, we have moved away from this
position over time. Therefore, I am going to go ahead and resolve the `b`
target type early on so that it is done uniformly.

(The older technique for managing this was always something of a hack
regardless; if we really wanted to avoid integrating coercion and inference we
needed to be more disciplined about it.)
This commit is contained in:
Niko Matsakis 2020-11-20 05:52:27 -05:00 committed by Mark Rousskov
parent 7e0ae7d89b
commit 5eca626e40
1 changed files with 2 additions and 5 deletions
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7
compiler/rustc_typeck/src/check/coercion.rs
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@ -147,6 +147,7 @@ fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
let a = self.shallow_resolve(a);
let b = self.shallow_resolve(b);
debug!("Coerce.tys({:?} => {:?})", a, b);
// Just ignore error types.
@ -162,8 +163,7 @@ fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
// 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() {
return if 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 {
@ -196,9 +196,6 @@ fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
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);