Auto merge of rust-lang#97480 - conradludgate:faster-format-literals, r=joshtriplett

improve format impl for literals

The basic idea of this change can be seen here https://godbolt.org/z/MT37cWoe1.

Updates the format impl to have a fast path for string literals and the default path for regular format args.

This change will allow `format!("string literal")` to be used interchangably with `"string literal".to_owned()`.

This would be relevant in the case of `f!"string literal"` being legal (rust-lang/rfcs#3267) in which case it would be the easiest way to create owned strings from literals, while also being just as efficient as any other impl

Author: bors

library/alloc/src/fmt.rs

@@ -604,9 +604,14 @@ use crate::string;
 #[cfg(not(no_global_oom_handling))]
 #[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[inline]
 pub fn format(args: Arguments<'_>) -> string::String {
-    let capacity = args.estimated_capacity();
-    let mut output = string::String::with_capacity(capacity);
-    output.write_fmt(args).expect("a formatting trait implementation returned an error");
-    output
+    fn format_inner(args: Arguments<'_>) -> string::String {
+        let capacity = args.estimated_capacity();
+        let mut output = string::String::with_capacity(capacity);
+        output.write_fmt(args).expect("a formatting trait implementation returned an error");
+        output
+    }
+
+    args.as_str().map_or_else(|| format_inner(args), crate::borrow::ToOwned::to_owned)
 }