Update vec.rs

[rick68]

Check the capacity of Vec<T> that is more than or equal to length.

[rust-highfive]

r? @huonw

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[brson]

This looks good to me - I think this invariant is strongly-implied by the existing docs. But it is not stated explicitly, do you mind adding a bullet to the 'Unsafety' section stating this invariant?

@alexcrichton does this debug assert seem appropriate to you? Do we have other debug asserts enforcing invariants like this in std?

[brson]

Hm, note that nobody will ever hit this because std is never compiled with debug asserts. This mostly serves as code docs.

[pnkfelix]

@brson std is never compiled with debug asserts? Even with a configure --enable-debug build? That is interesting if true.

[brson]

@pnkfelix oh, 'never' is too strong of course. More like users never see debug asserts in std.

[rick68]

@brson I just wanted this to be working in debug mode for the users, wondering if it won't work (libcollections is not compiled with support of debug_assert! macro)

[alexcrichton]

The from_raw_parts function is such a low-level function for manipulating vectors (it's basically exposing the representation) I don't think we should place many asserts around it. It's nice to have a clear connection between this function and the creation of a vector with literally zero extra overhead or checks. It's clearly very unsafe but I don't think that this assertion will buy us much in the long run.

[Gankra]

This also has exception-safety implications. e.g. if someone doesn't expect this to panic, but it suddenly does in debug, that's not great... although on the other hand it's basically UB to hit this assert anyway.

Also as has been noted, this will literally only run if someone builds their own custom compiler with debug assertions turned on. Using a stable/beta/nightly build this assertion will be stripped.

[brson]

@rick68 Unfortunately, because 'std' itself is compiled in release mode, even when users of std compile their projects in debug mode they will not get debug assertions that originate in std.

[rick68]

Thinking about generics, collections::vec::Vec can not just be used in std. If I want to use collections::vec::Vec to develop my own custom containers, this is able to access memory out of range.

But this is the risk what I have to take if I want to use unsafe methods.

[Gankra]

@rick68 yes most collections are generally unsafe.

Anyway, it seems like we agree this should be closed.