Tracking issue for RFC 803, "Type ascription"

[nikomatsakis]

Superseded by #101728 due to the landing of RFC 3307

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] -- permit coercion in type ascription #78248

[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