- only borrow the refcell once per loop
- avoid complex matches to reduce branch paths in the hot loop
- use a by-ref fast path that avoids mutations at the expense of having false negatives
make `define_opaque_types` fully explicit
based on the idea of #108389. Moved `define_opaque_types` into the actual operations, e.g. `eq`, instead of `infcx.at` because normalization doesn't use `define_opaque_types` and even creates it's own `At` with a different `define_opaque_types` internally.
Somewhat surprisingly, coherence actually relies on `DefineOpaqueTypes::Yes` for soundness which was revealed because I've incorrectly used `DefineOpaqueTypes::No` in `equate_impl_headers`. It feels concerning that even though this is the case, we still sometimes use `DefineOpaqueTypes::No` in coherence. I did not look into this as part of this PR as it is purely changing the structure of the code without changing behavior in any way.
r? ```@oli-obk```
Canonicalize root var when making response from new solver
During trait solving, if we equate two inference variables `?0` and `?1` but don't equate them with any rigid types, then `InferCtxt::probe_ty_var` will return `Err` for both of these. The canonicalizer code will then canonicalize the variables independently(!), and the response will not reflect the fact that these two variables have been made equal.
This hinders inference and I also don't think it's sound? I haven't thought too much about it past that, so let's talk about it.
r? ``@lcnr``
Better debug logs for borrowck constraint graph
It's really cumbersome to work with `RegionVar`s when trying to debug borrowck code or when trying to understand how the borrowchecker works. This PR collects some region information (behind `cfg(debug_assertions)`) for created `RegionVar`s (NLL region vars, this PR doesn't touch canonicalization) and prints the nodes and edges of the strongly connected constraints graph using representatives that use that region information (either lifetime names, locations in MIR or spans).
Optimize `mk_region`
PR #107869 avoiding some interning under `mk_ty` by special-casing `Ty` variants with simple (integer) bodies. This PR does something similar for regions.
r? `@compiler-errors`
Much like there are specialized variants of `mk_ty`. This will enable
some optimization in the next commit.
Also rename the existing `re_error*` functions as `mk_re_error*`, for
consistency.
remove unreachable error code `E0490`
AFAIK, the untested and undocumented error code `E0490` is now unreachable, it was from the days of the original borrow checker.
cc ``@GuillaumeGomez`` #61137
Move `check_region_obligations_and_report_errors` to `TypeErrCtxt`
Makes sense for this function to live with its sibling `resolve_regions_and_report_errors`, around which it's basically just a wrapper.
