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Auto merge of #74802 - Mark-Simulacrum:reland-74069, r=nnethercote

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Reland #74069

Investigation in #74716 has concluded that this PR is indeed not a regression (and in fact the rollup itself is not either).

This reverts the revert in #74611.

r? @nnethercote cc @eddyb
This commit is contained in:
bors 2020-08-11 21:23:00 +00:00
commit 873fc463bd
4 changed files with 51 additions and 7 deletions
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6
src/test/ui/print_type_sizes/niche-filling.stdout
View file

@ -8,12 +8,12 @@ print-type-size variant `Some`: 12 bytes
print-type-size field `.0`: 12 bytes
print-type-size variant `None`: 0 bytes
print-type-size type: `EmbeddedDiscr`: 8 bytes, alignment: 4 bytes
print-type-size discriminant: 1 bytes
print-type-size variant `Record`: 7 bytes
print-type-size field `.val`: 4 bytes
print-type-size field `.post`: 2 bytes
print-type-size field `.pre`: 1 bytes
print-type-size field `.post`: 2 bytes
print-type-size field `.val`: 4 bytes
print-type-size variant `None`: 0 bytes
print-type-size end padding: 1 bytes
print-type-size type: `MyOption<Union1<std::num::NonZeroU32>>`: 8 bytes, alignment: 4 bytes
print-type-size discriminant: 4 bytes
print-type-size variant `Some`: 4 bytes

25
src/test/ui/type-sizes.rs
View file

@ -5,6 +5,7 @@
#![feature(never_type)]
use std::mem::size_of;
use std::num::NonZeroU8;
struct t {a: u8, b: i8}
struct u {a: u8, b: i8, c: u8}
@ -102,6 +103,23 @@ enum Option2<A, B> {
None
}
// Two layouts are considered for `CanBeNicheFilledButShouldnt`:
// Niche-filling:
// { u32 (4 bytes), NonZeroU8 + tag in niche (1 byte), padding (3 bytes) }
// Tagged:
// { tag (1 byte), NonZeroU8 (1 byte), padding (2 bytes), u32 (4 bytes) }
// Both are the same size (due to padding),
// but the tagged layout is better as the tag creates a niche with 254 invalid values,
// allowing types like `Option<Option<CanBeNicheFilledButShouldnt>>` to fit into 8 bytes.
pub enum CanBeNicheFilledButShouldnt {
A(NonZeroU8, u32),
B
}
pub enum AlwaysTaggedBecauseItHasNoNiche {
A(u8, u32),
B
}
pub fn main() {
assert_eq!(size_of::<u8>(), 1 as usize);
assert_eq!(size_of::<u32>(), 4 as usize);
@ -145,4 +163,11 @@ pub fn main() {
assert_eq!(size_of::<Option<Option<(&(), bool)>>>(), size_of::<(bool, &())>());
assert_eq!(size_of::<Option<Option2<bool, &()>>>(), size_of::<(bool, &())>());
assert_eq!(size_of::<Option<Option2<&(), bool>>>(), size_of::<(bool, &())>());
assert_eq!(size_of::<CanBeNicheFilledButShouldnt>(), 8);
assert_eq!(size_of::<Option<CanBeNicheFilledButShouldnt>>(), 8);
assert_eq!(size_of::<Option<Option<CanBeNicheFilledButShouldnt>>>(), 8);
assert_eq!(size_of::<AlwaysTaggedBecauseItHasNoNiche>(), 8);
assert_eq!(size_of::<Option<AlwaysTaggedBecauseItHasNoNiche>>(), 8);
assert_eq!(size_of::<Option<Option<AlwaysTaggedBecauseItHasNoNiche>>>(), 8);
}