Add type_ascribe! macro as placeholder syntax for type ascription

compiler/rustc_builtin_macros/src/lib.rs

@@ -45,6 +45,7 @@ mod log_syntax;
 mod source_util;
 mod test;
 mod trace_macros;
+mod type_ascribe;
 mod util;
 
 pub mod asm;
@@ -92,6 +93,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         unreachable: edition_panic::expand_unreachable,
         stringify: source_util::expand_stringify,
         trace_macros: trace_macros::expand_trace_macros,
+        type_ascribe: type_ascribe::expand_type_ascribe,
     }
 
     register_attr! {

compiler/rustc_builtin_macros/src/type_ascribe.rs

@@ -0,0 +1,35 @@
+use rustc_ast::ptr::P;
+use rustc_ast::tokenstream::TokenStream;
+use rustc_ast::{token, Expr, ExprKind, Ty};
+use rustc_errors::PResult;
+use rustc_expand::base::{self, DummyResult, ExtCtxt, MacEager};
+use rustc_span::Span;
+
+pub fn expand_type_ascribe(
+    cx: &mut ExtCtxt<'_>,
+    span: Span,
+    tts: TokenStream,
+) -> Box<dyn base::MacResult + 'static> {
+    let (expr, ty) = match parse_ascribe(cx, tts) {
+        Ok(parsed) => parsed,
+        Err(mut err) => {
+            err.emit();
+            return DummyResult::any(span);
+        }
+    };
+
+    let asc_expr = cx.expr(span, ExprKind::Type(expr, ty));
+
+    return MacEager::expr(asc_expr);
+}
+
+fn parse_ascribe<'a>(cx: &mut ExtCtxt<'a>, stream: TokenStream) -> PResult<'a, (P<Expr>, P<Ty>)> {
+    let mut parser = cx.new_parser_from_tts(stream);
+
+    let expr = parser.parse_expr()?;
+    parser.expect(&token::Comma)?;
+
+    let ty = parser.parse_ty()?;
+
+    Ok((expr, ty))
+}

compiler/rustc_span/src/symbol.rs

@@ -1485,6 +1485,7 @@ symbols! {
         ty,
         type_alias_enum_variants,
         type_alias_impl_trait,
+        type_ascribe,
         type_ascription,
         type_changing_struct_update,
         type_id,

library/core/src/macros/mod.rs

@@ -1546,6 +1546,18 @@ pub(crate) mod builtin {
         /* compiler built-in */
     }
 
+    /// Unstable placeholder for type ascription.
+    #[rustc_builtin_macro]
+    #[unstable(
+        feature = "type_ascription",
+        issue = "23416",
+        reason = "placeholder syntax for type ascription"
+    )]
+    #[cfg(not(bootstrap))]
+    pub macro type_ascribe($expr:expr, $ty:ty) {
+        /* compiler built-in */
+    }
+
     /// Unstable implementation detail of the `rustc` compiler, do not use.
     #[rustc_builtin_macro]
     #[stable(feature = "rust1", since = "1.0.0")]

library/core/src/prelude/v1.rs

@@ -98,3 +98,11 @@ pub use crate::macros::builtin::cfg_accessible;
     reason = "`cfg_eval` is a recently implemented feature"
 )]
 pub use crate::macros::builtin::cfg_eval;
+
+#[unstable(
+    feature = "type_ascription",
+    issue = "23416",
+    reason = "placeholder syntax for type ascription"
+)]
+#[cfg(not(bootstrap))]
+pub use crate::macros::builtin::type_ascribe;

library/std/src/prelude/v1.rs

@@ -85,6 +85,15 @@ pub use core::prelude::v1::cfg_accessible;
 )]
 pub use core::prelude::v1::cfg_eval;
 
+// Do not `doc(no_inline)` either.
+#[unstable(
+    feature = "type_ascription",
+    issue = "23416",
+    reason = "placeholder syntax for type ascription"
+)]
+#[cfg(not(bootstrap))]
+pub use core::prelude::v1::type_ascribe;
+
 // The file so far is equivalent to src/libcore/prelude/v1.rs,
 // and below to src/liballoc/prelude.rs.
 // Those files are duplicated rather than using glob imports

src/test/ui/associated-consts/issue-93835.rs

@@ -1,9 +1,11 @@
+#![feature(type_ascription)]
+
 fn e() {
-    p:a<p:p<e=6>>
-    //~^ ERROR comparison operators
+    type_ascribe!(p, a<p:p<e=6>>);
+    //~^ ERROR cannot find type `a` in this scope
     //~| ERROR cannot find value
     //~| ERROR associated const equality
-    //~| ERROR associated const equality
+    //~| ERROR cannot find trait `p` in this scope
     //~| ERROR associated type bounds
 }
 

