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Don't allow flattened format_args in const. Fixes https://github.com/rust-lang/rust/issues/139136 Fixes https://github.com/rust-lang/rust/issues/139621 We allow `format_args!("a")` in const, but don't allow any format_args with arguments in const, such as `format_args!("{}", arg)`. However, we accidentally allow `format_args!("hello {}", "world")` in const, as it gets flattened to `format_args!("hello world")`. This also applies to panic in const. This wasn't supposed to happen. I added protection against this in the format args flattening code, ~~but I accidentally marked a function as const that shouldn't have been const~~ but this was removed in https://github.com/rust-lang/rust/pull/135139. This is a breaking change. The crater found no breakage, however. This breaks things like: ```rust const _: () = if false { panic!("a {}", "a") }; ``` and ```rust const F: std::fmt::Arguments<'static> = format_args!("a {}", "a"); ```