cleanup: merge RvalueScopes into ScopeTree

[dianne]

This gets rid of RvalueCandidate, inlines the definition of RvalueScopes into ScopeTree, and removes two RvalueScopes-specific modules, consolidating the scoping logic a bit. Removing the extra step of going from RvalueCandidates to RvalueScopes and removing the duplication between them in the incremental cache should also hopefully improve perf.

I've also taken the liberty of doing a bit of renaming and comment updates, changing some "rvalue scope"s to "extended temporary scope"s. This is a bit closer to the Reference's terminology and makes it clearer that it's specific to temporary lifetime extension. This isn't comprehensive. In particular, I've left record_rvalue_scope_if_borrow_expr untouched since #146098 gets rid of it.

Pulled out from #146098.

r? BoxyUwU as the reviewer of #146098 (though feel free to reassign/claim! this is just cleanup)

cc @dingxiangfei2009

[dianne]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[rust-bors]

⌛ Trying commit dc5405c with merge 5991162…

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Workflow: https://github.com/rust-lang/rust/actions/runs/19174102967

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

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 Documenting proc_macro_test v0.1.0 (/checkout/tests/rustdoc-gui/src/proc_macro_test)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.78s
   Generated /checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/proc_macro_test/index.html
npm WARN deprecated puppeteer@22.15.0: < 24.10.2 is no longer supported
npm ERR! code 127
npm ERR! git dep preparation failed
npm ERR! command /node/bin/node /node/lib/node_modules/npm/bin/npm-cli.js install --force --cache=/home/user/.npm --prefer-offline=false --prefer-online=false --offline=false --no-progress --no-save --no-audit --include=dev --include=peer --include=optional --no-package-lock-only --no-dry-run
npm ERR! npm WARN using --force Recommended protections disabled.
npm ERR! npm WARN deprecated inflight@1.0.6: This module is not supported, and leaks memory. Do not use it. Check out lru-cache if you want a good and tested way to coalesce async requests by a key value, which is much more comprehensive and powerful.
npm ERR! npm WARN deprecated glob@7.2.3: Glob versions prior to v9 are no longer supported
npm ERR! npm WARN deprecated glob@7.2.3: Glob versions prior to v9 are no longer supported
npm ERR! npm WARN deprecated debuglog@1.0.1: Package no longer supported. Contact Support at https://www.npmjs.com/support for more info.
npm ERR! npm WARN deprecated readdir-scoped-modules@1.1.0: This functionality has been moved to @npmcli/fs
npm ERR! npm WARN deprecated read-package-json@2.1.2: This package is no longer supported. Please use @npmcli/package-json instead.
npm ERR! npm WARN deprecated read-installed@4.0.3: This package is no longer supported.
npm ERR! npm ERR! code 127
npm ERR! npm ERR! path /home/user/.npm/_cacache/tmp/git-cloneXXXXXXzbG8Wz/node_modules/rollup
npm ERR! npm ERR! command failed
npm ERR! npm ERR! command sh -c patch-package
npm ERR! npm ERR! sh: 1: patch-package: not found
npm ERR! 
npm ERR! npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-11-07T17_18_37_777Z-debug-0.log

npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-11-07T17_18_30_634Z-debug-0.log
npm install did not exit successfully

thread 'main' (61600) panicked at src/tools/rustdoc-gui-test/src/main.rs:63:10:
unable to install browser-ui-test: Custom { kind: Other, error: "npm install returned exit code exit status: 127" }
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f/library/std/src/panicking.rs:698:5
   1: core::panicking::panic_fmt
             at /rustc/3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f/library/core/src/panicking.rs:80:14

[rust-bors]

💥 Test timed out after 21600s