Type ascription builtin is silently replaced by rustfmt(?)

[Mark-Simulacrum]

rustfmt complains about the type_ascribe macro asking for the builtin to be replaced with the ... : ... syntax:

Error: Diff in /checkout/library/core/src/macros/mod.rs at line 1712:
         reason = "placeholder syntax for type ascription"
     )]
     pub macro type_ascribe($expr:expr, $ty:ty) {
-        builtin # type_ascribe($expr, $ty)
+        $expr: $ty
     }

I've worked around this with #[rustfmt::skip] on the macro which seems to work, but filing an issue since it seems like somewhat odd behavior as well.

[Noratrieb]

looks like rustfmt wasn't updated to print type ascription correctly, this issue should be removed to the rustfmt repo probably

[ytmimi]

@Mark-Simulacrum The diff is helpful, but can you also provide a complete code snippet that we can use to reproduce the issue. A few more questions. What version of rustfmt were you using when you ran into this issue, and are you using any rustfmt configurations?

[ytmimi]

@Nilstrieb do you know how builtin # type_ascribe is represented in the AST?

for example, builtin # offset_of is represented by rustc_ast::ast::ExprKind::OffsetOf

[ytmimi]

@Nilstrieb @Mark-Simulacrum Okay, I think the issue here is that builtin # type_ascribe gets parsed as rustc_ast::ast::ExprKind::Type.

From what I can tell Here's where this get's parsed in rustc.

In this case rustfmt is handling this as expected. It sees a ExprKind::Type in the ast and outputs one. I think this issue should be moved back to rust-lang/rust and the fix probably involves creating some special builtin ast node, though I don't know enough about later phases of the compiler to know how that might complicate things after parsing.

[Noratrieb]

haha, looks like ast_pretty wasn't correctly updated then, fun. I don't think any AST changes are needed, it just needs to print the ExprKind::Type with the builtin syntax.

[ytmimi]

@Nilstrieb Do you have thoughts on how rustfmt can tell the difference between builtin # type_ascribe and expr: ty if they both get parsed as ExprKind::Type?

[Noratrieb]

the latter does not exist anymore

[ytmimi]

If ExprKind::Type exclusively represents builtin # type_ascrib then I agree we don't need to make any ast changes. Do you know when the switch from expr: ty -> builtin # type_ascribe occurred? Maybe we should also update the doc comment on ExprKind::Type to explain that as well.

[Noratrieb]

It was implemented by Yukang and me, with the biggest change in rust-lang/rust#109128

[ytmimi]

Thanks for the link!

[fmease]

And rust-lang/rust#122806 made the macro type_ascribe use builtin # type_ascribe.