src/test/ui/associated-consts/issue-93835.stderr

@@ -1,65 +1,40 @@
-error: comparison operators cannot be chained
-  --> $DIR/issue-93835.rs:2:8
-   |
-LL | fn e() {
-   |        - while parsing this struct
-LL |     p:a<p:p<e=6>>
-   |        ^        ^
-   |
-   = help: use `::<...>` instead of `<...>` to specify lifetime, type, or const arguments
-   = help: or use `(...)` if you meant to specify fn arguments
-
 error[E0425]: cannot find value `p` in this scope
-  --> $DIR/issue-93835.rs:2:5
-   |
-LL |     p:a<p:p<e=6>>
-   |     ^ not found in this scope
-   |
-help: you might have meant to write a `struct` literal
-   |
-LL ~ fn e() { SomeStruct {
-LL |     p:a<p:p<e=6>>
- ...
-LL |
-LL ~ }}
+  --> $DIR/issue-93835.rs:4:19
    |
-help: maybe you meant to write a path separator here
-   |
-LL |     p::a<p:p<e=6>>
-   |      ~~
-help: maybe you meant to write an assignment here
-   |
-LL |     let p:a<p:p<e=6>>
-   |     ~~~~~
+LL |     type_ascribe!(p, a<p:p<e=6>>);
+   |                   ^ not found in this scope
 
-error[E0658]: associated const equality is incomplete
-  --> $DIR/issue-93835.rs:2:13
+error[E0412]: cannot find type `a` in this scope
+  --> $DIR/issue-93835.rs:4:22
    |
-LL |     p:a<p:p<e=6>>
-   |             ^^^
+LL |     type_ascribe!(p, a<p:p<e=6>>);
+   |                      ^ not found in this scope
+
+error[E0405]: cannot find trait `p` in this scope
+  --> $DIR/issue-93835.rs:4:26
    |
-   = note: see issue #92827 <https://github.com/rust-lang/rust/issues/92827> for more information
-   = help: add `#![feature(associated_const_equality)]` to the crate attributes to enable
+LL |     type_ascribe!(p, a<p:p<e=6>>);
+   |                          ^ not found in this scope
 
 error[E0658]: associated const equality is incomplete
-  --> $DIR/issue-93835.rs:2:13
+  --> $DIR/issue-93835.rs:4:28
    |
-LL |     p:a<p:p<e=6>>
-   |             ^^^
+LL |     type_ascribe!(p, a<p:p<e=6>>);
+   |                            ^^^
    |
    = note: see issue #92827 <https://github.com/rust-lang/rust/issues/92827> for more information
    = help: add `#![feature(associated_const_equality)]` to the crate attributes to enable
 
 error[E0658]: associated type bounds are unstable
-  --> $DIR/issue-93835.rs:2:9
+  --> $DIR/issue-93835.rs:4:24
    |
-LL |     p:a<p:p<e=6>>
-   |         ^^^^^^^^
+LL |     type_ascribe!(p, a<p:p<e=6>>);
+   |                        ^^^^^^^^
    |
    = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information
    = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable
 
 error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0425, E0658.
-For more information about an error, try `rustc --explain E0425`.
+Some errors have detailed explanations: E0405, E0412, E0425, E0658.
+For more information about an error, try `rustc --explain E0405`.

src/test/ui/closures/issue-90871.rs

@@ -1,5 +1,7 @@
+#![feature(type_ascription)]
+
 fn main() {
-    2: n([u8; || 1])
+    type_ascribe!(2, n([u8; || 1]))
     //~^ ERROR cannot find type `n` in this scope
     //~| ERROR mismatched types
 }

src/test/ui/closures/issue-90871.stderr

@@ -1,21 +1,26 @@
 error[E0412]: cannot find type `n` in this scope
-  --> $DIR/issue-90871.rs:2:8
+  --> $DIR/issue-90871.rs:4:22
    |
-LL |     2: n([u8; || 1])
-   |        ^ expecting a type here because of type ascription
+LL |     type_ascribe!(2, n([u8; || 1]))
+   |                      ^ help: a trait with a similar name exists: `Fn`
+   |
+  ::: $SRC_DIR/core/src/ops/function.rs:LL:COL
+   |
+LL | pub trait Fn<Args: Tuple>: FnMut<Args> {
+   | -------------------------------------- similarly named trait `Fn` defined here
 
