Rollup merge of #53299 - MagnumOpus21:fix-macro-write, r=steveklabnik

Updated core/macros.rs to note it works in a no_std environment.

Fixes #45797
This PR updates the documentation of `write!` to note it works in a `no_std` environment, and adds an
example to showcase this. In a `no_std` environment, the author of the code is responsible for the
implementation of the `Write` trait. This example will work out of the box with `no_std`, but the
implementation of `Write` is expected to be provided by the user.

r? @steveklabnik

src/libcore/macros.rs

@@ -349,6 +349,26 @@ macro_rules! try {
 /// write!(&mut v, "s = {:?}", s).unwrap(); // uses io::Write::write_fmt
 /// assert_eq!(v, b"s = \"abc 123\"");
 /// ```
+///
+/// Note: This macro can be used in `no_std` setups as well
+/// In a `no_std` setup you are responsible for the
+/// implementation details of the components.
+///
+/// ```no_run
+/// # extern crate core;
+/// use core::fmt::Write;
+///
+/// struct Example;
+///
+/// impl Write for Example {
+///     fn write_str(&mut self, _s: &str) -> core::fmt::Result {
+///          unimplemented!();
+///     }
+/// }
+///
+/// let mut m = Example{};
+/// write!(&mut m, "Hello World").expect("Not written");
+/// ```
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! write {