odd suggestion for comma typo

[ehuss]

In an obscure situation with a typo (where there is a comma instead of a period), rustc will suggest replacing the comma with a semicolon, which will fail to compile.

let a = std::process::Command::new("echo")
    .arg("1")
    ,arg("2")
    .output();

Produces the following error and suggestion:

error: expected `;`, found ``,``
 --> src/main.rs:4:5
  |
4 |     ,arg("2")
  |     ^ help: change this to `;`: `;`

error[E0425]: cannot find function `arg` in this scope
 --> src/main.rs:4:6
  |
4 |     ,arg("2")
  |      ^^^ not found in this scope

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0425`.

Some observations:

• This must be a let binding.
• The method calls must be on separate lines (all on one line gives a different error).

I would expect this to provide the same error if it is all on one line ("expected one of ., ;, ?, or an operator"), possibly with a "maybe incorrect" suggestion to replace it with a period (maybe only if the following character is not whitespace?).

Note: Feel free to close this. This is really obscure, and I suspect it is not trivial to change, and it is fairly obvious what is wrong. I just thought it was strange, and I was doubly puzzled that the indentation mattered.

Meta

rustc 1.45.0-nightly (9912925 2020-05-10)

[estebank]

This is because the error recovery tries to figure out if you typoed something when trying to write ; to end an expression. This is probably easy to fix.

[chrissimpkins]

@rustbot claim

[chrissimpkins]

Thoughts about the better approach?:

error: expected `;`, found `,`
 --> src/main.rs:4:5
  |
4 |     ,arg("2")
  |     ^ unexpected token
  |
help: change this to `.`, `;`, `?`, or an operator
 --> src/main.rs:4:5
  |
4 |     ,arg("2")
  |     ^

vs.

error: expected `;`, found `,`
 --> src/main.rs:4:5
  |
4 |     ,arg("2")
  |     ^ unexpected token, expected one of `.`, `;`, `?`, or an operator
  |

[chrissimpkins]

#72348

[estebank]

Thoughts about the better approach?

I would prefer something closer to what we already see in other syntax errors:

error: expected one of `.`, `;`, `?`, or an operator, found `,`
 --> src/main.rs:4:5
  |
4 |     ,arg("2")
  |     ^ expected one of `.`, `;`, `?`, or an operator

We are not afraid of using vertical space to make errors clearer, but that vertical space still comes at a premium, as does verbosity. We try to lean towards the tersest possible output that can still convey the concept to the broadest set of people.