Type ascription misdiagnosis when trying to use serde_json::json!

[saethlin]

Hi @estebank I didn't forget about your encouragement on Twitter: https://twitter.com/ekuber/status/1255881826855313409?s=20

I forgot a comma at the end of the first key-value pair:

fn main() {
    let json = serde_json::json!({
        "domain_name": "redjack.com"
        "passive_provenance": [420, 69],
    });
}

On the latest nightly compiler this is explicitly diagnosed as an attempt to use type ascription:

error: expected type, found `"redjack.com"`
 --> src/main.rs:3:24
  |
3 |         "domain_name": "redjack.com"
  |                      - ^^^^^^^^^^^^^ 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

On stable, it's arguably much worse, the error only makes sense if you know the compiler thinks this is type ascription but feature is not mentioned:

error: expected type, found `"redjack.com"`
 --> src/main.rs:3:24
  |
3 |         "domain_name": "redjack.com"
  |                      - ^^^^^^^^^^^^^ expected type
  |                      |
  |                      tried to parse a type due to this

[estebank]

@dtolnay is this something that could be handled in serde_json? I had a cursory look and saw that there already macro cases exclusively for better diagnostics, but the macro was big enough that I couldn't disentangle how to write a macro case for ($expr : $expr*).

Ideally we would handle this in the compiler more gracefully, but changing the RHS here from a string literal to a type makes the compiler be a bit more helpful.

[dtolnay]

Yeah I opened a fix in serde-rs/json#688.

error: no rules expected the token `"passive_provenance"`
 --> $DIR/missing_comma.rs:4:9
  |
3 |         "domain_name": "redjack.com"
  |                                     - help: missing comma here
4 |         "passive_provenance": [420, 69],
  |         ^^^^^^^^^^^^^^^^^^^^ no rules expected this token in macro call

[estebank]

Closing as per the fix in serde and CCing #34255 which tracks the work around type ascription more generally.