Prevent type ascription misdiagnosis

src/macros.rs

@@ -224,6 +224,11 @@ macro_rules! json_internal {
         json_internal!(@object $object ($key) (: $($rest)*) (: $($rest)*));
     };
 
+    // Refuse to absorb colon token into key expression.
+    (@object $object:ident ($($key:tt)*) (: $($unexpected:tt)+) $copy:tt) => {
+        json_expect_expr_comma!($($unexpected)+);
+    };
+
     // Munch a token into the current key.
     (@object $object:ident ($($key:tt)*) ($tt:tt $($rest:tt)*) $copy:tt) => {
         json_internal!(@object $object ($($key)* $tt) ($($rest)*) ($($rest)*));
@@ -290,3 +295,9 @@ macro_rules! json_internal_vec {
 macro_rules! json_unexpected {
     () => {};
 }
+
+#[macro_export]
+#[doc(hidden)]
+macro_rules! json_expect_expr_comma {
+    ($e:expr ,) => {};
+}

tests/ui/missing_comma.stderr

@@ -1,10 +1,5 @@
-error: expected type, found `""`
- --> $DIR/missing_comma.rs:4:18
+error: no rules expected the token `"2"`
+ --> $DIR/missing_comma.rs:4:21
   |
 4 |     json!({ "1": "" "2": "" });
-  |                - ^^ expected type
-  |                |
-  |                tried to parse a type due to this type ascription
-  |
-  = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-  = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
+  |                     ^^^ no rules expected this token in macro call

tests/ui/parse_expr.stderr

@@ -1,7 +1,5 @@
-error: unexpected end of macro invocation
- --> $DIR/parse_expr.rs:4:5
+error: no rules expected the token `~`
+ --> $DIR/parse_expr.rs:4:19
   |
 4 |     json!({ "a" : ~ });
-  |     ^^^^^^^^^^^^^^^^^^^ missing tokens in macro arguments
-  |
-  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+  |                   ^ no rules expected this token in macro call