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A surprising enum size optimization in the Rust compiler

(jpfennell.com)
146 points returningfory2 | 3 comments | 07 Apr 25 22:30 UTC | HN request time: 1.841s | source
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lordnacho ◴[10 Apr 25 16:42 UTC] No.43645689[source]▶
Does the niche optimization require the compiler to know things about the type? So that only certain specializations will work?

Also, how do I get some code to do the memory layout vizualizer, perhaps one that is a bit more advanced and knows what pointers are?

replies(1): >>43645823 #
1. pornel ◴[10 Apr 25 16:54 UTC] No.43645823[source]▶
Internally the compiler tracks what "niches" exist in types, like minimum and maximum valid values, and takes the unused values for enum tags.

One thing it can't use is padding in structs, because references to individual fields must remain valid, and they don't guarantee that padding will be preserved.

replies(2): >>43645982 #>>43646104 #
2. ◴[10 Apr 25 17:08 UTC] No.43645982[source]▶
3. ◴[10 Apr 25 17:21 UTC] No.43646104[source]▶