Type ascription (tracking issue for RFC 803)

[nikomatsakis]

Tracking issue for rust-lang/rfcs#803. Important points:

• Implement the expr:Type syntax
• Ensure soundness with respect to ref positions:
  • let ref x = <expr> / let mut ref = <expr>
  • match <expr>: Type { ref x => ... }
  • (<expr>: Type).method_with_ref_self()
• Permit coercions like &[1, 2, 3]: &[u8]

[petrochenkov]

There's one thing making type ascription less convenient than it could potentially be. While it has the same operator priority as as (it is modeled after as in general), i.e. the highest from binary operators, it's still lower than member access . (dot).

Compare these two expressions

let v = v.iter().collect::<Vec<_>>().sort(); // Parens are not required
let v = (v.iter().collect(): Vec<_>).sort(); // Parens are required
// Formatted on several lines
let v = (v.iter().
           collect(): Vec<_>). // Looks bad, man
           sort();

I'm not sure if it is worth fixing or if it can be fixed without hacks, but it is worth discussing at least.

[hoodie]

BTW: This has is a breaking change for crates that implement a similar macro syntax already, such as prettytables.

[jonas-schievink]

@hoodie I opened #30531, since this seems worth tracking

[nrc]

cc @nrc

[vi]

Is it a good idea to only do ascription inside parentheses, so let a = x : T is not ascription yet, but let a = (x : T) is? This can allow more room for some further syntax additions.

[petrochenkov]

For the record: a patch implementing support for method chaining with type ascription #33380

[ottworks]

Why is this in the book if it won't compile? (1.9.0)

https://doc.rust-lang.org/book/closures.html

[hoodie]

@0TT what exactly?

[retep998]

@0TT Type ascription is specifically when adding a : T type annotation after an expression. Not in the parameters to a closure or a function, and not on a variable binding. Only when it follows an expression which I do not see any cases of on the page you linked.

[ottworks]

Oops. I accidentally typo'd a semicolon as a colon:

let plus_two = |x| {
    let mut result: i32 = x;

    result += 1:
    result += 1;

    result
};
assert_eq!(4, plus_two(2));

[hoodie]

that happens, how about an RFC to replace all semicolons with a less confusable character?

let plus_two = |x| {
    let mut result: i32 = x✓

    result += 1✓
    result += 1✓

    result
}✓
assert_eq!(4, plus_two(2))✓

makes the language even look safer, everything is checked after all 😀

[ottworks]

Should use emoji tbh. A language for the future.

[nrc]

re the soundness question in the OP, this comment is probably the best summary.

[crumblingstatue]

Before stabilizing this feature, I would like to see how this potentially impacts optional and named arguments, namely the expr : type syntax in function call argument position.

Could named arguments use this syntax if it wasn't for type ascription?

If yes, and no other syntaxes could be devised that are as optimal as name : type, I'd strongly recommend changing the syntax for type ascription, in favor of allowing the optimal syntax for named arguments.

I believe named arguments would be much more widely used than type ascription, and the syntax optimization for ergonomics should go there instead.

Ref rust-lang/rfcs#323

[aidanhs]

@nrc do you have a link a discussion about the widening integers ergonomics initiative?

@glaebhoerl so &x as *const u64 as *const u8 -> cast (cast &x: *const u64): *const u8?

I'm not a fan of that I must admit. I see as and ascription as being very different - as feels like a nod at the fact that Rust is a systems language and can cheaply convert some 'primitive' types (functions, pointers, numbers) into other representations, even when the translation may be lossy.

Working with pointers in Rust is already not a great experience, it's very syntax heavy. I'd prefer not to go the same direction for integers.

[glaebhoerl]

I see as and ascription as being very different

I completely agree, and I'm not sure what in my comment makes you think otherwise?

[aidanhs]

I think those couple of sentences muddied my comment, please ignore them - the primary point was the extra syntax and I got sidetracked.

[glaebhoerl]

Yeah, syntaxwise it's a legit tradeoff. I'm pretty sure there's also cases where the cast syntax would be more convenient (e.g. when it's actually sensible to leave off the target type).

