std: avoid tearing dbg! prints

[joboet]

Fixes #136703.

This is an alternative to #149859. Instead of formatting everything into a string, this PR makes multi-expression dbg! expand into multiple nested matches, with the final match containing a single eprint!. By using macro recursion and relying on hygiene, this allows naming every bound value in that eprint!.

Draft because I'll need to fix a clippy lint.

CC @orlp

r? libs

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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checking C++ file formatting
spellchecking files
building external tool typos from package [email protected]
finished building tool typos
error: `invokation` should be `invocation`
    ╭▸ library/std/src/macros.rs:392:64
    │
392 │             // The location returned here is that of the macro invokation, so
    ╰╴                                                               ━━━━━━━━━━
rerun tidy with `--extra-checks=spellcheck --bless` to fix typos
tidy [extra_checks]: checks with external tool 'typos' failed
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/yarn --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1320:29

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:02:42
  local time: Wed Dec 10 23:43:36 UTC 2025
  network time: Wed, 10 Dec 2025 23:43:36 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[orlp]

Neat solution :) I wasn't sure if we could use helper macros.

[orlp]

You have an extra closing square bracket here.