Slicing after str::starts_with performs unnecessary checks #72558
Description
Without unsafe (playground)
pub struct Foo<'a> { string: &'a str, } impl Foo<'_> { pub fn bar(&mut self) -> Option<&str> { if self.string.starts_with("[[") { let bracket = &self.string[..1]; self.string = &self.string[2..]; Some(bracket) } else { None } } }With unsafe (playground)
pub struct Foo<'a> { string: &'a str, } impl Foo<'_> { pub fn bar(&mut self) -> Option<&str> { if self.string.starts_with("[[") { let bracket = unsafe { self.string.get_unchecked(..1) }; self.string = unsafe { self.string.get_unchecked(2..) }; Some(bracket) } else { None } } }I'm not terribly familiar with assembly, but the compiler is clearly performing checks where they aren't necessary. Given the knowledge that the string starts with a literal (in this case two ASCII bytes), we should be able to slice without any additional performance costs.