[RalfJung]

Lossy conversion without an explicitly stated target type sounds like a huge footgun to me.

TBH I don't see why as and : should be confusing. Converting things and saying that things have certain types are two very different operations. It doesn't seem to hard to explain this.

[omarabid]

No progress on this one?

[scottmcm]

👍 to deprecating as in favour of more targeted things, like explicitly-introduced left-to-right coercion points. Personally, I think all the non-coercion things that as does would be better as functions/methods.

[SoniEx2]

hey can we remove type ascription in favor of an std::identity function?

std::identity::<Type>(stuff)

current alternatives include:

• (|x:ty|x)(expr) for use in macros.
• ???

if we had it in std, we wouldn't need to introduce new syntax, and we wouldn't need to deal with the soundness issues and stuff, and it could be freely used in macros.

[SimonSapin]

Both a function or a closure cause a value to be moved, which affects its life time more than just a type annotation would.

[SoniEx2]

#[inline(always)] and the compiler should be able to detect identity functions.

[vi]

Or identity can be a magical intrinsic.

[sinkuu]

The problem is that calling a function takes argument's ownerwhip. This cannot be cancelled by being an intrinsic (that is magical but stil has to appear to be a function), right?

let s = "foo".to_string();
(s : String).len();
println!("{}", s); // OK
identity::<String>(s).len();
println!("{}", s); // ERROR use of moved value: `s`

[SoniEx2]

It is meant for use without bindings.

If you wanna use it with bindings, just use let x: ty.

[sinkuu]

Oh, of course. So it may be temporary lifetime things (for x: RefCell<_>, *x.borrow() : T works, but identity::<T>(*x.borrow()) doesn't work).

[SoniEx2]

Won't NLLs solve that?

[scottmcm]

How about this as a more specific plan:

• Move towards as becoming the syntax for type ascription (with coercion support)
  • Reusing as has the nice property that things like my_ref as *const _ don't need to change
• Start linting to suggest methods instead of non-coercion as
  • error messages already do this, suggesting u64::from where infallible (not as u64)
• Over time, add more methods to cover the rest of the cases
  • For example there's now NonNull::cast; raw pointers could get the same (or similar)
  • And maybe wrapping_from between integers, or some other variant from IRLO (thread) (thread)
• Eventually (Rust 2021?) remove support for non-coercion as
  • Which is an allowed edition change as it's in the frontend and we'd be warning about any breaks

[Centril]

I think @scottmcm's plan sounds good. Repeating what I said on #rust-lang: I can live with the problem of not having 100% consistency personally. Some inconsistency is to me better than not having the feature at all. Unfortunately, with the current language and our backwards compatibility promises, we can't achieve 100% consistency with typing judgements in the language across the board and I'm not advocating that we write:

let foo as type = expr;

That said, I think the idea due to @cramertj to have some sort of expr.type!(MyType) or say expr.as!(MyType) or even expr.at!(MyType) (read as: "expr is typed at MyType") is interesting. With postfix macros as in rust-lang/rfcs#2442 you could build that.

An example:

let vec = iter.map(..).filter(..).collect().at!(Vec<u32>);

The benefit of post macros here is that it allows chaining... however, I think it is unlikely to have many type ascriptions per expression.

[Ichoran]

As a long-time user of Scala, I have only very rarely seen anyone confuse : Foo with .asInstanceOf[Foo]. Granted, as Foo is less different syntactically, but I still think it is very important for correctness to be able to distinguish between telling the compiler "I believe this type to be that, but check it for me" and "I want you to make this type be that if you can". That is, I want some way to avoid coercion, because coercion may be the wrong thing to do.

(Numeric types in Scala have a notion of "weak conformance" which means they can get coerced unexpectedly; this has turned into a a fruitful source of puzzlers. Rust has much less of an issue presently, but dropping the distinction between pure ascription and request for coercion is likely to erode the improvement. It would be especially pernicious if the very thing you would use to check your assumptions could break them instead!)