{String,Vec}::from_raw_parts docs need to be much clearer about incorrectness

[huonw]

The docs for these are currently fairly meagre:

unsafe fn from_raw_parts(buf: *mut u8, length: usize, capacity: usize) -> String

Creates a new String from a length, capacity, and pointer.

This is unsafe because:

• We call Vec::from_raw_parts to get a Vec<u8>;
• We assume that the Vec contains valid UTF-8.

unsafe fn from_raw_parts(ptr: *mut T, length: usize, capacity: usize) -> Vec

Creates a Vec<T> directly from the raw components of another vector.

This is highly unsafe, due to the number of invariants that aren't checked.

They need to be more specific about the details, e.g.:

• ptr needs to have been previously allocated via String/Vec (at least, it's highly likely to be incorrect if it wasn't)
• capacity needs to be the capacity that the pointer was allocated with.

Violating these may cause problems like corrupting the allocators internal datastructures.