x/tools/gopls: add some error handle postfix completion (rr, reterr, varCheckError)

[rogeryk]

Describe the solution you'd like

Go requires that you must explicit error handling, so many times you must write some boring error handling code, and postfix completion can ease the pain.
rr
In most cases, we just need check the error and return it.

This also depends on the return parameters of the current function, for example, when the function has return values other than error, the return values other than error will use default values.

reterr
Sometimes when calling a function, you just need to check if there is an error.

varCheckError
Get the return value after calling the function and check for errors. This is a more general version compared to reterr.

[muirdm]

There is already a fancy completion that writes the entire if err != nil { ... } for you. Do you see it? For example, completing at *HERE* below:

func foo() (int, error) {
  i, err := foo()
  if*HERE*
}

[rogeryk]

@muirdm Yes, It is same thing in the first case, but I think the latter two will still provide some convenience.
And i find that the rr and reterrr are the same postfix completion, so we can just focus on the latter two scenarios

[rogeryk]

@findleyr What do you think of this proposal? The pattern of error handle will not change anytime soon, so I think it's very valuable.

[gopherbot]

Change https://go.dev/cl/550455 mentions this issue: gopls/internal/lsp/source/completion: support postfix completion (rr, varCheckErr)

[findleyr]

@rogeryk I think this makes sense. I'll review your CL -- thanks!

[findleyr]

(although I'm not sure about varCheckErr -- let's discuss on the CL).

[findleyr]

In discussion on https://go.dev/cl/550455, we have settled on the more consistent snippet names iferr and variferr. I believe iferr is consistent with the preexisting vscode-go snippet: microsoft/vscode-go#1549 (comment).