It cannot be exactly the same, because we have needs rustc doesn't have (namely, accurate enumeration of all methods, not just with a specific name, for completions etc., while rustc also needs a best-effort implementation for diagnostics) but it is closer than the previous impl.
In addition we rewrite the closely related handling of operator inference and impl collection.
This in turn necessitate changing some other parts of inference in order to retain behavior. As a result, the behavior more closely matches rustc and is also more correct.
This fixes 2 type mismatches on self (1 remains) and 4 diagnostics (1 remains), plus some unknown types.
1. Err on unions on derive where it's required.
2. Err on `#[derive(Default)]` on enums without `#[default]` variant.
3. Don't add where bounds `T: Default` when expanding `Default` on enums (structs need that, enums not).
Also, because I was annoyed by that, in minicore, add a way to filter on multiple flags in the line-filter (`// :`). This is required for the `Debug` and `Hash` derives, because the derive should be in the prelude but the trait not.
Use `std::mem::{size_of, size_of_val, align_of, align_of_val}` from the
prelude instead of importing or qualifying them.
These functions were added to all preludes in Rust 1.80.
And add a new diagnostic for non-`Fn` parenthesized generic args.
Path lowering started to look like a mess, with each function carrying additional parameters for the diagnostic callback (since paths can occur both in type and in expression/pattern position, and their diagnostic handling is different) and the segment index, for the diagnostics report. So I refactored it from stateless functions on `TyLoweringContext` into stateful struct, `PathLoweringContext`, that tracks the process of lowering a path from resolution til assoc types selection.
Only in calls, because to support them in bounds we need support from Chalk. However we don't yet report error from bounds anyway, so this is less severe.
The returned future is shown in its name within inlay hints instead of as a nicer `impl Future`, but that can wait for another PR.
