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Add comments, rename variable

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This commit is contained in:
Aaron Hill 2019-10-01 21:44:13 -04:00
parent 9c5a5c471a
commit ba54ef8110
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GPG Key ID: B4087E510E98B164
1 changed files with 40 additions and 5 deletions
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45
src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs
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@ -157,7 +157,42 @@ impl<'tcx> RegionInferenceContext<'tcx> {
let target_scc = self.constraint_sccs.scc(target_region);
let mut range = 0..path.len();
let should_reverse = match from_region_origin {
// As noted above, when reporting an error, there is typically a chain of constraints
// leading from some "source" region which must outlive some "target" region.
// In most cases, we prefer to "blame" the constraints closer to the target --
// but there is one exception. When constraints arise from higher-ranked subtyping,
// we generally prefer to blame the source value,
// as the "target" in this case tends to be some type annotation that the user gave.
// Therefore, if we find that the region origin is some instantiation
// of a higher-ranked region, we start our search from the "source" point
// rather than the "target", and we also tweak a few other things.
//
// An example might be this bit of Rust code:
//
// ```rust
// let x: fn(&'static ()) = |_| {};
// let y: for<'a> fn(&'a ()) = x;
// ```
//
// In MIR, this will be converted into a combination of assignments and type ascriptions.
// In particular, the 'static is imposed through a type ascription:
//
// ```rust
// x = ...;
// AscribeUserType(x, fn(&'static ())
// y = x;
// ```
//
// We wind up ultimately with constraints like
//
// ```rust
// !a: 'temp1 // from the `y = x` statement
// 'temp1: 'temp2
// 'temp2: 'static // from the AscribeUserType
// ```
//
// and here we prefer to blame the source (the y = x statement).
let blame_source = match from_region_origin {
NLLRegionVariableOrigin::FreeRegion
| NLLRegionVariableOrigin::Existential { from_forall: false } => {
true
@ -173,7 +208,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
if should_reverse {
if blame_source {
match categorized_path[*i].0 {
ConstraintCategory::OpaqueType | ConstraintCategory::Boring |
ConstraintCategory::BoringNoLocation | ConstraintCategory::Internal => false,
@ -190,14 +225,14 @@ impl<'tcx> RegionInferenceContext<'tcx> {
}
};
let best_choice = if should_reverse {
let best_choice = if blame_source {
range.rev().find(find_region)
} else {
range.find(find_region)
};
debug!("best_blame_constraint: best_choice={:?} should_reverse={}",
best_choice, should_reverse);
debug!("best_blame_constraint: best_choice={:?} blame_source={}",
best_choice, blame_source);
if let Some(i) = best_choice {
if let Some(next) = categorized_path.get(i + 1) {