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Briefly touch on match ergonomics (#2581)

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This is done in the speaker notes as it's a relatively minor point, but
one that students should have in the back of their mind when they
wonder, "hey, how does a `&Foo` match against `Foo` patterns??"
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Dustin J. Mitchell
2025-01-22 11:47:23 -05:00
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6
src/pattern-matching/destructuring-structs.md
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@@ -14,6 +14,12 @@ Like tuples, Struct can also be destructured by matching:
- Change the literal values in `foo` to match with the other patterns. - Change the literal values in `foo` to match with the other patterns.
- Add a new field to `Foo` and make changes to the pattern as needed. - Add a new field to `Foo` and make changes to the pattern as needed.
- Try `match &foo` and check the type of captures. The pattern syntax remains
the same, but the captures become shared references. This is
[match ergonomics](https://rust-lang.github.io/rfcs/2005-match-ergonomics.html)
and is often useful with `match self` when implementing methods on an enum.
- The same effect occurs with `match &mut foo`: the captures become exclusive
references.
- The distinction between a capture and a constant expression can be hard to - The distinction between a capture and a constant expression can be hard to
spot. Try changing the `2` in the second arm to a variable, and see that it spot. Try changing the `2` in the second arm to a variable, and see that it
subtly doesn't work. Change it to a `const` and see it working again. subtly doesn't work. Change it to a `const` and see it working again.