Describe integer overflow semantics in the manual.

[nejucomo]

Please add a section on integer overflow (and underflow) semantics for signed/unsigned integers.

The only mention of "overflow" I see in the manual is 6.1.3.2.4 Behaviour not considered unsafe which doesn't define overflow semantics.

It would also help to augment 7.2.11.5 Type cast expressions to declare that the overflow semantics apply there with a cross-link. There may be other cases where it's not obvious that overflow semantics apply so those sections should cross-link also.

[Aatch]

The widening behaviour should also be documented in the Type Cast section, since it currently isn't. While the widening behaviour for unsigned integral values is fairly obvious, it would be worth noting the behaviour for signed integral values.

// A cast from a signed value widens with signed-extension
(-1i8 as i16, -1i8 as u16) // (-1i16, 65535u16)
// A cast from an unsigned value widens with zero-extension
(1u8 as i16, 1u8 s u16) // (1, 1)

[emberian]

Ever more relevant with overflow now being defined as a panic!

[steveklabnik]

https://github.com/rust-lang/rfcs/blob/master/text/0560-integer-overflow.md was accepted, and while this ticket is still valid, I have a 'go through all RFCs' bug over at #20137, so I'm closing this one, since that one is more comprehensive.