On Jan 16, 2014, at 8:13 AM, Tommy M. McGuire <[email protected]> wrote:
> let file = File::open_mode(&Path::new("test"), Truncate, Write);
>
> match file {
> Some(mut f) => f.write_str( "hello" ),
> None => fail!("not a file")
> }
This is fine, because you’re consuming `file` and moving the contained value
into a new mutable `f` variable. It’s basically the same as
if file.is_some() {
let mut f = file.unwrap();
…
} else {
fail!(“..”);
}
> works, while
>
> let mut file = File::open_mode(&Path::new("test"), Truncate, Write);
>
> match file {
> Some(mut f) => f.write_str( "hello" ),
> None => fail!("not a file")
> }
>
> results in a "variable does not need to be mutable" warning.
The warning here is because `file` doesn’t need to be mutable.
> Shouldn't the third option also fail (and possibly the second option
> succeed)?
The first two failed because the variable that needed to be mutable, `f`, was
not mutable the second two succeeded because it was.
As Simon has already pointed out, if you tried to use a by-ref binding it would
fail, because you can’t take a mutable borrow of an immutable variable. But you
didn’t do that, you moved the value, and moving from an immutable variable into
a mutable one is perfectly legal.
-Kevin
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