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Enable flatten-format-args by default. Part of https://github.com/rust-lang/rust/issues/99012. This enables the `flatten-format-args` feature that was added by https://github.com/rust-lang/rust/pull/106824: > 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. This is mostly an optimization, except that it will be visible through [`fmt::Arguments::as_str()`](https://doc.rust-lang.org/nightly/std/fmt/struct.Arguments.html#method.as_str). In https://github.com/rust-lang/rust/pull/106823, there was already a libs-api FCP about the documentation of `fmt::Arguments::as_str()` to allow it to give `Some` rather than `None` depending on optimizations like this. That was just a documentation update though. This PR is the one that actually makes the user visible change: ```rust assert_eq!(format_args!("abc").as_str(), Some("abc")); // Unchanged. assert_eq!(format_args!("ab{}", "c").as_str(), Some("abc")); // Was `None` before! ``` |
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