+1
I strongly dislike the current syntax, and I’d like to lay out my personal reasoning for why I believe using = (in some form) is better.
When you declare a struct like this:
struct Foo {
x: i32,
y: i32,
}
you’re basically declaring a new namespace of values like this (made-up syntax):
namespace Foo {
let x: i32;
let y: i32;
}
When you create a new struct, you’re creating an instance of that namespace of values, and you’re assigning values to each of the variables in the namespace:
let foo = Foo {
x = 1;
y = 2;
};
// or alternatively
let foo: Foo;
foo.x = 1;
foo.y = 2;
So why do we use : for this today, when what it is doing is assigning values to the yet-to-be-initialised variables inside the instance of the namespace (struct)? Saying that : fits with the ‘initialisation follows declaration’ pattern seems meaningless to me, because we don’t follow that with let, and no, let is not different. When declaring a struct field, the : functions to declare the type of the struct field. When declaring a variable, the : functions to declare the type of the variable. It follows pretty naturally (in my opinion) that = should be used to declare the value of a struct field in the same way that = declares the value of a variable.
Saying that struct initialisers declare a value instead of assigning one is also meaningless to me, as those two phrases mean the same thing. I can partially understand that argument, though, because you can use a struct initialiser as a value, whereas you can’t use let as one. However, = represents giving a value to a name. This is exactly what : in struct initialisers does: it gives a value to a name (a field of a struct).
I also like the idea of : being followed by a type everywhere in the Rust syntax. The only place (that I know of) that violates that in the entire Rust syntax (exempting macros, of course) is struct initialisers/patterns. : represents type ascription everywhere else. So for consistency, we should change : to only have one meaning: type ascription.
I’m not super fussed about the precise syntax to use (Foo { .x = 3 } or Foo { x = 3 } or something else; I personally like the former because it removes a lot of ambiguity) but I firmly believe that = is the One True Syntax to use in struct expressions.
