debug: support function calls via delve 'call'

[ekulabuhov]

Following up from microsoft/vscode-go#2655 as it was the most upvoted feature there and is still badly needed.

Description

Delve has added the call command to be able to make function calls while in debug.

Trying to execute any function in VSCode Debug Console results in:
Unable to eval expression: "function calls not allowed without using 'call'"

There was a patch attached in the linked issue that added basic support for this feature. I can add a PR rebased on the latest version of the code to see if it's possible to continue from there.

[stamblerre]

/cc @polinasok @quoctruong

[hyangah]

Thanks for moving the issue and creating a PR!

Because DAP doesn't have a separate method for Delve's call equivalent, the PR currently proposes to reuse the evaluateRequest and distinguish between the usual expression and the call request using a regular expression as a heuristic.

My only concern is the regexp matches the simple builtin functions and type assertion delve's expressions supports. As delve's expressions gets powerful, the set of expression can get wider and the current heuristic may be an issue. I didn't check the current state of Go2 yet, but the syntax is not fully finalized, so that's another thing we need to think about.

I wonder if we can ask users to be more explicit, e.g. adding a call suffix?

And, @polinasok - this feature needs to implemented in the delve dap. What do you think makes sense for call support?

[hyangah]

microsoft/vscode-go#2992 is about supporting other delve commands beyond the call command. Since VS Code UI will trigger the same evaluateRequest, I think other commands need to be handled in similar ways.

[hyangah]

Talked with @eliben and delve devs, we will focus on the call command support initially - some other delve cli commands do not make sense within evaluateRequest, so it needs more thought.