Functionality for int_format_into for integer types

[madhav-madhusoodanan]

ACP approval: rust-lang/libs-team#546 (comment)

Context

1. Implemented integer formatting into a fixed-size buffer without relying on fmt.
2. Added a NumBuffer type to help with this process.

Associated Issue

• Tracking Issue for integer formatting into a fixed-size buffer #138215

r? @hanna-kruppe

[rustbot]

Failed to set assignee to hanna-kruppe: invalid assignee

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[madhav-madhusoodanan]

@rustbot author

[madhav-madhusoodanan]

@rustbot label -O-hermit -O-SGX -O-solid -O-wasi -O-wasm -O-windows -O-unix

[madhav-madhusoodanan]

I was able to gain more clarity on what was needed, and I've moved the updates to the fmt module.

Primarily I've broken down impl_Display into impl_NumBuffer (the primitives) and impl_Display so that I can reuse functionality into the new macro impl_FormatInto.

Sorry for the long history in this PR. Please let me know if this looks good.

[hanna-kruppe]

Thank you for working on this! The bird's eye view of the diff is now basically what I'd expect. I left some smaller comments I've noticed while scrolling through. I don't expect I'll have the time to review the implementation details as carefully as unsafe code deserves, so please work out those details with a reviewer who actually has permissions on this repository.

[hanna-kruppe]

Likewise, none of these methods should be public I think. Or, if they need to be public e.g. for unit tests of the standard library, they should be behind a perma-unstable "for internal use only" feature gate, not tied to int_format_into / #138215.

[madhav-madhusoodanan]

Other than the new function, I don't see usage of the other functions in other crates.
Thank you for suggesting me.

[madhav-madhusoodanan]

Thank you for working on this! The bird's eye view of the diff is now basically what I'd expect. I left some smaller comments I've noticed while scrolling through. I don't expect I'll have the time to review the implementation details as carefully as unsafe code deserves, so please work out those details with a reviewer who actually has permissions on this repository.

I must thank you for patiently going through my updates multiple times. There was a lot for me to learn.

[scottmcm]

I'm going to re-roll to get someone hopefully more familiar with the formatting stuff.
r? libs

[rust-log-analyzer]

The job x86_64-gnu-llvm-18 failed! Check out the build log: (web) (plain)

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#19 exporting to docker image format
#19 sending tarball 19.9s done
#19 DONE 24.6s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Listening on address 127.0.0.1:4226
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'build.print-step-timings', '--enable-verbose-tests', '--set', 'build.metrics', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: build.build          := x86_64-unknown-linux-gnu
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10