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Add format_to! macro #76240

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Add format_to! macro #76240

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25 changes: 25 additions & 0 deletions library/alloc/src/macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -107,3 +107,28 @@ macro_rules! format {
res
}}
}

/// Like [`format!`], but appends to an existing string
///
///
/// [`format!`]: crate::format
///
/// # Examples
///
/// ```
/// #![feature(format_to)]
///
/// let mut buf = String::new();
/// format_to!(buf, "hello");
/// format_to!(buf, ", world!");
/// assert_eq!(buf, "hello, world!");
/// ```
#[macro_export]
#[unstable(feature = "format_to", issue = "none", reason = "new API")]
#[allow_internal_unstable(liballoc_internals)]
macro_rules! format_to {
($buf:expr $(, $($arg:tt)*)? ) => {{
// Redirect via method call syntax to get autoref behavior
$buf.__push_fmt($crate::__export::format_args!($($($arg)*)?));
}}
}
6 changes: 6 additions & 0 deletions library/alloc/src/string.rs
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Expand Up @@ -1585,6 +1585,12 @@ impl String {
let slice = self.vec.into_boxed_slice();
unsafe { from_boxed_utf8_unchecked(slice) }
}

#[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]
#[doc(hidden)]
pub fn __push_fmt(&mut self, args: fmt::Arguments<'_>) {
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Why can't it just be an inherent write_fmt method, so write!(...) always works on String instead of introducing a semi-hidden method and a whole new macro?

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That won’t be backward compatible: write_fmt already exists on String (via Write trait) and returns a result.

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@CryZe CryZe Sep 2, 2020 •
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Well I guess it would have to have the same signature. So I guess the unwrapping would unfortunately stay in that case. Though maybe it could use Result<(), !> and somehow be compatible and not require unwrapping... (Not sure if the stars align for that one though🤔)

fmt::Write::write_fmt(self, args).unwrap();
}
}

impl FromUtf8Error {
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3 changes: 3 additions & 0 deletions library/std/src/lib.rs
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Expand Up @@ -261,6 +261,7 @@
#![feature(external_doc)]
#![feature(fn_traits)]
#![feature(format_args_nl)]
#![feature(format_to)]
#![feature(gen_future)]
#![feature(generator_trait)]
#![feature(global_asm)]
Expand Down Expand Up @@ -373,6 +374,8 @@ pub use alloc_crate::boxed;
pub use alloc_crate::fmt;
#[stable(feature = "rust1", since = "1.0.0")]
pub use alloc_crate::format;
#[unstable(feature = "format_to", issue = "none", reason = "new API")]
pub use alloc_crate::format_to;
#[stable(feature = "rust1", since = "1.0.0")]
pub use alloc_crate::rc;
#[stable(feature = "rust1", since = "1.0.0")]
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