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nikomatsakis authored Aug 8, 2018
1 parent ffe336d commit e5f32ef
Showing 1 changed file with 24 additions and 0 deletions.
24 changes: 24 additions & 0 deletions src/librustc_mir/borrow_check/nll/region_infer/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -645,6 +645,30 @@ impl<'tcx> RegionInferenceContext<'tcx> {
}
}

/// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
/// prove to be satisfied. If this is a closure, we will attempt to
/// "promote" this type-test into our `ClosureRegionRequirements` and
/// hence pass it up the creator. To do this, we have to phrase the
/// type-test in terms of external free regions, as local free
/// regions are not nameable by the closure's creator.
///
/// Promotion works as follows: we first check that the type `T`
/// contains only regions that the creator knows about. If this is
/// true, then -- as a consequence -- we know that all regions in
/// the type `T` are free regions that outlive the closure body. If
/// false, then promotion fails.
///
/// Once we've promoted T, we have to "promote" `'X` to some region
/// that is "external" to the closure. Generally speaking, a region
/// may be the union of some points in the closure body as well as
/// various free lifetimes. We can ignore the points in the closure
/// body: if the type T can be expressed in terms of external regions,
/// we know it outlives the points in the closure body. That
/// just leaves the free regions.
///
/// The idea then is to lower the `T: 'X` constraint into multiple
/// bounds -- e.g., if `'X` is the union of two free lifetimes,
/// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
fn try_promote_type_test<'gcx>(
&self,
infcx: &InferCtxt<'_, 'gcx, 'tcx>,
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