Parameter patterns are dropped *after* arguments

STR

struct NoisyDrop(&'static str);
impl Drop for NoisyDrop {
    fn drop(&mut self) {
        println!("dropping {}", self.0);
    }
}

fn argument_buf((_,v,_): (NoisyDrop, NoisyDrop, NoisyDrop)) {
    println!("in argument_buf");
}
fn main() {
    argument_buf((NoisyDrop("x"), NoisyDrop("y"), NoisyDrop("z")));
}

Actual Result

in argument_buf
dropping x
dropping z
dropping y

Explanation

When the function exits, the argument arg0 is dropped first, followed by the variable var0 aka y. This is the reverse of what happened in old trans, and looks quite unnatural. OTOH, it is quite similar to let-bindings, so maybe that is for the best.

Relation to `match` statements

I don't think that lifetimes in match patterns are a good example for anything (see #46525), but a match statement drops the discriminant last:

    match (NoisyDrop("x"), NoisyDrop("y"), NoisyDrop("z")) {
        (_, v, _) => println!("in match")
    }

prints

in match
dropping y
dropping x
dropping z

Really, I'd expect x and z to be dropped at the start of the function (same as let bindings).

@arielb1

@arielb1 What's the status of this?

This does look somewhat expected to me - _ never binds anything, so the value is dropped as soon as the pattern is matched. v keeps the only bound value alive until the function exits, so dropping y is printed later.

@Centril

@Centril

This still exists. With NLL it can't cause any soundness problems, but it just feels weird to me. Whatever we choose, this needs to be documented in the reference near the temporary lifetimes documentation.

@jonas-schievink

Not quite. The drop of both the arguments and the patterns happens at the function end, not at function start (see my edited post).

@matthewjasper

This still exists. With NLL it can't cause any soundness problems, but it just feels weird to me. Whatever we choose, this needs to be documented in the reference near the temporary lifetimes documentation.

iirc @matthewjasper has a PR to document this, rust-lang/reference#514 (I need to get back to it soon).

Also, to fix a regression, we recently changed the drop semantics so that the drop order is y x z rather than x y z. This is seen in https://play.rust-lang.org/?version=beta&mode=debug&edition=2018&gist=9d2cd068bf948d37a988f70f7d1bce59. Is that what you expect?

Sure enough, this was fixed in #56044, with a test. Closing.

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Parameter patterns are dropped after arguments #36088

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Relation to `match` statements

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Parameter patterns are dropped *after* arguments #36088

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Relation to match statements

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Stebalien commented on Aug 29, 2016

Centril commented on Feb 18, 2019

jonas-schievink commented on Feb 18, 2019

arielb1 commented on Feb 25, 2019

Centril commented on Feb 25, 2019

arielb1 commented on Feb 28, 2019

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Parameter patterns are dropped after arguments #36088

Relation to `match` statements