fix: avoid crash on empty file formatting (hashicorp#578)

Author: radeksimko

internal/hcl/diff.go

@@ -112,9 +112,25 @@ func diffLines(filename string, beforeLines, afterLines source.Lines) filesystem
 			}
 
 			if c.Tag == OpInsert {
-				insertRng := &hcl.Range{}
-				insertRng.Start = beforeLines[beforeStart-1].Range().End
-				insertRng.End = beforeLines[beforeStart-1].Range().End
+				insertRng := &hcl.Range{
+					Filename: filename,
+					Start:    hcl.InitialPos,
+					End:      hcl.InitialPos,
+				}
+
+				if beforeStart == beforeEnd {
+					line := beforeLines[beforeStart]
+					insertRng = line.Range().Ptr()
+				} else {
+					for i, line := range beforeLines[beforeStart:beforeEnd] {
+						if i == 0 {
+							insertRng = line.Range().Ptr()
+							continue
+						}
+						insertRng.End = line.Range().End
+					}
+				}
+
 				var newBytes []byte
 
 				for _, line := range afterLines[afterStart:afterEnd] {

internal/hcl/diff_test.go

@@ -147,8 +147,9 @@ ccc`,
 				&fileChange{
 					newText: "\n",
 					rng: &hcl.Range{
-						Start: hcl.Pos{Line: 3, Column: 1, Byte: 30},
-						End:   hcl.Pos{Line: 3, Column: 1, Byte: 30},
+						Filename: "test.tf",
+						Start:    hcl.Pos{Line: 3, Column: 1, Byte: 30},
+						End:      hcl.Pos{Line: 3, Column: 1, Byte: 30},
 					},
 				},
 			},
@@ -177,6 +178,22 @@ ccc`,
 				},
 			},
 		},
+		{
+			"empty to newline",
+			``,
+			`
+`,
+			filesystem.DocumentChanges{
+				&fileChange{
+					newText: "\n",
+					rng: &hcl.Range{
+						Filename: "test.tf",
+						Start:    hcl.Pos{Line: 1, Column: 1, Byte: 0},
+						End:      hcl.Pos{Line: 1, Column: 1, Byte: 0},
+					},
+				},
+			},
+		},
 	}
 
 	for i, tc := range testCases {