Implement `int_format_into` feature #142098

GuillaumeGomez · 2025-06-05T21:33:04Z

I took over #138338 with @madhav-madhusoodanan's approval.

Since #136264, a lot of changes happened so I made use of them to reduce the number of changes.

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

Associated Issue

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

r? @hanna-kruppe

rustbot · 2025-06-05T21:33:07Z

Failed to set assignee to hanna-kruppe: invalid assignee

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GuillaumeGomez · 2025-06-05T21:33:47Z

library/alloctests/tests/num.rs

+        assert_eq!(ret, value);
+
+        let mut buffer = NumBuffer::<$int>::new();
+        assert_eq!(value.format_into(&mut buffer), s.as_str());


Since format_into is not part of a trait, I needed to switch from a function to a macro to avoid the generics limitations.

You could write and implement a trait right here, in the test - right?

I could indeed, but would be better than the current code?

Less churn; other than that I don't have strong feelings.

If someone else agrees with you I'll make the change (lazyness++ 😆).

library/core/src/fmt/num_buffer.rs

GuillaumeGomez · 2025-06-05T21:38:08Z

r? @Amanieu

library/core/src/fmt/num.rs

GuillaumeGomez · 2025-06-06T12:31:13Z

Applied suggestions and opened another PR for the number of digits of u128 computation to not add even more changes to this PR.

library/core/src/fmt/num.rs

bors · 2025-06-07T04:19:14Z

☔ The latest upstream changes (presumably #142133) made this pull request unmergeable. Please resolve the merge conflicts.

GuillaumeGomez · 2025-06-10T14:29:34Z

Fixed merge conflict.

GuillaumeGomez · 2025-06-17T22:07:41Z

Maybe you want to take a look at this one too @tgross35 ? :)

GuillaumeGomez · 2025-06-19T11:12:19Z

I improved docs for the new API and rebased too.

…egers, r=tgross35 Use a distinct `ToString` implementation for `u128` and `i128` Part of #135543. Follow-up of #136264. When working on #142098, I realized that `i128` and `u128` could also benefit from a distinct `ToString` implementation so here it. The last commit is just me realizing that I forgot to add the format tests for `usize` and `isize`. Here is the bench comparison: | bench name | last nightly | with this PR | diff | |-|-|-|-| | bench_i128 | 29.25 ns/iter (+/- 0.66) | 17.52 ns/iter (+/- 0.7) | -40.1% | | bench_u128 | 34.06 ns/iter (+/- 0.21) | 16.1 ns/iter (+/- 0.6) | -52.7% | I used this code to test: ```rust #![feature(test)] extern crate test; use test::{Bencher, black_box}; #[inline(always)] fn convert_to_string<T: ToString>(n: T) -> String { n.to_string() } macro_rules! decl_benches { ($($name:ident: $ty:ident,)+) => { $( #[bench] fn $name(c: &mut Bencher) { c.iter(|| convert_to_string(black_box({ let nb: $ty = 20; nb }))); } )+ } } decl_benches! { bench_u128: u128, bench_i128: i128, } ```

bors · 2025-06-20T06:43:19Z

☔ The latest upstream changes (presumably #142294) made this pull request unmergeable. Please resolve the merge conflicts.

GuillaumeGomez · 2025-06-20T12:44:18Z

Fixed merge conflict.

bors · 2025-06-20T23:48:39Z

☔ The latest upstream changes (presumably #142795) made this pull request unmergeable. Please resolve the merge conflicts.

…egers, r=tgross35 Use a distinct `ToString` implementation for `u128` and `i128` Part of rust-lang/rust#135543. Follow-up of rust-lang/rust#136264. When working on rust-lang/rust#142098, I realized that `i128` and `u128` could also benefit from a distinct `ToString` implementation so here it. The last commit is just me realizing that I forgot to add the format tests for `usize` and `isize`. Here is the bench comparison: | bench name | last nightly | with this PR | diff | |-|-|-|-| | bench_i128 | 29.25 ns/iter (+/- 0.66) | 17.52 ns/iter (+/- 0.7) | -40.1% | | bench_u128 | 34.06 ns/iter (+/- 0.21) | 16.1 ns/iter (+/- 0.6) | -52.7% | I used this code to test: ```rust #![feature(test)] extern crate test; use test::{Bencher, black_box}; #[inline(always)] fn convert_to_string<T: ToString>(n: T) -> String { n.to_string() } macro_rules! decl_benches { ($($name:ident: $ty:ident,)+) => { $( #[bench] fn $name(c: &mut Bencher) { c.iter(|| convert_to_string(black_box({ let nb: $ty = 20; nb }))); } )+ } } decl_benches! { bench_u128: u128, bench_i128: i128, } ```

library/core/src/fmt/num_buffer.rs

ChayimFriedman2 · 2025-06-22T01:12:43Z

library/core/src/fmt/num_buffer.rs

+    /// Returns the length of the buffer.
+    #[unstable(feature = "int_format_into", issue = "138215")]
+    pub const fn len(&self) -> usize {
+        self.buf.len()


This will always return 40, is this intended?

For now it's always 40 but I do hope in the future it will depend on the integer size when associated const support will be more advanced.

…egers, r=tgross35 Use a distinct `ToString` implementation for `u128` and `i128` Part of rust-lang/rust#135543. Follow-up of rust-lang/rust#136264. When working on rust-lang/rust#142098, I realized that `i128` and `u128` could also benefit from a distinct `ToString` implementation so here it. The last commit is just me realizing that I forgot to add the format tests for `usize` and `isize`. Here is the bench comparison: | bench name | last nightly | with this PR | diff | |-|-|-|-| | bench_i128 | 29.25 ns/iter (+/- 0.66) | 17.52 ns/iter (+/- 0.7) | -40.1% | | bench_u128 | 34.06 ns/iter (+/- 0.21) | 16.1 ns/iter (+/- 0.6) | -52.7% | I used this code to test: ```rust #![feature(test)] extern crate test; use test::{Bencher, black_box}; #[inline(always)] fn convert_to_string<T: ToString>(n: T) -> String { n.to_string() } macro_rules! decl_benches { ($($name:ident: $ty:ident,)+) => { $( #[bench] fn $name(c: &mut Bencher) { c.iter(|| convert_to_string(black_box({ let nb: $ty = 20; nb }))); } )+ } } decl_benches! { bench_u128: u128, bench_i128: i128, } ```

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jun 5, 2025

GuillaumeGomez commented Jun 5, 2025

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Functionality for int_format_into for integer types #138338

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GuillaumeGomez changed the title ~~Int format into~~ Implement int_format_into feature Jun 6, 2025

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Use a distinct ToString implementation for u128 and i128 #142294

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GuillaumeGomez added 3 commits June 23, 2025 13:50

Implement int_format_into feature

0a99434

Add test for int_format_into feature

5dce91f

Use slice_buffer_to_str in GenericRadix::fmt_int

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Implement int_format_into feature #142098

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Implement `int_format_into` feature #142098

Implement `int_format_into` feature #142098

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