Encode spans relative to the enclosing item -- enable on nightly
Follow-up to #84373 with the flag `-Zincremental-relative-spans` set by default.
This PR seeks to remove one of the main shortcomings of incremental: the handling of spans.
Changing the contents of a function may require redoing part of the compilation process for another function in another file because of span information is changed.
Within one file: all the spans in HIR change, so typechecking had to be re-done.
Between files: spans of associated types/consts/functions change, so type-based resolution needs to be re-done (hygiene information is stored in the span).
The flag `-Zincremental-relative-spans` encodes local spans relative to the span of an item, stored inside the `source_span` query.
Trap: stashed diagnostics are referenced by the "raw" span, so stealing them requires to remove the span's parent.
In order to avoid too much traffic in the span interner, span encoding uses the `ctxt_or_tag` field to encode:
- the parent when the `SyntaxContext` is 0;
- the `SyntaxContext` when the parent is `None`.
Even with this, the PR creates a lot of traffic to the Span interner, when a Span has both a LocalDefId parent and a non-root SyntaxContext. They appear in lowering, when we add a parent to all spans, including those which come from macros, and during inlining when we mark inlined spans.
The last commit changes how queries of `LocalDefId` manage their cache. I can put this in a separate PR if required.
Possible future directions:
- validate that all spans are marked in HIR validation;
- mark macro-expanded spans relative to the def-site and not the use-site.
Document read_line gotchas
1. The "You do not need to clear the buffer before appending" advice is ambiguous, because it depends what you use this function for. For a rather common case of reading individual lines in a loop, it _is_ necessary to clear the buffer.
2. The docs warn about a DoS risk. I've added a hint how to mitigate unbounded memory growth.
Added link from Targets to Platform Support in the book
If you search the web for "rust targets", the first result is the [targets page](https://doc.rust-lang.org/nightly/rustc/targets/index.html). However, usually when searching for this I'm interested in seeing the available triples with host information, so I just added a link to the correct page.
The entire `Targets` chapter could probably be combined into one page, since its three subchapters each only have a tiny section (I'll do this if requested)
refactor: merge error code `E0465` into `E0464`
`E0465` is an undocumented and untested error code that is functionally identical to `E0464`. This PR merges `E0465` into `E0464`, thus documenting and testing another error code (#61137).
r? `@GuillaumeGomez` (not sure if you want to review this but it's relevant to my other PRs that you have reviewed)
