100% CPU usage when running doc tests

## The Problem
Since a while, and I don't know since when, running any variant of `cargo test --doc` puts my CPU at 100% so hard, that it crashes my PC if I don't Ctrl+C early enough. Running other tests works just fine, namely unit tests, I haven't tried running any integration tests.

**I expected to see this happen:**
It runs the doc tests basically as fast as regular tests.

**Instead, this happened:**
My PC locks up and sometimes crashes.

## Meta
### Toolchains
I tried this on various versions, and it has persisted across all of them:
- `1.82-nightly`
  ```
  rustc 1.82.0-nightly (612a33f20 2024-07-29)
  binary: rustc
  commit-hash: 612a33f20b9b2c27380edbc4b26a01433ed114bc
  commit-date: 2024-07-29
  host: x86_64-unknown-linux-gnu
  release: 1.82.0-nightly
  LLVM version: 18.1.7
  ```
- `1.80-stable`
  ```
  rustc 1.80.0 (051478957 2024-07-21)
  binary: rustc
  commit-hash: 051478957371ee0084a7c0913941d2a8c4757bb9
  commit-date: 2024-07-21
  host: x86_64-unknown-linux-gnu
  release: 1.80.0
  LLVM version: 18.1.7
  ```
- `1.79.1-stable`
  - Don't have it installed anymore.
- `rustc 1.74.1`
  ```
  rustc 1.74.1 (a28077b28 2023-12-04)
  binary: rustc
  commit-hash: a28077b28a02b92985b3a3faecf92813155f1ea1
  commit-date: 2023-12-04
  host: x86_64-unknown-linux-gnu
  release: 1.74.1
  LLVM version: 17.0.4
  ```

### Repositories
I have tried this on various repositories like:
- `cargo +1.80 test --doc --workspace` on [typst-test#a8aea584](https://github.com/tingerrr/typst-test/tree/a8aea584), fails on the `typst-test-lib` crate
- `cargo +1.74.1 test --doc --workspace` on [typst-test#599717fc](https://github.com/tingerrr/typst-test/tree/599717fc), fails on the `typst-test-lib` crate
- `cargo +1.80 test --doc --workspace` on [helix#3fcf168c](https://github.com/helix-editor/helix/tree/3fcf168c), fails mostly on the `helix-tui` crate
- `cargo +1.80 test --doc --workspace` on [typst#c4dd6fa0](https://github.com/typst/typst/tree/c4dd6fa0), largely doesn't fail, due to the lack of runnable tests

### Hardware
This happens consistently on two PCs:

<details><summary>PC 1</summary>
<p>

`Linux twinkbook 6.10.2-arch1-1 #1 SMP PREEMPT_DYNAMIC Sat, 27 Jul 2024 16:49:55 +0000 x86_64 GNU/Linux`

Output of `lscpu`:
```
Architecture:             x86_64
  CPU op-mode(s):         32-bit, 64-bit
  Address sizes:          39 bits physical, 48 bits virtual
  Byte Order:             Little Endian
CPU(s):                   8
  On-line CPU(s) list:    0-7
Vendor ID:                GenuineIntel
  Model name:             11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
    CPU family:           6
    Model:                140
    Thread(s) per core:   2
    Core(s) per socket:   4
    Socket(s):            1
    Stepping:             1
    CPU(s) scaling MHz:   24%
    CPU max MHz:          4700,0000
    CPU min MHz:          400,0000
    BogoMIPS:             5608,00
    Flags:                fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse
                          36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb
                           rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopo
                          logy nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64
                           monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1
                           sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdr
                          and lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l2 cdp_l2 ssbd ibrs
                           ibpb stibp ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad fsg
                          sbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid rdt_a avx512f avx51
                          2dq rdseed adx smap avx512ifma clflushopt clwb intel_pt avx512cd sha_
                          ni avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves split_lock_detect
                           user_shstk dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp
                          _epp hwp_pkg_req vnmi avx512vbmi umip pku ospke avx512_vbmi2 gfni vae
                          s vpclmulqdq avx512_vnni avx512_bitalg avx512_vpopcntdq rdpid movdiri
                           movdir64b fsrm avx512_vp2intersect md_clear ibt flush_l1d arch_capab
                          ilities
Virtualization features:
  Virtualization:         VT-x
Caches (sum of all):
  L1d:                    192 KiB (4 instances)
  L1i:                    128 KiB (4 instances)
  L2:                     5 MiB (4 instances)
  L3:                     12 MiB (1 instance)
NUMA:
  NUMA node(s):           1
  NUMA node0 CPU(s):      0-7
Vulnerabilities:
  Gather data sampling:   Mitigation; Microcode
  Itlb multihit:          Not affected
  L1tf:                   Not affected
  Mds:                    Not affected
  Meltdown:               Not affected
  Mmio stale data:        Not affected
  Reg file data sampling: Not affected
  Retbleed:               Not affected
  Spec rstack overflow:   Not affected
  Spec store bypass:      Mitigation; Speculative Store Bypass disabled via prctl
  Spectre v1:             Mitigation; usercopy/swapgs barriers and __user pointer sanitization
  Spectre v2:             Mitigation; Enhanced / Automatic IBRS; IBPB conditional; RSB filling;
                           PBRSB-eIBRS SW sequence; BHI SW loop, KVM SW loop
  Srbds:                  Not affected
  Tsx async abort:        Not affected
```

