Commits on May 26, 2020

1. Display information about captured variable in FnMut error …

   Fixes rust-lang#69446
   
   When we encounter a region error involving an `FnMut` closure, we
   display a specialized error message. However, we currently do not
   tell the user which upvar was captured. This makes it difficult to
   determine the cause of the error, especially when the closure is large.
   
   This commit records marks constraints involving closure upvars
   with `ConstraintCategory::ClosureUpvar`. When we decide to 'blame'
   a `ConstraintCategory::Return`, we additionall store
   the captured upvar if we found a `ConstraintCategory::ClosureUpvar` in
   the path.
   
   When generating an error message, we point to relevant spans if we have
   closure upvar information available. We further customize the message if
   an `async` closure is being returned, to make it clear that the captured
   variable is being returned indirectly.

   

   Aaron1011 committed May 26, 2020
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