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document bind_by_move_pattern_guards #720

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Nov 30, 2019
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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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15 changes: 13 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 +126,16 @@ let message = match maybe_digit {
> assert_eq!(i.get(), 2);
> ```

A pattern guard may refer to the variables bound within the pattern they follow.
Before evaluating the guard, a shared reference is taken to the part of the
scrutinee the variable matches on. While evaluating the guard,
this shared reference is then used when accessing the variable.
Only when the guard evaluates to true is the value moved, or copied,
from the scrutinee into the variable. This allows shared borrows to be used
inside guards without moving out of the scrutinee in case guard fails to match.
Moreover, by holding a shared reference while evaluating the guard,
mutation inside guards is also prevented.

## Attributes on match arms

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