Query system cycle errors should be extendable with notes

[tristanburgess]

Relevant PR: #53316
Relevant Issue: #52985

The query system automatically detects and emits a cycle error if a cycle occurs when dependency nodes are added to the query DAG. This error is extensible with a custom main message defined as below to help human readability,

rust/src/librustc/ty/query/config.rs

Line 93 in a385095

impl<'tcx> QueryDescription<'tcx> for queries::normalize_ty_after_erasing_regions<'tcx> {

but is otherwise closed for modification outside of the query::plumbing module:

rust/src/librustc/ty/query/plumbing.rs

Line 248 in b239743

pub(super) fn report_cycle(self, CycleError { usage, cycle: stack }: CycleError<'gcx>)

It would be nice to have a mechanism in addition that allows custom notes and suggestions to be added to these errors to help illustrate why a cycle occurred, not just where. It may be possible to expose the DiagnosticBuilder or provide wrappers for methods like span_suggestion() and span_note()

[tristanburgess]

It looks like one possible way to do this specifically for normalize_ty_after_erasing_regions is to add a try_normalize_ty_after_erasing_regions function to this list

rust/src/librustc/ty/query/mod.rs

Line 668 in b239743

impl<'a, 'tcx, 'lcx> TyCtxt<'a, 'tcx, 'lcx> {

, which would return a DiagnosticBuilder on cycle which can be modified before emitting, but there should be a cleaner way to do this kind of thing than to have to create new functions for each query that wants to offer this kind of control.