</p>
</details>

<details><summary>PC 2</summary>
<p>

`Linux hsh 6.10.0-arch1-2 #1 SMP PREEMPT_DYNAMIC Mon, 22 Jul 2024 17:28:23 +0000 x86_64 GNU/Linux
`

Output of `lscpu`:
```
Architecture:             x86_64
  CPU op-mode(s):         32-bit, 64-bit
  Address sizes:          39 bits physical, 48 bits virtual
  Byte Order:             Little Endian
CPU(s):                   8
  On-line CPU(s) list:    0-7
Vendor ID:                GenuineIntel
  Model name:             Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz
    CPU family:           6
    Model:                158
    Thread(s) per core:   2
    Core(s) per socket:   4
    Socket(s):            1
    Stepping:             9
    CPU(s) scaling MHz:   18%
    CPU max MHz:          4500.0000
    CPU min MHz:          800.0000
    BogoMIPS:             8403.00
    Flags:                fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xto
                          pology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm
                           abm 3dnowprefetch cpuid_fault pti ssbd ibrs ibpb stibp tpr_shadow flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid mpx rdseed adx smap clflushopt intel_pt xsaveopt xsavec
                          xgetbv1 xsaves dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp vnmi md_clear flush_l1d arch_capabilities
Virtualization features:
  Virtualization:         VT-x
Caches (sum of all):
  L1d:                    128 KiB (4 instances)
  L1i:                    128 KiB (4 instances)
  L2:                     1 MiB (4 instances)
  L3:                     8 MiB (1 instance)
NUMA:
  NUMA node(s):           1
  NUMA node0 CPU(s):      0-7
Vulnerabilities:
  Gather data sampling:   Mitigation; Microcode
  Itlb multihit:          KVM: Mitigation: VMX disabled
  L1tf:                   Mitigation; PTE Inversion; VMX conditional cache flushes, SMT vulnerable
  Mds:                    Mitigation; Clear CPU buffers; SMT vulnerable
  Meltdown:               Mitigation; PTI
  Mmio stale data:        Mitigation; Clear CPU buffers; SMT vulnerable
  Reg file data sampling: Not affected
  Retbleed:               Mitigation; IBRS
  Spec rstack overflow:   Not affected
  Spec store bypass:      Mitigation; Speculative Store Bypass disabled via prctl
  Spectre v1:             Mitigation; usercopy/swapgs barriers and __user pointer sanitization
  Spectre v2:             Mitigation; IBRS; IBPB conditional; STIBP conditional; RSB filling; PBRSB-eIBRS Not affected; BHI Not affected
  Srbds:                  Mitigation; Microcode
  Tsx async abort:        Mitigation; TSX disabled
```

</p>
</details>

I'll happily add more info if requested.

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100% CPU usage when running doc tests #128538

tingerrr
openedon Aug 2, 2024

The Problem

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