u8::to_string() specialisation (far less asm).

[gilescope]

#73533 pointed out that u8::to_string() was heavy on the codegen: https://rust.godbolt.org/z/1MnhWY

Interestingly whether we use [0;1] or [0;4] (the latter is used for the char specialisation) it seems to make no difference to the generated code.

(I tried putting an unstable attribute but apparently it's not effective there so I put a high since value for now. Please point me in the right direction as to how I should put that attribute on?)

[rust-highfive]

r? @dtolnay

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[rust-log-analyzer]

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

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

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
    Checking rand v0.7.3
    Checking core v0.0.0 (/checkout/library/core)
    Checking alloc v0.0.0 (/checkout/library/alloc)
    Checking std v0.0.0 (/checkout/library/std)
error[E0277]: the trait bound `u8: Neg` is not satisfied
    |
    |
434 |     assert_eq!((-1).to_string(), "-1");
    |                ^^^^ the trait `Neg` is not implemented for `u8`
error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `alloc`
error: could not compile `alloc`

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed
command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "check" "--target" "i686-pc-windows-gnu" "-Zbinary-dep-depinfo" "-j" "16" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace compiler-builtins-c" "--manifest-path" "/checkout/library/test/Cargo.toml" "--all-targets" "-p" "test" "-p" "term" "-p" "std" "-p" "alloc" "-p" "unwind" "-p" "proc_macro" "-p" "panic_unwind" "-p" "panic_abort" "-p" "core" "--message-format" "json-render-diagnostics"
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu --all-targets
Build completed unsuccessfully in 0:00:46

[gilescope]

Ah wait. My head has been in utf8 land too long. u8 should be formatted as a number. Scratch this.