1 files changed, 95 insertions, 0 deletions
diff --git a/gopls/internal/lsp/analysis/nonewvars/nonewvars.go b/gopls/internal/lsp/analysis/nonewvars/nonewvars.go
new file mode 100644
index 000000000..6937b36d1
--- /dev/null
+++ b/gopls/internal/lsp/analysis/nonewvars/nonewvars.go
@@ -0,0 +1,95 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package nonewvars defines an Analyzer that applies suggested fixes
+// to errors of the type "no new variables on left side of :=".
+package nonewvars
+
+import (
+	"bytes"
+	"go/ast"
+	"go/format"
+	"go/token"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/analysis/passes/inspect"
+	"golang.org/x/tools/go/ast/inspector"
+	"golang.org/x/tools/internal/analysisinternal"
+)
+
+const Doc = `suggested fixes for "no new vars on left side of :="
+
+This checker provides suggested fixes for type errors of the
+type "no new vars on left side of :=". For example:
+	z := 1
+	z := 2
+will turn into
+	z := 1
+	z = 2
+`
+
+var Analyzer = &analysis.Analyzer{
+	Name:             "nonewvars",
+	Doc:              Doc,
+	Requires:         []*analysis.Analyzer{inspect.Analyzer},
+	Run:              run,
+	RunDespiteErrors: true,
+}
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
+	if len(pass.TypeErrors) == 0 {
+		return nil, nil
+	}
+
+	nodeFilter := []ast.Node{(*ast.AssignStmt)(nil)}
+	inspect.Preorder(nodeFilter, func(n ast.Node) {
+		assignStmt, _ := n.(*ast.AssignStmt)
+		// We only care about ":=".
+		if assignStmt.Tok != token.DEFINE {
+			return
+		}
+
+		var file *ast.File
+		for _, f := range pass.Files {
+			if f.Pos() <= assignStmt.Pos() && assignStmt.Pos() < f.End() {
+				file = f
+				break
+			}
+		}
+		if file == nil {
+			return
+		}
+
+		for _, err := range pass.TypeErrors {
+			if !FixesError(err.Msg) {
+				continue
+			}
+			if assignStmt.Pos() > err.Pos || err.Pos >= assignStmt.End() {
+				continue
+			}
+			var buf bytes.Buffer
+			if err := format.Node(&buf, pass.Fset, file); err != nil {
+				continue
+			}
+			pass.Report(analysis.Diagnostic{
+				Pos:     err.Pos,
+				End:     analysisinternal.TypeErrorEndPos(pass.Fset, buf.Bytes(), err.Pos),
+				Message: err.Msg,
+				SuggestedFixes: []analysis.SuggestedFix{{
+					Message: "Change ':=' to '='",
+					TextEdits: []analysis.TextEdit{{
+						Pos: err.Pos,
+						End: err.Pos + 1,
+					}},
+				}},
+			})
+		}
+	})
+	return nil, nil
+}
+
+func FixesError(msg string) bool {
+	return msg == "no new variables on left side of :="
+}