Provide a VM Service / DDS API for evaluating expressions based on their location

[DanTup]

Consider the following code:

var aaa = 1;

f() {
  var aaa = 2;
  debugger();
}

If I stop at debugger() and hover over aaa on the first line, I will see a value of 2 because we always just evaluate the expression in the current frame. It would be nice if we could provide a location and ask for an evaluation at a given location, which automatically handles selecting the right frame based on the hover location.

It would even be nice if we didn't need to provide an expression at all, but just a location, since VS Code often expands to something that is not valid (an API for "evaluate whatever expression is at [location]"), which causes issues for evaluating expressions in parens (see Dart-Code/Dart-Code#5362).

There's some more discussion about this in Dart-Code/Dart-Code#5362

cc @bkonyi @derekxu16

[dart-github-bot]

Summary: Debugger hover shows incorrect value due to frame selection. A new VM service API is requested to evaluate expressions based on their source location, simplifying debugging.

[a-siva]

Not sure if the VM or CFE will be able to reliably figure out the expression to evaluate based on the location information. Ideally the IDE is in the best position to figure out the correct expression to evaluate.

[DanTup]

Ah, maybe in that case we need to delegate this to the language server. There's a VS Code API for that, but it's not yet supported in LSP (microsoft/language-server-protocol#1596). I thought since we might want to send the location to the debugger anyway, it could do both together. Probably we'll need both things though (this VM/DDS service to handle scoping, and the EvaluatableExpressions in LSP to handle getting the right expression for a location).

[DanTup]

Sorry, I got confused while writing the above. I thought there were two issues for:

• using the location to get the expression
• using the location to get the scope/frame correct

But actually, it was just this one issue. So if we can't use the location to get the correct expression here, this issue should remain open about using the location for the correct scope/frame :-)

I've updated my comment above to avoid confusion.

[FMorschel]

I'm not sure if this is an issue on itself or if it is the same as this one, but see this screenshot:

I can evaluate element from the comment (it is declared below).