ui_test: provide a way to match against labels

[RalfJung]

With a stderr like this

full stderr:
error[E0080]: evaluation of constant value failed
  --> tests/fail/const-ub-checks.rs:6:9
   |
LL |         ptr.read();
   |         ^^^^^^^^^^ accessing memory with alignment 1, but alignment 4 is required

I'd like to be able to match not only against "evaluation of constant value failed", but also against "accessing memory with alignment 1, but alignment 4 is required", to ensure that it actually detected the right kind of problem.

This is a prerequisite for #2200.

[RalfJung]

The JSON from that message looks like this:

{
  "message": "evaluation of constant value failed",
  "code": {
    "code": "E0080",
    "explanation": "A constant value failed to get evaluated.\n\nErroneous code example:\n\n```compile_fail,E0080\nenum Enum {\n    X = (1 << 500),\n    Y = (1 / 0),\n}\n```\n\nThis error indicates that the compiler was unable to sensibly evaluate a\nconstant expression that had to be evaluated. Attempting to divide by 0\nor causing an integer overflow are two ways to induce this error.\n\nEnsure that the expressions given can be evaluated as the desired integer type.\n\nSee the [Custom Discriminants][custom-discriminants] section of the Reference\nfor more information about setting custom integer types on fieldless enums\nusing the [`repr` attribute][repr-attribute].\n\n[custom-discriminants]: https://doc.rust-lang.org/reference/items/enumerations.html#custom-discriminant-values-for-field-less-enumerations\n[repr-attribute]: https://doc.rust-lang.org/reference/type-layout.html#reprc-enums\n"
  },
  "level": "error",
  "spans": [
    {
      "file_name": "tests/fail/const-ub-checks.rs",
      "byte_start": 145,
      "byte_end": 155,
      "line_start": 6,
      "line_end": 6,
      "column_start": 9,
      "column_end": 19,
      "is_primary": true,
      "text": [
        {
          "text": "        ptr.read(); //~ERROR: evaluation of constant value failed",
          "highlight_start": 9,
          "highlight_end": 19
        }
      ],
      "label": "accessing memory with alignment 1, but alignment 4 is required",
      "suggested_replacement": null,
      "suggestion_applicability": null,
      "expansion": null
    }
  ],
  "children": [],
  "rendered": "error[E0080]: evaluation of constant value failed\n --> tests/fail/const-ub-checks.rs:6:9\n  |\n6 |         ptr.read(); //~ERROR: evaluation of constant value failed\n  |         ^^^^^^^^^^ accessing memory with alignment 1, but alignment 4 is required\n\n"
}

[oli-obk]

should we just try the main message first and then the label, or should we explicitly opt into checking the label?

[RalfJung]

I think explicit is better than implicit for a test suite, so I was thinking of something like //~LABEL: ....