Commits on Sep 16, 2021

1. Add ConstraintCategory::Usage for handling aggregate construction …

   In some cases, we emit borrowcheck diagnostics pointing
   at a particular field expression in a struct expression
   (e.g. `MyStruct { field: my_expr }`). However, this
   behavior currently relies on us choosing the
   `ConstraintCategory::Boring` with the 'correct' span.
   When adding additional variants to `ConstraintCategory`,
   (or changing existing usages away from `ConstraintCategory::Boring`),
   the current behavior can easily get broken, since a non-boring
   constraint will get chosen over a boring one.
   
   To make the diagnostic output less fragile, this commit
   adds a `ConstraintCategory::Usage` variant. We use this variant
   for the temporary assignments created for each field of
   an aggregate we are constructing.
   
   Using this new variant, we can emit a message mentioning
   "this usage", emphasizing the fact that the error message
   is related to the specific use site (in the struct expression).
   
   This is preparation for additional work on improving NLL error messages
   (see rust-lang#57374)

   

   Aaron1011 committed Sep 16, 2021
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