Ensure we can optimize out does-nothing `drop`s like `array::IntoIter<NotNeedsDrop, N>`
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Description
Noticed this looking at #139415.
We already do this for !needs_drop types, but std::array::IntoIter<&mut usize, 1> does have a Drop, and thus needs_drop is true because it's not trivially obvious that the Drop::drop won't actually do anything.
But if you look at https://rust.godbolt.org/z/raWrq6rsY you can see that we're emitting the call to the drop.
This probably is about inlining drop shims in MIR? Might be another way, though, if somehow we could notice in cg_ssa.
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Research direction
Start with the related issue #139415 and reproduce the linked Godbolt example for std::array::IntoIter<&mut usize, 1>. Investigate the suggested MIR drop-shim inlining and cg_ssa alternatives, then verify that the generated code no longer emits a call for this no-op drop case.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- compilers, performance
- Issue type
- Bug
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100