This commit adds logging statements to `promote_consts` and
`qualify_consts` to make it easier to understand what it is doing.

This commit adds a test that ensures that a cast in a static doesn't
stop const promotion within the static.

This commit refactors `PlaceContext` to split it into four different
smaller enums based on if the context represents a mutating use,
non-mutating use, maybe-mutating use or a non-use (this is based on the
recommendation from @oli-obk on Zulip[1]).

This commit then introduces a `PlaceContext::AscribeUserTy` variant.
`StatementKind::AscribeUserTy` is now correctly mapped to
`PlaceContext::AscribeUserTy` instead of `PlaceContext::Validate`.
`PlaceContext::AscribeUserTy` can also now be correctly categorized as a
non-use which fixes an issue with constant promotion in statics after a
cast introduces a `AscribeUserTy` statement.

[1]: https://rust-lang.zulipchat.com/#narrow/stream/122657-wg-nll/subject/.2355288.20cast.20fails.20to.20promote.20to.20'static/near/136536949

Previously this threshold when testing was 100ns, but the Windows
documentation states:

> which is a high resolution (<1us) time stamp

which presumably means that we could have up to 1us resolution, which
means that 100ns doesn't capture "equivalent" time intervals due to
various bits of rounding here and there.

It's hoped that this..

Closes rust-lang#56034

Give each PathSegment a NodeId

Store a resolved def on hir::PathSegment

save-analysis: remove hacky, unnecessary code now that we have spans for every ident

dump data for prefix path segments

dump refs for path segments in save-analysis

Requires adding path segments to the hir map

Fix tests and rustdoc

save-analysis: handle missing field names

FIxes rust-lang/rls#1031

rebasing and reviewer changes

Primarily refactoring `(Ident, Option<NodeId>)` to `Segment`

Fix tests and assertions; add some comments

more reviewer changes

Fixes rust-lang#55376

…d defs

Follow-up to
rust-lang@c2bb7ca.

This will be part of our strategy for shipping renamed versions of these
components for the Rust 2018 edition.

Closes rust-lang#55967

Fixes rust-lang#55001, rust-lang#54744

Previously, SelectionContext would unconditionally cache the selection
result for an obligation. This worked fine for most users of
SelectionContext, but it caused an issue when used by Rustdoc's blanket
impl finder.

The issue occured when SelectionContext chose a ParamCandidate which
contained inference variables. Since inference variables can change
between calls to select(), it's not safe to cache the selection result -
the chosen candidate might not be applicable for future results, leading
to an ICE when we try to run confirmation.

This commit prevents SelectionContext from caching any ParamCandidate
that contains inference variables. This should always be completely
safe, as trait selection should never depend on a particular result
being cached.

I've also added some extra debug!() statements, which I found helpful in
tracking down this bug.

Needed by rustfmt!