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Add initial implementation of HIR-based WF checking for diagnostics

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During well-formed checking, we walk through all types 'nested' in
generic arguments. For example, WF-checking `Option<MyStruct<u8>>`
will cause us to check `MyStruct<u8>` and `u8`. However, this is done
on a `rustc_middle::ty::Ty`, which has no span information. As a result,
any errors that occur will have a very general span (e.g. the
definintion of an associated item).

This becomes a problem when macros are involved. In general, an
associated type like `type MyType = Option<MyStruct<u8>>;` may
have completely different spans for each nested type in the HIR. Using
the span of the entire associated item might end up pointing to a macro
invocation, even though a user-provided span is available in one of the
nested types.

This PR adds a framework for HIR-based well formed checking. This check
is only run during error reporting, and is used to obtain a more precise
span for an existing error. This is accomplished by individually
checking each 'nested' type in the HIR for the type, allowing us to
find the most-specific type (and span) that produces a given error.

The majority of the changes are to the error-reporting code. However,
some of the general trait code is modified to pass through more
information.

Since this has no soundness implications, I've implemented a minimal
version to begin with, which can be extended over time. In particular,
this only works for HIR items with a corresponding `DefId` (e.g. it will
not work for WF-checking performed within function bodies).
This commit is contained in:
Aaron Hill 2021-04-04 16:55:39 -04:00
parent 59d92bd017
commit a765333738
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23 changed files with 350 additions and 66 deletions
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14
compiler/rustc_middle/src/query/mod.rs
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@ -1713,4 +1713,18 @@ rustc_queries! {
query limits(key: ()) -> Limits {
desc { "looking up limits" }
}
/// Performs an HIR-based well-formed check on the item with the given `HirId`. If
/// we get an `Umimplemented` error that matches the provided `Predicate`, return
/// the cause of the newly created obligation.
///
/// This is only used by error-reporting code to get a better cause (in particular, a better
/// span) for an *existing* error. Therefore, it is best-effort, and may never handle
/// all of the cases that the normal `ty::Ty`-based wfcheck does. This is fine,
/// because the `ty::Ty`-based wfcheck is always run.
query diagnostic_hir_wf_check(key: (ty::Predicate<'tcx>, hir::HirId)) -> Option<traits::ObligationCause<'tcx>> {
eval_always
no_hash
desc { "performing HIR wf-checking for predicate {:?} at item {:?}", key.0, key.1 }
}
}