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ignore higher-ranked object bound conditions created by WF #59132

Merged
merged 2 commits into from
Mar 13, 2019
Merged

ignore higher-ranked object bound conditions created by WF #59132

merged 2 commits into from
Mar 13, 2019

Commits on Mar 12, 2019

  1. add a useful debug printout

    @nikomatsakis
    nikomatsakis committed Mar 12, 2019
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  2. ignore higher-ranked WF requirements for trait objects 

    In the `issue-53548` test added in this commit, the `Box<dyn Trait>`
type is expanded to `Box<dyn Trait + 'static>`, but the generator
"witness" that results is `for<'r> { Box<dyn Trait + 'r> }`. The WF
code was encountering an ICE (when debug-assertions were enabled) and
an unexpected compilation error (without debug-asserions) when trying
to process this `'r` region bound. In particular, to be WF, the region
bound must meet the requirements of the trait, and hence we got
`for<'r> { 'r: 'static }`. This would ICE because the `Binder`
constructor we were using was assering that no higher-ranked regions
were involved (because the WF code is supposed to skip those). The
error (if debug-asserions were disabled) came because we obviously
cannot prove that `'r: 'static` for any region `'r`.  Pursuant with
our "lazy WF" strategy for higher-ranked regions, the fix is not to
require that `for<'r> { 'r: 'static }` holds (this is also analogous
to what we would do for higher-ranked regions appearing within the
trait in other positions).
    @nikomatsakis
    nikomatsakis committed Mar 12, 2019
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