gopls/internal/analysis/modernize: fix bad edit in var ( ... ) decl

The TextEdit.End offset was wrong in the case where the var decl
has an explicit close paren.

+ test

Fixes golang/go#75318

Change-Id: I5caa20dbc81a0a239bcdab354949e184135b11ad
Reviewed-on: https://go-review.googlesource.com/c/tools/+/701780
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

gopls/internal/analysis/modernize/stringsbuilder.go

@@ -179,7 +179,7 @@ nextcand:
 				// delete "= expr"
 				edits = append(edits, analysis.TextEdit{
 					Pos: init,
-					End: decl.End(),
+					End: spec.End(),
 				})
 			}
 

gopls/internal/analysis/modernize/testdata/src/stringsbuilder/stringsbuilder.go

@@ -88,3 +88,14 @@ func _(x string) string {
 	}
 	return s
 }
+
+// Regression test for bug in a GenDecl with parens.
+func issue75318(slice []string) string {
+	var (
+		msg string
+	)
+	for _, s := range slice {
+		msg += s // want "using string \\+= string in a loop is inefficient"
+	}
+	return msg
+}

gopls/internal/analysis/modernize/testdata/src/stringsbuilder/stringsbuilder.go.golden

@@ -93,3 +93,14 @@ func _(x string) string {
 	}
 	return s
 }
+
+// Regression test for bug in a GenDecl with parens.
+func issue75318(slice []string) string {
+	var (
+		msg strings.Builder
+	)
+	for _, s := range slice {
+		msg.WriteString(s) // want "using string \\+= string in a loop is inefficient"
+	}
+	return msg.String()
+}