Work-around optimiser deficiencies in Range iterator.
#26390
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The conditional mutation of the previous implementation resulted in poor code, making it unconditional makes `Range` less well behaved as an Iterator (but still legal) but also makes it fast. The intention is that this change will be reverted when rustc/LLVM handle the best-behaved implementation better. cc rust-lang#24660
| // mishandles the version that places the mutation inside the | ||
| // `if`: it seems to optimise the `Option<i32>` in a way that | ||
| // confuses it. | ||
| let mut n = &self.start + &A::one(); |
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Unfortunately I think doing this addition outside the guard causes the panic semantics of this iterator to change, which I believe will be a regression. For example this code does not panic today, but I suspect it will panic with this implementation:
fn main() {
for i in 255u8..255 {
println!("{}", i);
}
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Yea, didn't notice that corner case...
Anyway, closing is perfectly alright with me, I just wanted to move the PR forward :)
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Hm ok, I think for now we may want to not merge this due to that, but perhaps we can think of a new idea in the future!
|
Closing because I think this may be backwards incompatible |
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closes #24705
the issue in the other PR was that the inclusive range iterator depended on internals of the normal range iterator
which then causes the match arm reachability to fail
This was not detected by the test-suite, because the failure happens during stage1 compilation of the standard libraries.
r? @aturon
cc @huonw @pythonesque @pnkfelix