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False positive on collapsible_if with if let #1197

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llogiq opened this issue Aug 27, 2016 · 3 comments
Closed

False positive on collapsible_if with if let #1197

llogiq opened this issue Aug 27, 2016 · 3 comments

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@llogiq
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llogiq commented Aug 27, 2016

This is from linefeed:

fn read_init(&mut self) {
    if let Some(path) = env_init_file() {
        // If `INPUTRC` is present, even if invalid, parse nothing else.
        // Thus, an empty `INPUTRC` will inhibit loading configuration.
        self.read_init_file_if_exists(Some(path));
    } else {
        if !self.read_init_file_if_exists(user_init_file()) {
            self.read_init_file_if_exists(system_init_file());
        }
    }
}

If you try to collapse this, rustc won't have it:

error[E0301]: cannot mutably borrow in a pattern guard
    --> src/reader.rs:2116:20
     |
2116 |         } else if !self.read_init_file_if_exists(user_init_file()) {
     |                    ^^^^ borrowed mutably in pattern guard

error: aborting due to previous error

The difference appears to be that the else if part of an if let desugars to a guard. And guards appear to have different borrowing rules than simple if expressions (I'm not sure why). Perhaps that is a bug in borrowck, but as long as it is there, we should at least note it in the docs.
@mcarton
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mcarton commented Aug 27, 2016 •
edited
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Commits that seems to add if let desugaring: rust-lang/rust@0e6ff43#diff-7fde1475846894e59317d75947bdb921R72.

Desugar ExprIfLet
From: `if let <pat> = <expr> <body> [<elseopt>]`
to:

  match <expr> {
    <pat> => <body>,
    [_ if <elseopt_if_cond> => <elseopt_if_body>,]
    _ => [<elseopt> | ()]
  }

There isn't much explanation about why this is done that way. But it certainly seems surprising because it leads to very unintuitive errors. I think we should report that on rustc's issue tracker.
See also rust-lang/rust#28449.

@Arnavion
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rust-lang/rust#28449 is closed. rust-lang/rust#37510 is another issue for it that's still open.

@flip1995 flip1995 mentioned this issue Jan 15, 2020
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@camsteffen
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This must be fixed by now, especially with NLL.

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4 participants
@Arnavion @mcarton @llogiq @camsteffen