Add goimports functionality to the DocumentFormatting

[JeanMertz]

Not sure if this is something you'd consider, but it would be nice if it was possible to auto add missing imports.

I use this diff locally, which seems to do the trick, but I haven't tested this thoroughly yet:

diff --git a/langserver/format.go b/langserver/format.go
index 7678fc9..6ab56bd 100644
--- a/langserver/format.go
+++ b/langserver/format.go
@@ -4,13 +4,13 @@ import (
        "bytes"
        "context"
        "fmt"
-       "go/ast"
        "go/parser"
        "go/printer"
        "go/token"
        "path"
 
        "golang.org/x/tools/go/buildutil"
+       "golang.org/x/tools/imports"
 
        "github.com/sourcegraph/go-langserver/langserver/util"
        "github.com/sourcegraph/go-langserver/pkg/lsp"
@@ -33,8 +33,6 @@ func (h *LangHandler) handleTextDocumentFormatting(ctx context.Context, conn jso
                return nil, err
        }
 
-       ast.SortImports(fset, file)
-
        var buf bytes.Buffer
        cfg := printer.Config{Mode: printer.UseSpaces | printer.TabIndent, Tabwidth: 8}
        err = cfg.Fprint(&buf, fset, file)
@@ -43,6 +41,12 @@ func (h *LangHandler) handleTextDocumentFormatting(ctx context.Context, conn jso
        }
 
        b := buf.Bytes()
+
+       b, err = imports.Process("", b, nil)
+       if err != nil {
+               return nil, err
+       }
+
        orig, err := h.readFile(ctx, params.TextDocument.URI)
        if err != nil {
                return nil, err

[keegancsmith]

I'm happy with changing our default to using goimports. If someone prefers plain old go fmt then we can provide an option later. Can you send a PR?

[lloiser]

#83 is probably also needed to get this working, right?

[JeanMertz]

Interesting @lloiser. I guess that answers the issue I posted here:

sublimelsp/LSP#319

So yes, the above "works", but as explained in the above issue, it's not very user friendly (but the current gofmt isn't either, since it suffers from the same problem).

[maximbaz]

I think having goimports functionality is very useful. Since #83 is merged and you have the working patch, @JeanMertz would you please create the PR?

[otto-md]

It would be very nice if it was possible to also configure the LocalPrefix if this is implemented.