Expose Dart's "why not promoted" information in VS Code

[devoncarew]

As of dart-lang/sdk@a42244f, the analyzer now generates context messages in some circumstances explaining why type promotion failed. We would like these messages to show up in VS Code in a way that's easily discoverable.

See also dart-lang/sdk#44897.

(Danny, I'll give you some context on this when we sync up next)

[devoncarew]

Ah, this work might already have been done?

@DanTup, I'll close this issue (but please re-open if you think there's something to do here).

[DanTup]

If the info is in the correction or contextMessages then yep, it shows up here. The correction is appended to the main message, and the context messages are relatedInformation.

Non-LSP:

Dart-Code/src/extension/providers/dart_diagnostic_provider.ts

Lines 52 to 55 in 6966fba

	if (error.correction)
	diag.message += `\n${error.correction}`;
	if (error.contextMessages && error.contextMessages.length)
	diag.relatedInformation = error.contextMessages.map(DartDiagnosticProvider.createRelatedInformation);

LSP:
https://github.com/dart-lang/sdk/blob/f306f4c4d41c8f2af2a3f3f30880703d43a8c5af/pkg/analysis_server/lib/src/lsp/mapping.dart#L935-L945

I'm not totally familiar with the "why not promoted" info (your screenshot doesn't look like it's that?) but if there's an example I can use to trigger it I can test in both LSP/non-LSP.

[stereotype441]

@DanTup

I'm not totally familiar with the "why not promoted" info (your screenshot doesn't look like it's that?) but if there's an example I can use to trigger it I can test in both LSP/non-LSP.

You should be able to test using this example (it works with the command-line analyzer as of dart-lang/sdk@a42244f):

f(int? i, int? j) {
  if (i == null) return; // (1)
  i = j;                 // (2)
  i.isEven;              // (3)
}

On the line marked (3) there should be an error message saying The property 'isEven' can't be unconditionally accessed because the receiver can be 'null'. There should be an associated context message for the line marked (2) saying Variable 'i' could be null due to a write occurring here.

Two minor notes:

• We're contemplating changing the context message to just Variable 'i' could be null due to a write. (rationale here)
• In the future we may also add a context message for the line marked (1) to tell the user the location of the promotion that failed (see Explore the possibility of showing the user promotion attempt locations dart-lang/sdk#44899)

[DanTup]

@stereotype441 thanks! In that case, it appears to be working as expected (both under LSP and the original protocol):

Clicking on the main error ("The property 'isEven'") navigates to line (3) were the red squiggle is, and clicking the nested message "Variable 'i' could be null..." navigates to line (2).

[stereotype441]

@DanTup Great! Thanks for checking.