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Don't use match-destructuring for derived ops on structs. #98446

Merged
merged 3 commits into from
Jul 4, 2022
Merged

Don't use match-destructuring for derived ops on structs. #98446

merged 3 commits into from
Jul 4, 2022

Commits on Jul 4, 2022

  1. Add an interesting case to the deriving-all-codegen.rs test.

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    nnethercote committed Jul 4, 2022
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  2. Comment fixes. 

    Remove an out-of-date sentence, and fix a typo.
    @nnethercote
    nnethercote committed Jul 4, 2022
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  3. Don't use match-destructuring for derived ops on structs. 

    All derive ops currently use match-destructuring to access fields. This
is reasonable for enums, but sub-optimal for structs. E.g.:
```
fn eq(&self, other: &Point) -> bool {
    match *other {
	Self { x: ref __self_1_0, y: ref __self_1_1 } =>
	    match *self {
		Self { x: ref __self_0_0, y: ref __self_0_1 } =>
		    (*__self_0_0) == (*__self_1_0) &&
			(*__self_0_1) == (*__self_1_1),
	    },
    }
}
```
This commit changes derive ops on structs to use field access instead, e.g.:
```
fn eq(&self, other: &Point) -> bool {
    self.x == other.x && self.y == other.y
}
```
This is faster to compile, results in smaller binaries, and is simpler to
generate. Unfortunately, we have to keep the old pattern generating code around
for `repr(packed)` structs because something like `&self.x` (which doesn't show
up in `PartialEq` ops, but does show up in `Debug` and `Hash` ops) isn't
allowed. But this commit at least changes those cases to use let-destructuring
instead of match-destructuring, e.g.:
```
fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {
    {
	let Self(ref __self_0_0) = *self;
	{ ::core::hash::Hash::hash(&(*__self_0_0), state) }
    }
}
```
There are some unnecessary blocks remaining in the generated code, but I
will fix them in a follow-up PR.
    @nnethercote
    nnethercote committed Jul 4, 2022
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