Clarify niche optimization on enums with reprs #315
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A repr on an enum with fields suppresses the optimization, but a repr on a fieldless enum does not.
tmiasko
reviewed
Oct 9, 2021
| @@ -110,6 +110,8 @@ assert_eq!(8, size_of::<MyOption<&u16>>()); | |||
| assert_eq!(16, size_of::<MyReprOption<&u16>>()); | |||
| ``` | |||
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| This optimization still applies to fieldless enums with an explicit `repr(u*)`, `repr(i*)`, or `repr(C)`. | |||
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Which optimization does this refer to, niche optimization? Niche optimization uses niche exposed by a data full variant to encode the discriminant. It does not apply to enums without any data.
matthiaskrgr
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Oct 12, 2021
Update books ## nomicon 5 commits in 2747c4bb2cbc0639b733793ddb0bf4e9daa2634e..2d66852a27c5d0ec50ae021820d1de22caa2b1bd 2021-09-19 17:33:32 +0900 to 2021-10-07 19:00:37 +0900 - Fix typo/minor grammar error in subtyping.md (rust-lang/nomicon#317) - doc: clarify `thread::scoped::JoinGuard` chapter (rust-lang/nomicon#313) - Clarify niche optimization on enums with reprs (rust-lang/nomicon#315) - Update rc decrement snipped (rust-lang/nomicon#316) - Remove useless `unsafe`, `mut` and ptr casts in example in `send-and-sync.md` (rust-lang/nomicon#308) ## reference 2 commits in 13747275bd14c2d2b453100498532f9ae5504769..b5c68b02984f74e99d1f1b332029e05f607e2660 2021-09-24 17:44:04 +0900 to 2021-10-02 08:11:35 -0700 - Use subtrait/supertrait, not sub-trait, super-trait. (rust-lang/reference#1095) - Fixed link typo in Generics.md (rust-lang/reference#1094) ## rust-by-example 3 commits in 28aca4a36962c709bce301c03114b5589381dfb8..9a60624fcad0140826c44389571dc622917cd632 2021-09-25 08:19:51 -0300 to 2021-10-04 08:13:53 -0300 - Change 1..n + 1 to 1..=n (rust-lang/rust-by-example#1467) - Close rust-lang/rust-by-example#1464 (rust-lang/rust-by-example#1465) - Fix incorrect formatted print hint; closes rust-lang/rust-by-example#1459. (rust-lang/rust-by-example#1466) ## rustc-dev-guide 11 commits in d1f03cb..fba15a4 2021-09-24 12:00:29 +0900 to 2021-10-12 08:52:21 +0900 - Update some docs from 2021-10 date triage (rust-lang/rustc-dev-guide#1230) - Recommend `rustfmt --edition=2021` - Update sanitizer documentation - Mention needs-sanitizer-hwaddress directive - Address review comments - Document tracing awesomeness - Move log/tracing instructions to its own file - Update src/closure.md - Fix accidental HTML tag - Update links to `rustc_mir` in Two Phase Borrows (rust-lang/rustc-dev-guide#1226) - Update some docs from 2021-10 date triage (rust-lang/rustc-dev-guide#1224) ## edition-guide 1 commits in 2d9b1b9da706de24650fdc5c3b0182f55c82115d..7c0088ca744d293a5f4b1e2ac378e7c23d30fe55 2021-08-31 10:44:09 +0200 to 2021-10-05 13:28:05 +0200 - Clarify cargo resolver behavior in a workspace. (rust-lang/edition-guide#267) ## embedded-book 2 commits in 4c76da9ddb4650203c129fceffdea95a3466c205..270fccd339e5972d9c900e788f197e81a0bcd956 2021-09-12 12:43:03 +0000 to 2021-10-06 16:28:48 +0000 - Update section 1.4 to mention that some additional packages required (rust-embedded/book#304) - Change `-file-headers` to `--file-headers` (rust-embedded/book#303)
the8472
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Oct 12, 2021
Update books ## nomicon 5 commits in 2747c4bb2cbc0639b733793ddb0bf4e9daa2634e..2d66852a27c5d0ec50ae021820d1de22caa2b1bd 2021-09-19 17:33:32 +0900 to 2021-10-07 19:00:37 +0900 - Fix typo/minor grammar error in subtyping.md (rust-lang/nomicon#317) - doc: clarify `thread::scoped::JoinGuard` chapter (rust-lang/nomicon#313) - Clarify niche optimization on enums with reprs (rust-lang/nomicon#315) - Update rc decrement snipped (rust-lang/nomicon#316) - Remove useless `unsafe`, `mut` and ptr casts in example in `send-and-sync.md` (rust-lang/nomicon#308) ## reference 2 commits in 13747275bd14c2d2b453100498532f9ae5504769..b5c68b02984f74e99d1f1b332029e05f607e2660 2021-09-24 17:44:04 +0900 to 2021-10-02 08:11:35 -0700 - Use subtrait/supertrait, not sub-trait, super-trait. (rust-lang/reference#1095) - Fixed link typo in Generics.md (rust-lang/reference#1094) ## rust-by-example 3 commits in 28aca4a36962c709bce301c03114b5589381dfb8..9a60624fcad0140826c44389571dc622917cd632 2021-09-25 08:19:51 -0300 to 2021-10-04 08:13:53 -0300 - Change 1..n + 1 to 1..