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Improve io::Error::downcast #149521

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@a1phyr a1phyr commented Dec 1, 2025 •
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Rewrite this function to help to compiler understand what is going on here.

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 1, 2025
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rustbot commented Dec 1, 2025

r? @joboet

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joboet commented Dec 2, 2025

The downcast change looks fine, but I'm not convinced about the box removal being beneficial: wouldn't this increase the size of io::Result to three words instead of two?

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a1phyr commented Dec 3, 2025

Wow, I somehow managed to convince myself that the ErrorData enum was boxed in this case to make Error a single word...

Given the current situation, removing this box is probably not beneficial indeed. I'll remove the second commit.

@a1phyr a1phyr changed the title io::Error: avoid double-boxing of "custom" variant on 32 bits targets io::Error::downcast: avoid reallocation in case of failure Dec 3, 2025
@a1phyr a1phyr changed the title io::Error::downcast: avoid reallocation in case of failure Improve io::Error::downcast: Dec 3, 2025
@a1phyr a1phyr changed the title Improve io::Error::downcast: Improve io::Error::downcast Dec 3, 2025
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joboet commented Dec 3, 2025

@bors r+

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bors commented Dec 3, 2025

📌 Commit 7de190a has been approved by joboet

It is now in the queue for this repository.

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 3, 2025
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