Consider returning &c_char from CString::as_ptr

[Ms2ger]

This should make it a lot harder to run into the bug mentioned in its documentation (let p = foo.to_c_str().as_ptr();).

[pcwalton]

+1

[metajack]

We hit this a ton in Servo in this latest rust upgrade, which caused many use after free bugs in otherwise safe code.

I think as_ptr() should be removed, and replaced with as_ref(), so the lifetime annotations stick and prevent this easy-to-make error.

[pcwalton]

cc @aturon

[pcwalton]

nominated for 1.0 P-backcompat-libs

[aturon]

The return type should be &[c_char] rather than &c_char, I'm assuming?

Just to clarify, the as_bytes and as_bytes_no_nul methods already provide this functionality.

I'd like to understand better how CString is being used in Servo. Within libstd, the predominant use is cases where you need to get a *const i8 to pass along to some C bindings. (When consuming C strings, we generally use from_c_str, which has recently been renamed to from_buf.)

I could imagine trying to do something more RAII-ish here, and return an object that derefs into *const i8, which would probably help catch some of these bugs.

Thoughts?

cc @alexcrichton

[metajack]

Here are some examples:

https://github.com/servo/servo/blob/rustup-20140716/src/components/script/dom/bindings/utils.rs#L241-L244

https://github.com/servo/servo/blob/rustup-20140716/src/components/script/dom/bindings/utils.rs#L241-L244

https://github.com/servo/rust-mozjs/blob/rustup-20140716/rust.rs#L137-L151

https://github.com/servo/rust-opengles/blob/rustup-20140716/gl2.rs#L381

[alexcrichton]

I believe the idea is that &c_char will implicitly coerce to *const c_char whereas &[c_char] will require another method call, sort of the difference between

foo(s.as_ref())
// vs
foo(s.as_bytes().as_ptr())

I'd be ok moving this over to &c_char, although I'd like to see the impact first to make sure it doesn't turn up too many surprises.

[pnkfelix]

P-backcompat-libs, but not 1.0: this is in part of the stdlib that is still tagged with experimental stability attribute.