=n (rust-lang/rust-by-example#1467) - Close rust-lang/rust-by-example#1464 (rust-lang/rust-by-example#1465) - Fix incorrect formatted print hint; closes rust-lang/rust-by-example#1459. (rust-lang/rust-by-example#1466) ## rustc-dev-guide 11 commits in d1f03cb..fba15a4 2021-09-24 12:00:29 +0900 to 2021-10-12 08:52:21 +0900 - Update some docs from 2021-10 date triage (rust-lang/rustc-dev-guide#1230) - Recommend `rustfmt --edition=2021` - Update sanitizer documentation - Mention needs-sanitizer-hwaddress directive - Address review comments - Document tracing awesomeness - Move log/tracing instructions to its own file - Update src/closure.md - Fix accidental HTML tag - Update links to `rustc_mir` in Two Phase Borrows (rust-lang/rustc-dev-guide#1226) - Update some docs from 2021-10 date triage (rust-lang/rustc-dev-guide#1224) ## edition-guide 1 commits in 2d9b1b9da706de24650fdc5c3b0182f55c82115d..7c0088ca744d293a5f4b1e2ac378e7c23d30fe55 2021-08-31 10:44:09 +0200 to 2021-10-05 13:28:05 +0200 - Clarify cargo resolver behavior in a workspace. (rust-lang/edition-guide#267) ## embedded-book 2 commits in 4c76da9ddb4650203c129fceffdea95a3466c205..270fccd339e5972d9c900e788f197e81a0bcd956 2021-09-12 12:43:03 +0000 to 2021-10-06 16:28:48 +0000 - Update section 1.4 to mention that some additional packages required (rust-embedded/book#304) - Change `-file-headers` to `--file-headers` (rust-embedded/book#303)
the8472
added a commit
to the8472/rust
that referenced
this pull request
Oct 12, 2021
Update books ## nomicon 5 commits in 2747c4bb2cbc0639b733793ddb0bf4e9daa2634e..2d66852a27c5d0ec50ae021820d1de22caa2b1bd 2021-09-19 17:33:32 +0900 to 2021-10-07 19:00:37 +0900 - Fix typo/minor grammar error in subtyping.md (rust-lang/nomicon#317) - doc: clarify `thread::scoped::JoinGuard` chapter (rust-lang/nomicon#313) - Clarify niche optimization on enums with reprs (rust-lang/nomicon#315) - Update rc decrement snipped (rust-lang/nomicon#316) - Remove useless `unsafe`, `mut` and ptr casts in example in `send-and-sync.md` (rust-lang/nomicon#308) ## reference 2 commits in 13747275bd14c2d2b453100498532f9ae5504769..b5c68b02984f74e99d1f1b332029e05f607e2660 2021-09-24 17:44:04 +0900 to 2021-10-02 08:11:35 -0700 - Use subtrait/supertrait, not sub-trait, super-trait. (rust-lang/reference#1095) - Fixed link typo in Generics.md (rust-lang/reference#1094) ## rust-by-example 3 commits in 28aca4a36962c709bce301c03114b5589381dfb8..9a60624fcad0140826c44389571dc622917cd632 2021-09-25 08:19:51 -0300 to 2021-10-04 08:13:53 -0300 - Change 1..n + 1 to 1..=n (rust-lang/rust-by-example#1467) - Close rust-lang/rust-by-example#1464 (rust-lang/rust-by-example#1465) - Fix incorrect formatted print hint; closes rust-lang/rust-by-example#1459. (rust-lang/rust-by-example#1466) ## rustc-dev-guide 11 commits in d1f03cb..fba15a4 2021-09-24 12:00:29 +0900 to 2021-10-12 08:52:21 +0900 - Update some docs from 2021-10 date triage (rust-lang/rustc-dev-guide#1230) - Recommend `rustfmt --edition=2021` - Update sanitizer documentation - Mention needs-sanitizer-hwaddress directive - Address review comments - Document tracing awesomeness - Move log/tracing instructions to its own file - Update src/closure.md - Fix accidental HTML tag - Update links to `rustc_mir` in Two Phase Borrows (rust-lang/rustc-dev-guide#1226) - Update some docs from 2021-10 date triage (rust-lang/rustc-dev-guide#1224) ## edition-guide 1 commits in 2d9b1b9da706de24650fdc5c3b0182f55c82115d..7c0088ca744d293a5f4b1e2ac378e7c23d30fe55 2021-08-31 10:44:09 +0200 to 2021-10-05 13:28:05 +0200 - Clarify cargo resolver behavior in a workspace. (rust-lang/edition-guide#267) ## embedded-book 2 commits in 4c76da9ddb4650203c129fceffdea95a3466c205..270fccd339e5972d9c900e788f197e81a0bcd956 2021-09-12 12:43:03 +0000 to 2021-10-06 16:28:48 +0000 - Update section 1.4 to mention that some additional packages required (rust-embedded/book#304) - Change `-file-headers` to `--file-headers` (rust-embedded/book#303)
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