Implement `int_format_into` feature by GuillaumeGomez · Pull Request #142098 · rust-lang/rust

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 · 2025-07-01T12:00:46Z

@Amanieu I went to the end of the usage of get_unchecked everywhere in #143277 and basically, there is absolutely no difference, the compiler is already optimizing all the code correctly.

hanna-kruppe · 2025-07-01T12:14:10Z

Note that there were some recent PRs removing unsafe code from int Display impls that didn’t make a difference (#135265, #136594).

GuillaumeGomez · 2025-07-01T12:16:02Z

Yeah I will just revert the get_unchecked calls I added here as well. No point in keeping them.

GuillaumeGomez · 2025-07-01T12:25:30Z

And done.

Amanieu · 2025-07-02T06:13:33Z

library/core/src/fmt/num_buffer.rs

+    /// Initializes internal buffer.
+    #[unstable(feature = "int_format_into", issue = "138215")]
+    pub const fn new() -> Self {
+        // FIXME: Once const generics feature is working, use `T::BUF_SIZE` instead of 40.


Can you add a const { assert!(T::BUF_SIZE <= 40) } here.

Amanieu · 2025-07-02T06:14:17Z

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()


Maybe return T::BUF_SIZE here for forward-compatiblity.

Amanieu · 2025-07-02T06:14:58Z

library/core/src/fmt/num.rs

@@ -248,6 +253,13 @@ macro_rules! impl_Display {
                issue = "none"
            )]
            pub fn _fmt<'a>(self, buf: &'a mut [MaybeUninit::<u8>]) -> &'a str {


This needs to be an unsafe function because it makes assumptions on the size of buf.

Amanieu · 2025-07-02T06:17:17Z

library/core/src/fmt/num.rs

                    }
                }
            }
+            cur


Should this be curr? How does this even compile?

Oh, that's because of optimize_for_size. I'm surprised it's not tested in CI.

Amanieu · 2025-07-02T06:17:42Z

library/core/src/fmt/num.rs

@@ -336,20 +421,25 @@ macro_rules! impl_Display {
            unsafe {


Unsafe is not needed here since get_unchecked is not used.

Amanieu · 2025-07-02T06:18:51Z

library/core/src/fmt/num.rs

@@ -598,12 +688,20 @@ impl u128 {
        issue = "none"
    )]
    pub fn _fmt<'a>(self, buf: &'a mut [MaybeUninit<u8>]) -> &'a str {


Same here, this needs to be unsafe because it assumes the slice size. Or _fmt_inner should be made safe.

…n-128-integers, r=tgross35 Use a distinct `ToString` implementation for `u128` and `i128` Part of rust-lang#135543. Follow-up of rust-lang#136264. When working on rust-lang#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, } ```

GuillaumeGomez · 2025-07-04T09:05:36Z

Applied suggestions.

Amanieu · 2025-07-07T23:37:59Z

@bors r+

bors · 2025-07-07T23:38:35Z

📌 Commit 9943c19 has been approved by Amanieu

It is now in the queue for this repository.

Rollup of 7 pull requests Successful merges: - #142098 (Implement `int_format_into` feature) - #143567 (Point to correct argument in Func Call when Self type fails trait bound) - #143570 (consider nested cases for duplicate RPITIT) - #143571 (remove `has_nested` from builtin candidates) - #143586 (Fix wrong cache event query key) - #143589 (const-block-as-pattern: do not refer to no-longer-existing nightly feature) - #143608 (Fix in std::String docs) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #142098 - GuillaumeGomez:int_format_into, r=Amanieu Implement `int_format_into` feature 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 - #138215 r? `@hanna-kruppe`

…Amanieu Implement `int_format_into` feature I took over rust-lang#138338 with `@madhav-madhusoodanan's` approval. Since rust-lang#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 - rust-lang#138215 r? `@hanna-kruppe`

RalfJung · 2025-07-28T07:42:41Z

library/core/src/fmt/num_buffer.rs

+#[unstable(feature = "int_format_into", issue = "138215")]
+#[derive(Debug)]
+pub struct NumBuffer<T: NumBufferTrait> {
+    // FIXME: Once const generics feature is working, use `T::BUF_SIZE` instead of 40.


AFAIK that's way off... the current plan, last I heard from @BoxyUwU, is to allow this only for associated consts whose values are restricted to be plain consts or other type-system-safe consts. Your associated consts use arithmetic and function calls.

However we could hard-code the right value for each type so I guess there's at least some, albeit ugly, part forward here.

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 July 3, 2025 18:57

Implement int_format_into feature

82c6037

Add test for int_format_into feature

7e23144

Use slice_buffer_to_str in GenericRadix::fmt_int

9943c19

GuillaumeGomez force-pushed the int_format_into branch from fb17077 to 9943c19 Compare July 3, 2025 16:57

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 7, 2025

matthiaskrgr mentioned this pull request Jul 8, 2025

Rollup of 7 pull requests #143621

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bors merged commit d41f046 into rust-lang:master Jul 8, 2025
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rustbot added this to the 1.90.0 milestone Jul 8, 2025

hanna-kruppe mentioned this pull request Jul 8, 2025

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

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GuillaumeGomez deleted the int_format_into branch July 8, 2025 09:29

GuillaumeGomez mentioned this pull request Jul 8, 2025

Make int_format_into API more flexible #143636

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nazo6 mentioned this pull request Jul 11, 2025

On some targets, specifying optimize_for_size in build-std-features causes the core crate build to fail #143804

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sammysheep mentioned this pull request Jul 12, 2025

M4 max shows unexpected results for nightly format_into() vs itoa crate sammysheep/itoa-feedback#1

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