Document expression evaluation order

[Havvy]

See also, https://internals.rust-lang.org/t/rust-expression-order-of-evaluation/2605/32

[fweimer]

The referenced discussion is about evaluation order. Operator precedence is something different, it's about whether A ⊕ B ⊙ C is interpreted as (A ⊕ B) ⊙ C or A ⊕ (B ⊙ C). That difference applies independently of the order of evaluation of A, B, and C.

[Havvy]

This is what opening issues at 1AM does. Title fixed. Operator precedence is already documented.

[eloraiby]

referring rust-lang/rust#72763: Is this the intended behaviour for a for x in 0..function(mutable state) { .. } ?

[ehuss]

@eloraiby Yes. The range is an expression that is constructs a Range value, which captures the bounds when it is constructed.

[John-Nagle]

I brought this up in the help forums, related to order of structure initialization.

Tuples are guaranteed to be initialized in order. How about structs? Doing some marshalling. So I have things like

let vector3d = Vector3d{x: getnextvalue(); y: getnextvalue(); z: getnextvalue()};

where "getnextvalue()" is reading a value from some stream and thus has side effects.

How about the nested case?

let place = Place(pos:  Vector3d{x: getnextvalue(); y: getnextvalue(); z: getnextvalue()),
                         rot: Quaternion(x: getnextvalue(); y: getnextvalue(); z: getnextvalue(), s: getnextvalue()));

Consensus seems to be that the structure field initializers are executed left to right, as with tuples and arrays, but no spec actually says that. It's probably worth nailing down, so some later optimization doesn't break that assumption. For example, if I have

struct Vector3 {x: f32, y: f32, z: f32}

and then initialize with

let v = Vector3(z: getnextvalue(), y: getnextvalue(), x: getnextvalue());
the compiler might conceivably do the initialization in structure order rather than initializer order. Since this sort of thing would lead to subtle optimizer-related bugs, it's probably best to prohibit it.

A Reddit discussion four years ago indicated confusion over this issue. Nobody is totally sure.