Add format_to! macro

It's a version of `format!` which allows appending to an existing
`String`.

Unlike `write!`, it doesn't require a trait import and unwrap.

See

https://internals.rust-lang.org/t/add-an-easier-way-to-append-t-display-to-string-to-stdlib/10607

for some preliminary discussion.

See

https://github.com/rust-analyzer/rust-analyzer/blob/3ffa915cbcf4d7a3988142cd94da0463acc87c8a/crates/stdx/src/macros.rs#L12-L19

for an equivalent macro used in rust-analyzer quite a bit.

Author: matklad

library/alloc/src/macros.rs

@@ -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)*)?));
+    }}
+}

library/alloc/src/string.rs

@@ -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<'_>) {
+        fmt::Write::write_fmt(self, args).unwrap();
+    }
 }
 
 impl FromUtf8Error {

library/std/src/lib.rs

@@ -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)]
@@ -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")]