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document bind_by_move_pattern_guards #720

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matthewjasper merged 2 commits into from
Nov 30, 2019
Merged

document bind_by_move_pattern_guards #720

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c03b84c
document `bind_by_move_pattern_guards`
Centril Nov 20, 2019
2bb5698
further improve match guard bind-by-move docs
Centril Nov 30, 2019
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14 changes: 12 additions & 2 deletions src/expressions/match-expr.md
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Expand Up @@ -91,10 +91,11 @@ Every binding in each `|` separated pattern must appear in all of the patterns
in the arm. Every binding of the same name must have the same type, and have
the same binding mode.

## Match guards

Match arms can accept _match guards_ to further refine the
criteria for matching a case. Pattern guards appear after the pattern and
consist of a bool-typed expression following the `if` keyword. A pattern guard
may refer to the variables bound within the pattern they follow.
consist of a `bool`-typed expression following the `if` keyword.

When the pattern matches successfully, the pattern guard expression is executed.
If the expression evaluates to true, the pattern is successfully matched against.
Expand Down Expand Up @@ -125,6 +126,15 @@ let message = match maybe_digit {
> assert_eq!(i.get(), 2);
> ```

A pattern guard may refer to the variables bound within the pattern they follow.
When such a variable's binding mode is by-value,
a shared reference is taken to it before evaluating the guard.
When accessing the variable in the guard by shared reference,
the shared reference taken before evaluating the guard is used.
Only when the guard expression evaluates to true is the by-value variable
moved, or copied, into the arm's body. This allows shared borrows to be used
inside guards without moving out of the scrutinee in case guard fails to match.

## Attributes on match arms

Outer attributes are allowed on match arms. The only attributes that have
Expand Down