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with #![no_std] enabled, 'panic!("{{}}")' and 'panic!("{{}}",)' are different #48042
Description
Discovered while trying to implement fixes for #46241. Possible back-compat hazard for trailing comma support, but hopefully not.
macro_rules! panic {
() => (/* default message */);
($msg:expr) => (/* use msg as literal */);
($fmt:expr, $($arg:tt)+) => (/* use format_args! */);
}macro_rules! panic {
() => (/* default message */);
($msg:expr) => (/* use msg as literal */);
($fmt:expr, $($arg:tt)*) => (/* use format_args! */);
}Can you spot the difference?
The definition in std fails to support trailing commas after a single string literal:
fn main() {
panic!("a");
panic!("a",); //~ ERROR unexpected end of macro invocation
panic!("{}");
panic!("{}",); //~ ERROR unexpected end of macro invocation
}The definition in core accepts a trailing comma, but follows a completely different code-path with potentially different semantics.
#![no_std]
fn main() {
panic!("a");
panic!("a",);
panic!("{}");
panic!("{}",); //~ ERROR 1 positional argument in format string, but no arguments were given
}Fortuitously, it seems to me that it libcore's definition produces equivalent behavior for panic!(<literal>,) and panic!(<literal>) in all cases where both forms successfully compile, meaning it should hopefully be safe to change panic!(<literal>,) to behave like panic!(<literal>).
Activity
ExpHP commentedon Feb 7, 2018
Crap. Found an example of the sort of back-compat hazard I was talking about.
In today's rust, the following two programs are different:
[-]with #![no_std] enabled, 'panic!(<literal>,)' compiles and is not necessarily the same as 'panic!(<literal>)'[/-][+]with #![no_std] enabled, 'panic!(<literal>,)' compiles and is different from 'panic!(<literal>)'[/+][-]with #![no_std] enabled, 'panic!(<literal>,)' compiles and is different from 'panic!(<literal>)'[/-][+]with #![no_std] enabled, 'panic!("{{}}")' and 'panic!("{{}}",)' are different[/+]Rollup merge of rust-lang#48056 - ExpHP:macro-commas, r=dtolnay
Auto merge of #48056 - ExpHP:macro-commas, r=dtolnay