Auto merge of #90473 - joshtriplett:stabilize-format-args-capture, r=Mark-Simulacrum

stabilize format args capture

Works as expected, and there are widespread reports of success with it, as well as interest in it.

RFC: rust-lang/rfcs#2795
Tracking issue: https://github.com/rust-lang/rust/issues/67984

Addressing items from the tracking issue:

- We don't support capturing arguments from a non-literal format string like `format_args!(concat!(...))`. We could add that in a future enhancement, or we can decide that it isn't supported (as suggested in https://github.com/rust-lang/rust/issues/67984#issuecomment-801394736 ).
- I've updated the documentation.
- `panic!` now supports capture as well.
- There are potentially opportunities to further improve diagnostics for invalid usage, such as if it looks like the user tried to use an expression rather than a variable. However, such cases are all already caught and provide reasonable syntax errors now, and we can always provided even friendlier diagnostics in the future.

compiler/rustc_typeck/src/lib.rs

@@ -58,7 +58,7 @@ This API is completely unstable and subject to change.
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(bool_to_option)]
 #![feature(crate_visibility_modifier)]
-#![feature(format_args_capture)]
+#![cfg_attr(bootstrap, feature(format_args_capture))]
 #![feature(if_let_guard)]
 #![feature(in_band_lifetimes)]
 #![feature(is_sorted)]