Implement forced keywords (k#)

[fmease]

Part of #153839. CC @dianne

Re. draft status: Needs one final self-review pass.

---

What won't be done in this PR:

1. implementing forced keyword lifetimes ('k#static)
2. migrating away from built-in syntax builtin # $ident($($tt)*) / introducing k#-exclusive (weak) keywords
3. allowing the user to force edition-dependent keywords to be keywords even in editions where they're usually not a (strong) keyword (today, that only affects gen anyway IINM)
4. extending the proc_macro::Ident API to allow users to programmatically create forced keywords (might never be added)

_{(No LLM was or will be used by me during the entire creation process of this PR)}

[fmease]

@bors try @rust-timer queue

[fmease]

sussy, needs re-audit

View changes since the review

[fmease]

ditto

View changes since the review

[fmease]

I've intentionally not marked it internal even though this feature is only backed by a T-compiler MCP and not by a T-lang RFC or in-tree experiment:

I would find it a bit weird if we (in a hypothetical future) told the users to "please try out" "k#only bounds" or "&k#own" types & exprs where the corresponding feature will obviously be non-internal.

However, I'm fine with switching it to internal (or incomplete).

View changes since the review

[fmease]

We can't test the roundtripping in the current form of this PR because I haven't added any k#-exclusive weak keywords where it'd be observable if we accidentally dropped k# during encoding&decoding.

I have however tested locally that it works by introducing a new dummy keyword. A hypothetical UI test would have a setup like

use proc_macro::TokenStream;

#[proc_macro]
pub fn perform(_: TokenStream) -> TokenStream {
    let stream: TokenStream = "const _: Option<k#never> = None;").parse().unwrap(); // positive
    //let stream: TokenStream = "fn k#branded() {}").parse().unwrap(); // negative

    stream.into_iter().collect() // forces encoding+decoding
}

View changes since the review

[rust-bors]

☀️ Try build successful (CI)
Build commit: 4eb8eac (4eb8eac3e01e4dbc9d58546df979f51a2ac2e5d5)
Base parent: b751e7a (b751e7a48501be14dcd57b4a430b73a0e51a0c52)

[rust-timer]

Finished benchmarking commit (4eb8eac): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.2%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -3.0%, secondary 1.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.8%	[2.5%, 3.1%]	2
Improvements ✅ (primary)	-3.0%	[-4.1%, -1.9%]	3
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-3.0%	[-4.1%, -1.9%]	3

Cycles

Results (primary 2.1%, secondary -1.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.2%	[-4.1%, -2.3%]	2
All ❌✅ (primary)	2.1%	[2.1%, 2.1%]	1

Binary size

Results (primary 0.0%, secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.1%]	4
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.0%	[-0.1%, -0.0%]	4
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	2
All ❌✅ (primary)	0.0%	[-0.1%, 0.1%]	8

Bootstrap: 477.576s -> 469.017s (-1.79%)
Artifact size: 400.35 MiB -> 400.38 MiB (0.01%)

[rust-bors]

☔ The latest upstream changes (presumably #161801) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.