Debugging experience for patterns needs a cleanup

dart-lang / webdev

A CLI for Dart web development.

https://pub.dev/packages/webdev

212 stars 75 forks source link

Debugging experience for patterns needs a cleanup #1974

Closed annagrin closed 1 year ago

annagrin commented 1 year ago

Stepping into the statement on the breakpoint line seems to stuck forever on the same spot
- I suppose it goes through all ddc statements the patterns has been translated to
- Can we remove unnecessary locations in source maps?
Compiler-generated locals appear in the variables window

nshahan commented 1 year ago

@bkonyi Are there any expectations for how stepping through patterns like this works when debugging on the VM?

bkonyi commented 1 year ago

I'm not sure what the behavior will be like as it depends on where we insert breakpoints into generated code. Maybe @alexmarkov will have a better idea or know who will?

alexmarkov commented 1 year ago

Debugger behavior depends on how patterns are lowered in the front-end and how source positions are assigned to the AST nodes.

However, VM has a logic to avoid stopping at the same position repeatedly:

https://github.com/dart-lang/sdk/blob/b8dc6762921aa2fd3ef3c7415d3d7f946ee6f21b/runtime/vm/debugger.cc#L3809-L3814

So even if there are many AST nodes with the same source position, VM would stop at the first one and then skip the rest of the stops, until the source position changes.

nshahan commented 1 year ago

Looking at the screenshot, there are a few initial questions that come to my mind.

If you set a breakpoint and pause on line 15 then step where would you expect the next pause to be?
If you set a breakpoint on line 16 and the debugger pauses there do we expect the evaluation to be paused before the when?
Is there any point that you can hover over variables in the patterns like [var a, int n]to see values?

I don't expect these all to be answered already but I'd like to get the discussion started and I'm not sure who should be involved.

annagrin commented 1 year ago

I am going to split this into a few things:

[x] (DDC) do not show temp variables generated by CFE https://github.com/dart-lang/sdk/issues/51554 https://github.com/dart-lang/sdk/issues/51589

https://github.com/dart-lang/webdev/issues/2041 https://github.com/dart-lang/webdev/pull/2042
[ ] (dwds) minimize the number of debugger steps through CFE code https://github.com/dart-lang/webdev/issues/2034 https://github.com/dart-lang/webdev/pull/2043
[ ] (dwds, DevTools, dart-code?) cleanup hover over and evaluation behavior if needed

annagrin commented 1 year ago

All done!