Add new failed_merged_doctest_compilation rustdoc lint

[GuillaumeGomez]

Finally took the time to write this lint. We need to have a compiler context in order to be able to emit the lint, but I'm not sure if DiagCtxtHandle doesn't provide it somehow... Anyway, I'm expecting performance regression because we keep the compiler and its internals around for much longer, stacking doctests memory on top of the allocated memory to the current crate (which is why I'd really love to see if it's possible to emit lints from DiagCtxtHandle somehow, gonna investigate).

Also: both lint name and lint message are very much up to debate.

r? @fmease

[rustbot]

fmease is currently at their maximum review capacity.
They may take a while to respond.

[GuillaumeGomez]

@bors2 try @rust-timer queue

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

error: doctest failed, to rerun pass `-p alloc --doc`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustdoc --edition=2024 --crate-type lib --color always --crate-name alloc --test library/alloc/src/lib.rs --test-run-directory /checkout/library/alloc --target x86_64-unknown-linux-gnu -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/release/deps -L native=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/build/compiler_builtins-ab4e95d684d268d9/out --test-args -Z --test-args unstable-options --test-args --format --test-args json --warn=unexpected_cfgs --check-cfg 'cfg(no_global_oom_handling)' --check-cfg 'cfg(no_rc)' --check-cfg 'cfg(no_sync)' --extern 'priv:alloc=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps/liballoc-7ba7b7a48df3e66b.rlib' --extern 'priv:compiler_builtins=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps/libcompiler_builtins-646e9c051dbeece2.rlib' --extern core=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps/libcore-116781e8a0b7fefc.rlib -C embed-bitcode=no --cfg 'feature="compiler-builtins-c"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("compiler-builtins-c", "compiler-builtins-mem", "compiler-builtins-no-f16-f128", "optimize_for_size"))' -Csymbol-mangling-version=legacy '--check-cfg=cfg(feature,values(any()))' -Zunstable-options -Dwarnings '-Wrustdoc::invalid_codeblock_attributes' --crate-version '1.93.0-nightly (e7571561e 2025-11-12)' '-Zcrate-attr=doc(html_root_url="https://doc.rust-lang.org/nightly/")' '-Zcrate-attr=warn(rust_2018_idioms)' --error-format human -Zunstable-options` (exit status: 1)
note: test exited abnormally; to see the full output pass --no-capture to the harness.
Bootstrap failed while executing `test --stage 1 core alloc std test proc_macro`
Build completed unsuccessfully in 0:06:41
  local time: Wed Nov 12 14:43:10 UTC 2025
  network time: Wed, 12 Nov 2025 14:43:10 GMT
##[error]Process completed with exit code 1.

[GuillaumeGomez]

Another thing I thought: should we display errors?

[rust-bors]

☀️ Try build successful (CI)
Build commit: fa55e90 (fa55e9058b427aeb73ae208b2105b8e21713b3b3, parent: 95aeb4696545eb4c9cbb68516f2912770e3846ea)

[rust-timer]

Finished benchmarking commit (fa55e90): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (primary 1.5%)

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

	mean	range	count
Regressions ❌ (primary)	1.5%	[1.5%, 1.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.5%	[1.5%, 1.5%]	1

Cycles

Results (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)	4.4%	[4.4%, 4.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.8%, -2.1%]	4
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 474.976s -> 474.831s (-0.03%)
Artifact size: 391.06 MiB -> 391.05 MiB (-0.00%)