internal/lsp/source: omit assign stmt LHS in RHS completion

In the below example:

    fooBar := fooB<>

We will no longer suggest "fooBar" at <>.

Fixes golang/go#39203.

Change-Id: Ie1450397ce3de4f21fb0862c1a4f0fe2812325fd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/275693
Run-TryBot: Muir Manders <muir@mnd.rs>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Trust: Rebecca Stambler <rstambler@golang.org>
Trust: Robert Findley <rfindley@google.com>

internal/lsp/source/completion/completion.go

@@ -1284,12 +1284,10 @@ func (c *completer) lexical(ctx context.Context) error {
 				}
 			}
 
-			// Don't use LHS of value spec in RHS.
-			if vs := enclosingValueSpec(c.path); vs != nil {
-				for _, ident := range vs.Names {
-					if obj.Pos() == ident.Pos() {
-						continue Names
-					}
+			// Don't use LHS of decl in RHS.
+			for _, ident := range enclosingDeclLHS(c.path) {
+				if obj.Pos() == ident.Pos() {
+					continue Names
 				}
 			}
 

internal/lsp/source/completion/util.go

@@ -195,10 +195,21 @@ func enclosingSelector(path []ast.Node, pos token.Pos) *ast.SelectorExpr {
 	return nil
 }
 
-func enclosingValueSpec(path []ast.Node) *ast.ValueSpec {
+// enclosingDeclLHS returns LHS idents from containing value spec or
+// assign statement.
+func enclosingDeclLHS(path []ast.Node) []*ast.Ident {
 	for _, n := range path {
-		if vs, ok := n.(*ast.ValueSpec); ok {
-			return vs
+		switch n := n.(type) {
+		case *ast.ValueSpec:
+			return n.Names
+		case *ast.AssignStmt:
+			ids := make([]*ast.Ident, 0, len(n.Lhs))
+			for _, e := range n.Lhs {
+				if id, ok := e.(*ast.Ident); ok {
+					ids = append(ids, id)
+				}
+			}
+			return ids
 		}
 	}
 

internal/lsp/testdata/assign/assign.go.in

@@ -16,3 +16,11 @@ func _() {
 func _() {
 	var a string = a //@complete(" //")
 }
+
+func _() {
+	fooBar := fooBa //@complete(" //"),item(assignFooBar, "fooBar", "", "var")
+	abc, fooBar := 123, fooBa //@complete(" //", assignFooBar)
+	{
+		fooBar := fooBa //@complete(" //", assignFooBar)
+	}
+}

internal/lsp/testdata/summary.txt.golden

@@ -1,7 +1,7 @@
 -- summary --
 CallHierarchyCount = 1
 CodeLensCount = 5
-CompletionsCount = 255
+CompletionsCount = 258
 CompletionSnippetCount = 88
 UnimportedCompletionsCount = 5
 DeepCompletionsCount = 5