Rust expression order of evaluation

[nikomatsakis]

@fweimer

The expectation is that first the value a is received, then the value b. But with a non-determinate evaluation order, one has to introduce temporaries.

More to the point, in a struct literal, the fields are evaluated in the order you write them; if a panic occurs before the struct is completely build, the intermediate values are dropped in the reverse order (once the struct is fully built, the fields are dropped in the order they are written in the struct declaration, iirc).