Allow Rule.custom to specify severity when reporting multiple

[blended-bram]

Is your feature request related to a problem? Please describe.

It is annoying that reaching for Rule.custom is not enough to hand-pick the severity. In particular when mixing logic that may produce both errors and warnings. This results in unneeded complexity (as the same logic to inspect the value is split across different rules).
Multiple rules are needed and need to be nested where already using Rule.custom should permit maximal customization.

Creating custom warnings, mixed with errors, currently requires:

Rule.all([
  Rule.custom(/* warnings impl */).warning(),
  Rule.custom(/* errors impl */),
])

I guess one could make a helper function that splits an implementation:

function makeCustomRule(Rule: Rule, fn: (value, context) => ValidationWithSeverity[]): Rule {
  return Rule.all([
    Rule.custom((value, context) => fn(value, context).filter(x => (x.severity ?? 'error') == 'error')).error(),
    Rule.custom((value, context) => fn(value, context).filter(x => (x.severity ?? 'error') == 'warning')).warning(),
  ])
}

Describe the solution you'd like

interface ValidationError {
  // New addition:
  severity?: 'warning' | 'error' /* | ... */

  // Current operational fields:
  message: string;
  path?: Path;
}

Then use it like so:

Rule.custom((value) => {
  const failures: ValidationError[] = [];
  if (/* ... */) {
    failures.push({ message: 'My warning', severity: 'warning' }) // Overrides severity of the rule definition.
  }
  if (/* ... */) {
    failures.push({ message: 'My error' }) // no severity field: use severity from rule definition
  }
})

[cngonzalez]

Hi @blended-bram ! Yes, I understand the duplicated logic probably feels a bit frustrating. I'm forwarding this feature request to our studio team. Thanks very much for submitting it!