[CFE] Incremental compilation fails for optimized-away constants

[annagrin]

Example

void main() {
  const int p = 1;
  print(p); // breakpoint here
}

Javascript from initial compilation

For main function only, is here to illustrate that kernel representation has variable 'p' optimized away:

main.main = function main$() {
  core.print(1);
};

Result of evaluating ‘p’ at the breakpoint above

CompilationError: Getter not found: 'p'

Suggestions

Is it possible to introduce debug mode in CFE where the information about optimized variables (such as 'p=1') is preserved so the incremental compiler can use it?

[annagrin]

Added an example to expression evaluation tests in https://dart-review.googlesource.com/c/sdk/+/153941

@jensjoha @johnniwinther this is a fairly common evaluation problem I see in my examples and in flutter samples, is there a setting to work around removed constants?

[johnniwinther]

We have a keepFields flag in the constant transformer that lets you preserve constant field declarations. We should add a similar flag for local constant variables. I'll look into it.

[johnniwinther]

I've add keepLocals support in https://dart-review.googlesource.com/c/sdk/+/153965

@annagrin try making DevCompilerConstantsBackend.keepLocals to return true to see if this fixes this use case.