Summary

Rust currently forbids pattern guards on match arms with move-bound variables. Allowing them would increase the applicability of pattern guards.

Motivation

Currently, if you attempt to use guards on a match arm with a move-bound variable, e.g.

struct A { a: Box<int> }

fn foo(n: int) {
    let x = A { a: box n };
    let y = match x {
        A { a: v } if *v == 42 => v,
        _ => box 0
    };
}

you get an error:

test.rs:6:16: 6:17 error: cannot bind by-move into a pattern guard
test.rs:6         A { a: v } if *v == 42 => v,
                         ^

This should be permitted in cases where the guard only accesses the moved value by reference or copies out of derived paths.

This allows for succinct code with less pattern matching duplication and a minimum number of copies at runtime. The lack of this feature was encountered by @kmcallister when developing Servo's new HTML 5 parser.

Detailed design

This change requires all occurrences of move-bound pattern variables in the guard to be treated as paths to the values being matched before they are moved, rather than the moved values themselves. Any moves of matched values into the bound variables would occur on the control flow edge between the guard and the arm's expression. There would be no changes to the handling of reference-bound pattern variables.

The arm would be treated as its own nested scope with respect to borrows, so that pattern-bound variables would be able to be borrowed and dereferenced freely in the guard, but these borrows would not be in scope in the arm's expression. Since the guard dominates the expression and the move into the pattern-bound variable, moves of either the match's head expression or any pattern-bound variables in the guard would trigger an error.

The following examples would be accepted:

struct A { a: Box<int> }

impl A {
    fn get(&self) -> int { *self.a }
}

fn foo(n: int) {
    let x = A { a: box n };
    let y = match x {
        A { a: v } if *v == 42 => v,
        _ => box 0
    };
}

fn bar(n: int) {
    let x = A { a: box n };
    let y = match x {
        A { a: v } if x.get() == 42 => v,
        _ => box 0
    };
}

fn baz(n: int) {
    let x = A { a: box n };
    let y = match x {
        A { a: v } if *v.clone() == 42 => v,
        _ => box 0
    };
}

This example would be rejected, due to a double move of v:

struct A { a: Box<int> }

fn foo(n: int) {
    let x = A { a: box n };
    let y = match x {
        A { a: v } if { drop(v); true } => v,
        _ => box 0
    };
}

This example would also be rejected, even though there is no use of the move-bound variable in the first arm's expression, since the move into the bound variable would be moving the same value a second time:

enum VecWrapper { A(Vec<int>) }

fn foo(x: VecWrapper) -> uint {
    match x {
        A(v) if { drop(v); false } => 1,
        A(v) => v.len()
    }
}

There are issues with mutation of the bound values, but that is true without the changes proposed by this RFC, e.g. Rust issue #14684. The general approach to resolving that issue should also work with these proposed changes.

This would be implemented behind a feature(bind_by_move_pattern_guards) gate until we have enough experience with the feature to remove the feature gate.

Drawbacks

The current error message makes it more clear what the user is doing wrong, but if this change is made the error message for an invalid use of this feature (even if it were accidental) would indicate a use of a moved value, which might be more confusing.

This might be moderately difficult to implement in rustc.

Alternatives

As far as I am aware, the only workarounds for the lack of this feature are to manually expand the control flow of the guard (which can quickly get messy) or use unnecessary copies.

Unresolved questions

This has nontrivial interaction with guards in arbitrary patterns as proposed in #99.

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