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Flatten/inline format_args!() and (string and int) literal arguments into format_args!() Implements https://github.com/rust-lang/rust/issues/78356 Gated behind `-Zflatten-format-args=yes`. Part of #99012 This change inlines string literals, integer literals and nested format_args!() into format_args!() during ast lowering, making all of the following pairs result in equivalent hir: ```rust println!("Hello, {}!", "World"); println!("Hello, World!"); ``` ```rust println!("[info] {}", format_args!("error")); println!("[info] error"); ``` ```rust println!("[{}] {}", status, format_args!("error: {}", msg)); println!("[{}] error: {}", status, msg); ``` ```rust println!("{} + {} = {}", 1, 2, 1 + 2); println!("1 + 2 = {}", 1 + 2); ``` And so on. This is useful for macros. E.g. a `log::info!()` macro could just pass the tokens from the user directly into a `format_args!()` that gets efficiently flattened/inlined into a `format_args!("info: {}")`. It also means that `dbg!(x)` will have its file, line, and expression name inlined: ```rust eprintln!("[{}:{}] {} = {:#?}", file!(), line!(), stringify!(x), x); // before eprintln!("[example.rs:1] x = {:#?}", x); // after ``` Which can be nice in some cases, but also means a lot more unique static strings than before if dbg!() is used a lot.