Hi rust-dev,
How should these errors be dealt with?
For example, given the following code:
fn takes_an_option(_x: option<int>) {}
fn takes_a_rec(x: @{mut a: option<int>}) {
takes_an_option(x.a);
}
fn main() {
let x = @{mut a: some(4)};
takes_a_rec(x);
}
rustc produces:
test.rs:6:20: 6:23 error: illegal borrow unless pure: creating immutable
alias to aliasable, mutable memory
test.rs:6 takes_an_option(x.a);
^~~
test.rs:6:4: 6:19 note: impure due to access to impure function
test.rs:6 takes_an_option(x.a);
^~~~~~~~~~~~~~~
error: aborting due to previous error
I can think of 3 ways of fixing the issue:
1. Declaring takes_an_option as pure. This can't be done in a library
controlled by
someone else, though, and it also can't be done if the function isn't
actually pure.
2. Declaring the argument mode for takes_an_option to be ++ (pass by
copy). Again
this can only be done if the library isn't someone else's, but I can't
see any other
downsides apart from more sigil-filled source code.
3. Copying the argument explicitly when invoking the function:
takes_an_option(copy a.x);
This doesn't require owning the library.
Are there other ways of fixing the issue?
I also have read that argument modes are being removed. How will that
affect this situation?
If the single remaining argument mode is by-copy, does that mean this
issue will go away?
Thanks
Gareth
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