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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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GPG key ID: B4087E510E98B164
23 changed files with 350 additions and 66 deletions
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7
compiler/rustc_infer/src/traits/mod.rs
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@ -70,6 +70,10 @@ pub struct FulfillmentError<'tcx> {
/// obligation error caused by a call argument. When this is the case, we also signal that in
/// this field to ensure accuracy of suggestions.
pub points_at_arg_span: bool,
/// Diagnostics only: the 'root' obligation which resulted in
/// the failure to process `obligation`. This is the obligation
/// that was initially passed to `register_predicate_obligation`
pub root_obligation: PredicateObligation<'tcx>,
}
#[derive(Clone)]
@ -122,8 +126,9 @@ impl<'tcx> FulfillmentError<'tcx> {
pub fn new(
obligation: PredicateObligation<'tcx>,
code: FulfillmentErrorCode<'tcx>,
root_obligation: PredicateObligation<'tcx>,
) -> FulfillmentError<'tcx> {
FulfillmentError { obligation, code, points_at_arg_span: false }
FulfillmentError { obligation, code, points_at_arg_span: false, root_obligation }
}
}