`src/doc/reference` doctests is failing in PR CI

[jieyouxu]

Failure seen in #149574 (comment), I don't think my changes are related?

##[group]Testing stage2 mdbook src/doc/reference (x86_64-unknown-linux-gnu)
[2025-12-03T02:14:07Z WARN  mdbook::preprocess::cmd] The command wasn't found, is the "spec" preprocessor installed?
[2025-12-03T02:14:07Z WARN  mdbook::preprocess::cmd] 	Command: cargo run --release --manifest-path mdbook-spec/Cargo.toml
[2025-12-03T02:14:14Z ERROR mdbook::book] rustdoc returned an error:
    
    --- stdout
    
    running 12 tests
    test items/constant-items.md - Constant_items (line 108) ... ok
    test items/constant-items.md - Constant_items (line 127) ... ok
    test items/constant-items.md - Constant_items (line 115) ... ok
    test items/constant-items.md - Constant_items (line 138) ... ok
    test items/constant-items.md - Constant_items (line 151) ... ok
    test items/constant-items.md - Constant_items (line 165) ... ok
    test items/constant-items.md - Constant_items (line 54) ... ok
    test items/constant-items.md - Constant_items (line 33) ... ok
    test items/constant-items.md - Constant_items::Constants_with_destructors (line 191) ... ok
    test items/constant-items.md - Constant_items (line 62) ... ok
    test items/constant-items.md - Constant_items::Unnamed_constant (line 216) ... ok
    test items/constant-items.md - Constant_items::Unnamed_constant (line 227) ... ok
    
    test result: ok. 12 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
    
    
    running 5 tests
    test items/constant-items.md - Constant_items (line 84) - compile fail ... ok
    test items/constant-items.md - Constant_items (line 173) - compile fail ... ok
    test items/constant-items.md - Constant_items (line 97) - compile fail ... FAILED
    test items/constant-items.md - Constant_items (line 71) - compile fail ... FAILED
    test items/constant-items.md - Constant_items::Evaluation (line 244) - compile fail ... ok
    
    failures:
    
    ---- items/constant-items.md - Constant_items (line 97) stdout ----
    Test compiled successfully, but it's marked `compile_fail`.
    ---- items/constant-items.md - Constant_items (line 71) stdout ----
    Test compiled successfully, but it's marked `compile_fail`.
    
    failures:
        items/constant-items.md - Constant_items (line 71)
        items/constant-items.md - Constant_items (line 97)
    
    test result: FAILED. 3 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.12s
    
    all doctests ran in 0.49s; merged doctests compilation took 0.37s
    
    --- stderr
    
Error: One or more tests failed
	finished in 34.603 seconds
##[endgroup]

[jieyouxu]

cc @ehuss in case you have any guesses. I'll try rerunning PR CI, but just wanted to open an issue since I didn't expect that failure.

(I can't look into this rn.)