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Auto merge of rust-lang#121001 - nyurik:optimize-core-fmt, r=<try>
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perf: improve write_fmt to handle simple strings

In case format string has no arguments, simplify its implementation with a direct call to `output.write_str(value)`. This builds on `@dtolnay` original [suggestion](serde-rs/serde#2697 (comment)). This does not change any expectations because the original `fn write()` implementation calls `write_str` for parts of the format string.

```rust
write!(f, "text")  ->  f.write_str("text")
```

```diff
 /// [`write!`]: crate::write!
+#[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn write(output: &mut dyn Write, args: Arguments<'_>) -> Result {
+    if let Some(s) = args.as_str() { output.write_str(s) } else { write_internal(output, args) }
+}
+
+/// Actual implementation of the [`write`], but without the simple string optimization.
+fn write_internal(output: &mut dyn Write, args: Arguments<'_>) -> Result {
     let mut formatter = Formatter::new(output);
     let mut idx = 0;
```

* Hopefully it will improve the simple case for the rust-lang#99012
* Another related (original?) issues rust-lang#10761
* Previous similar attempt to fix it by by `@Kobzol` rust-lang#100700

CC: `@m-ou-se` as probably the biggest expert in everything `format!`
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bors committed Feb 20, 2024
2 parents 0b9f6ad + 1eee9f5 commit f2db114
Showing 1 changed file with 6 additions and 0 deletions.
6 changes: 6 additions & 0 deletions library/core/src/fmt/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1102,8 +1102,14 @@ pub trait UpperExp {
/// ```
///
/// [`write!`]: crate::write!
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn write(output: &mut dyn Write, args: Arguments<'_>) -> Result {
if let Some(s) = args.as_str() { output.write_str(s) } else { write_internal(output, args) }
}

/// Actual implementation of the [`write`], but without the simple string optimization.
fn write_internal(output: &mut dyn Write, args: Arguments<'_>) -> Result {
let mut formatter = Formatter::new(output);
let mut idx = 0;

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