 error[E0308]: mismatched types
-  --> $DIR/issue-90871.rs:2:15
+  --> $DIR/issue-90871.rs:4:29
    |
-LL |     2: n([u8; || 1])
-   |               ^^^^ expected `usize`, found closure
+LL |     type_ascribe!(2, n([u8; || 1]))
+   |                             ^^^^ expected `usize`, found closure
    |
    = note: expected type `usize`
-           found closure `[closure@$DIR/issue-90871.rs:2:15: 2:17]`
+           found closure `[closure@$DIR/issue-90871.rs:4:29: 4:31]`
 help: use parentheses to call this closure
    |
-LL |     2: n([u8; (|| 1)()])
-   |               +    +++
+LL |     type_ascribe!(2, n([u8; (|| 1)()]))
+   |                             +    +++
 
 error: aborting due to 2 previous errors
 

src/test/ui/coercion/coerce-expect-unsized-ascribed.rs

@@ -6,27 +6,27 @@
 use std::fmt::Debug;
 
 pub fn main() {
-    let _ = box { [1, 2, 3] }: Box<[i32]>; //~ ERROR mismatched types
-    let _ = box if true { [1, 2, 3] } else { [1, 3, 4] }: Box<[i32]>; //~ ERROR mismatched types
-    let _ = box match true { true => [1, 2, 3], false => [1, 3, 4] }: Box<[i32]>;
+    let _ = type_ascribe!(box { [1, 2, 3] }, Box<[i32]>); //~ ERROR mismatched types
+    let _ = type_ascribe!(box if true { [1, 2, 3] } else { [1, 3, 4] }, Box<[i32]>); //~ ERROR mismatched types
+    let _ = type_ascribe!(box match true { true => [1, 2, 3], false => [1, 3, 4] }, Box<[i32]>);
     //~^ ERROR mismatched types
-    let _ = box { |x| (x as u8) }: Box<dyn Fn(i32) -> _>; //~ ERROR mismatched types
-    let _ = box if true { false } else { true }: Box<dyn Debug>; //~ ERROR mismatched types
-    let _ = box match true { true => 'a', false => 'b' }: Box<dyn Debug>; //~ ERROR mismatched types
+    let _ = type_ascribe!(box { |x| (x as u8) }, Box<dyn Fn(i32) -> _>); //~ ERROR mismatched types
+    let _ = type_ascribe!(box if true { false } else { true }, Box<dyn Debug>); //~ ERROR mismatched types
+    let _ = type_ascribe!(box match true { true => 'a', false => 'b' }, Box<dyn Debug>); //~ ERROR mismatched types
 
-    let _ = &{ [1, 2, 3] }: &[i32]; //~ ERROR mismatched types
-    let _ = &if true { [1, 2, 3] } else { [1, 3, 4] }: &[i32]; //~ ERROR mismatched types
-    let _ = &match true { true => [1, 2, 3], false => [1, 3, 4] }: &[i32];
+    let _ = type_ascribe!(&{ [1, 2, 3] }, &[i32]); //~ ERROR mismatched types
+    let _ = type_ascribe!(&if true { [1, 2, 3] } else { [1, 3, 4] }, &[i32]); //~ ERROR mismatched types
+    let _ = type_ascribe!(&match true { true => [1, 2, 3], false => [1, 3, 4] }, &[i32]);
     //~^ ERROR mismatched types
-    let _ = &{ |x| (x as u8) }: &dyn Fn(i32) -> _; //~ ERROR mismatched types
-    let _ = &if true { false } else { true }: &dyn Debug; //~ ERROR mismatched types
-    let _ = &match true { true => 'a', false => 'b' }: &dyn Debug; //~ ERROR mismatched types
+    let _ = type_ascribe!(&{ |x| (x as u8) }, &dyn Fn(i32) -> _); //~ ERROR mismatched types
+    let _ = type_ascribe!(&if true { false } else { true }, &dyn Debug); //~ ERROR mismatched types
+    let _ = type_ascribe!(&match true { true => 'a', false => 'b' }, &dyn Debug); //~ ERROR mismatched types
 
-    let _ = Box::new([1, 2, 3]): Box<[i32]>; //~ ERROR mismatched types
-    let _ = Box::new(|x| (x as u8)): Box<dyn Fn(i32) -> _>; //~ ERROR mismatched types
+    let _ = type_ascribe!(Box::new([1, 2, 3]), Box<[i32]>); //~ ERROR mismatched types
+    let _ = type_ascribe!(Box::new(|x| (x as u8)), Box<dyn Fn(i32) -> _>); //~ ERROR mismatched types
 
-    let _ = vec![
+    let _ = type_ascribe!(vec![
         Box::new(|x| (x as u8)),
         box |x| (x as i16 as u8),
-    ]: Vec<Box<dyn Fn(i32) -> _>>;
+    ], Vec<Box<dyn Fn(i32) -> _>>);
 }