Slicing after str::starts_with performs unnecessary checks #72558
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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.
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Area: str and StringCategory: An issue proposing an enhancement or a PR with one.Category: An issue highlighting optimization opportunities or PRs implementing suchIssue: Problems and improvements with respect to performance of generated code.Relevant to the compiler team, which will review and decide on the PR/issue.
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