Tracking Issue for De-RFC 3307: Remove type ascription

This supersedes #23416

This is a tracking issue for the RFC "3307" (rust-lang/rfcs#3307).

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

Implement the RFC:
- Remove type ascription from the parser and diagnostics
- @rust-lang/compiler: Do y'all have a preference as to how far the removal should go? Unparseable AST nodes could be left in the code, for example (perhaps only instantiable by internal macro)
Adjust documentation: Remove it from the unstable book
(No stabilization PR necessary)
Remove the feature gate.

Unresolved Questions

Should this be completely removed by the compiler, or left behind in a way that cannot be directly accessed through Rust syntax (or requires using a wrapper macro)?
[ ]

Implementation history

Remove type ascription

@rustbot

@rustbot claim

I'll remove all the parsing and diagnostic logic/hacks but leave the AST nodes and everything after it behind for now.

Rollup merge of rust-lang#104614 - Nilstrieb:type-ascribe!, r=TaKO8Ki

Rollup merge of rust-lang#105614 - lukas-code:ascription, r=Nilstrieb

Would it make sense to possibly replace type ascription by an unstable macro in the standard library?

macro_rules! type_ascribe {
    ($e:expr, $t:ty) => {{
        let val: $t = $e;
        val
    }};
}

I noticed #104614 added a magic macro that preserves the old semantics, but maybe we can implement this entirely in userland?

That macro shortens the lifetime of temporaries created in the expression.

Would a function eliviate this problems with temporaries?
Or am I missing more temporary stuff?

pub fn type_ascribe<T>(value: T) -> T { value }

#[allow(unused_macros)]
macro_rules! type_ascribe {
    ($e:expr, $t:ty) => {{
        let val: $t = $e;
        val
    }};
}

pub fn main() {
    // macro dosn't work because of temporaries
    // println!("{}", type_ascribe!(String::from("hello world").split_once(' '), _).unwrap().0)
    
    //  identity function with turbo fish works
    println!("{}", type_ascribe::<_>(String::from("hello world").split_once(' ')).unwrap().0)
}

playground

That can be wrapped in a macro, of course:

pub fn type_ascribe<T>(value: T) -> T { value }

#[allow(unused_macros)]
macro_rules! type_ascribe {
    ($e:expr, $t:ty) => {type_ascribe::<$t>($e)};
}

pub fn test(x: usize) {
    println!("{}", type_ascribe!(String::from("hello world").split_once(' '), _).unwrap().0);
    let v = type_ascribe!([[0, 1, 2, 3], [4, 5, 6, 7]][x], [u32; 4]);
}

@estebank

This has taken a lot more time than I thought since lots of diagnostics were relying on it in subtle ways and require restructuring. I don't have enough time to work on this and also the other things I'm working on so I'm dropping my assignment here, feel free to take it. There's a WIP PR #106826 which may or may not be useful.
@estebank maybe you're interested in working on it, given all the pains you've had with type ascription :D
@rustbot release-assignment

Auto merge of rust-lang#109128 - chenyukang:yukang/remove-type-ascrip…

@chenyukang

@chenyukang is there still something to do here after #109128?

I will take a check in the following days, I remember there are still some macros left.

@lcnr

@lcnr now we still keep export a build-in macro for type ascription.

rust/library/core/src/macros/mod.rs

Lines 1858 to 1868 in 8a70352

    
               /// Unstable placeholder for type ascription. 
        
               #[allow_internal_unstable(builtin_syntax)] 
        
               #[unstable( 
        
                   feature = "type_ascription", 
        
                   issue = "23416", 
        
                   reason = "placeholder syntax for type ascription" 
        
               )] 
        
               #[rustfmt::skip] 
        
               pub macro type_ascribe($expr:expr, $ty:ty) { 
        
                   builtin # type_ascribe($expr, $ty) 
        
               }

and there are also some tests left for it:

https://github.com/rust-lang/rust/blob/8a703520e80d87d4423c01f9d4fbc9e5f6533a02/tests/ui/type/type-ascription-soundness.rs#L1-L14

I'm not sure can we delete them now, seems https://github.com/rust-lang/rfcs/blob/master/text/3307-de-rfc-type-ascription.md referred It's quite possible that in the future someone will have an RFC written to reintroduce this feature

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Tracking Issue for De-RFC 3307: Remove type ascription #101728

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	/// Unstable placeholder for type ascription.
	#[allow_internal_unstable(builtin_syntax)]
	#[unstable(
	feature = "type_ascription",
	issue = "23416",
	reason = "placeholder syntax for type ascription"
	)]
	#[rustfmt::skip]
	pub macro type_ascribe($expr:expr, $ty:ty) {
	builtin # type_ascribe($expr, $ty)
	}

Tracking Issue for De-RFC 3307: Remove type ascription #101728

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SoniEx2 commented on Sep 12, 2022

Noratrieb commented on Nov 16, 2022

RalfJung commented on Jan 10, 2023

bjorn3 commented on Jan 10, 2023

Skgland commented on Jan 12, 2023

RalfJung commented on Jan 12, 2023

Noratrieb commented on Feb 13, 2023

lcnr commented on Feb 14, 2026

chenyukang commented on Feb 15, 2026

chenyukang commented on Mar 25, 2026

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