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rust-bors merged 1 commit into from
Jun 20, 2026
+13 −14
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Improve NumBufferTrait associated items #157976

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4 changes: 2 additions & 2 deletions library/core/src/fmt/num.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -758,7 +758,7 @@ impl u128 {
/// ```
#[stable(feature = "int_format_into", since = "CURRENT_RUSTC_VERSION")]
pub fn format_into(self, buf: &mut NumBuffer<Self>) -> &str {

@JonathanBrouwer JonathanBrouwer Jun 21, 2026 •
edited by rustbot
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Well, it's being used here at least, isn't it?

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@GuillaumeGomez GuillaumeGomez Jun 21, 2026

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That's the API added alongside NumBuffer. It's not used anywhere (yet?) in rustc/rustdoc/std.

@JonathanBrouwer JonathanBrouwer Jun 21, 2026

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Ah ok, then I have no clue what's causing this, weird

@GuillaumeGomez GuillaumeGomez Jun 21, 2026

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Welcome to the club. :')

let diff = buf.capacity() - U128_MAX_DEC_N;
let diff = buf.buf.len() - U128_MAX_DEC_N;
// FIXME: Once const generics are better, use `NumberBufferTrait::BUF_SIZE` as generic const
// for `fmt_u128_inner`.
//
Expand Down Expand Up @@ -790,7 +790,7 @@ impl i128 {
/// ```
#[stable(feature = "int_format_into", since = "CURRENT_RUSTC_VERSION")]
pub fn format_into(self, buf: &mut NumBuffer<Self>) -> &str {
let diff = buf.capacity() - U128_MAX_DEC_N;
let diff = buf.buf.len() - U128_MAX_DEC_N;
// FIXME: Once const generics are better, use `NumberBufferTrait::BUF_SIZE` as generic const
// for `fmt_u128_inner`.
//
Expand Down
23 changes: 11 additions & 12 deletions library/core/src/fmt/num_buffer.rs
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Expand Up @@ -3,9 +3,12 @@ use crate::mem::MaybeUninit;
/// Trait used to describe the maximum number of digits in decimal base of the implemented integer.
#[unstable(feature = "fmt_internals", issue = "none")]
pub trait NumBufferTrait {
/// Maximum number of digits in decimal base of the implemented integer.
/// Used for initializing the `NumberBuffer` value.
#[unstable(feature = "fmt_internals", issue = "none")]
const BUF_SIZE: usize;
const DEFAULT: Self::Buf;
/// The actual underlying type.
#[unstable(feature = "fmt_internals", issue = "none")]
type Buf: AsRef<[MaybeUninit<u8>]> + AsMut<[MaybeUninit<u8>]>;
}

macro_rules! impl_NumBufferTrait {
Expand All @@ -14,11 +17,13 @@ macro_rules! impl_NumBufferTrait {
#[stable(feature = "int_format_into", since = "CURRENT_RUSTC_VERSION")]
impl NumBufferTrait for $signed {
// `+ 2` and not `+ 1` to include the `-` character.
const BUF_SIZE: usize = $signed::MAX.ilog(10) as usize + 2;
const DEFAULT: Self::Buf = [MaybeUninit::<u8>::uninit(); $signed::MAX.ilog(10) as usize + 2];
type Buf = [MaybeUninit<u8>; $signed::MAX.ilog(10) as usize + 2];
}
#[stable(feature = "int_format_into", since = "CURRENT_RUSTC_VERSION")]
impl NumBufferTrait for $unsigned {
const BUF_SIZE: usize = $unsigned::MAX.ilog(10) as usize + 1;
const DEFAULT: Self::Buf = [MaybeUninit::<u8>::uninit(); $unsigned::MAX.ilog(10) as usize + 1];
type Buf = [MaybeUninit<u8>; $unsigned::MAX.ilog(10) as usize + 1];
}
)*
}
Expand Down Expand Up @@ -52,9 +57,7 @@ impl_NumBufferTrait! {
/// ```
#[stable(feature = "int_format_into", since = "CURRENT_RUSTC_VERSION")]
pub struct NumBuffer<T: NumBufferTrait> {
// FIXME: Once const generics feature is working, use `T::BUF_SIZE` instead of 40.
pub(crate) buf: [MaybeUninit<u8>; 40],
// FIXME: Remove this field once we can actually use `T`.
pub(crate) buf: T::Buf,
phantom: core::marker::PhantomData<T>,
}

Expand All @@ -72,10 +75,6 @@ impl<T: NumBufferTrait> NumBuffer<T> {
#[rustc_const_stable(feature = "int_format_into", since = "CURRENT_RUSTC_VERSION")]
pub const fn new() -> Self {
// FIXME: Once const generics feature is working, use `T::BUF_SIZE` instead of 40.
NumBuffer { buf: [MaybeUninit::<u8>::uninit(); 40], phantom: core::marker::PhantomData }
}

pub(crate) const fn capacity(&self) -> usize {
self.buf.len()
NumBuffer { buf: T::DEFAULT, phantom: core::marker::PhantomData }
}
}
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