command/views/json: Never generate invalid diagnostic snippet offsets

internal/command/format/diagnostic.go

@@ -250,6 +250,21 @@ func appendSourceSnippets(buf *bytes.Buffer, diag *viewsjson.Diagnostic, color *
 			}
 		}
 
+		// If either start or end is out of range for the code buffer then
+		// we'll cap them at the bounds just to avoid a panic, although
+		// this would happen only if there's a bug in the code generating
+		// the snippet objects.
+		if start < 0 {
+			start = 0
+		} else if start > len(code) {
+			start = len(code)
+		}
+		if end < 0 {
+			end = 0
+		} else if end > len(code) {
+			end = len(code)
+		}
+
 		before, highlight, after := code[0:start], code[start:end], code[end:]
 		code = fmt.Sprintf(color.Color("%s[underline]%s[reset]%s"), before, highlight, after)
 

internal/command/views/json/diagnostic.go

@@ -221,12 +221,23 @@ func NewDiagnostic(diag tfdiags.Diagnostic, sources map[string][]byte) *Diagnost
 			// to the code snippet string.
 			start := highlightRange.Start.Byte - codeStartByte
 			end := start + (highlightRange.End.Byte - highlightRange.Start.Byte)
-			if start > len(codeStr) {
+
+			// We can end up with some quirky results here in edge cases like
+			// when a source range starts or ends at a newline character,
+			// so we'll cap the results at the bounds of the highlight range
+			// so that consumers of this data don't need to contend with
+			// out-of-bounds errors themselves.
+			if start < 0 {
+				start = 0
+			} else if start > len(codeStr) {
 				start = len(codeStr)
 			}
-			if end > len(codeStr) {
+			if end < 0 {
+				end = 0
+			} else if end > len(codeStr) {
 				end = len(codeStr)
 			}
+
 			diagnostic.Snippet.HighlightStartOffset = start
 			diagnostic.Snippet.HighlightEndOffset = end
 

internal/command/views/json/diagnostic_test.go

@@ -29,7 +29,8 @@ func TestNewDiagnostic(t *testing.T) {
   }
 }
 `),
-		"short.tf": []byte("bad source code"),
+		"short.tf":       []byte("bad source code"),
+		"odd-comment.tf": []byte("foo\n\n#\n"),
 		"values.tf": []byte(`[
   var.a,
   var.b,
@@ -285,6 +286,51 @@ func TestNewDiagnostic(t *testing.T) {
 				},
 			},
 		},
+		"error whose range starts at a newline": {
+			&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Invalid newline",
+				Detail:   "How awkward!",
+				Subject: &hcl.Range{
+					Filename: "odd-comment.tf",
+					Start:    hcl.Pos{Line: 2, Column: 5, Byte: 4},
+					End:      hcl.Pos{Line: 3, Column: 1, Byte: 6},
+				},
+			},
+			&Diagnostic{
+				Severity: "error",
+				Summary:  "Invalid newline",
+				Detail:   "How awkward!",
+				Range: &DiagnosticRange{
+					Filename: "odd-comment.tf",
+					Start: Pos{
+						Line:   2,
+						Column: 5,
+						Byte:   4,
+					},
+					End: Pos{
+						Line:   3,
+						Column: 1,
+						Byte:   6,
+					},
+				},
+				Snippet: &DiagnosticSnippet{
+					Code:      `#`,
+					StartLine: 2,
+					Values:    []DiagnosticExpressionValue{},
+
+					// Due to the range starting at a newline on a blank
+					// line, we end up stripping off the initial newline
+					// to produce only a one-line snippet. That would
+					// therefore cause the start offset to naturally be
+					// -1, just before the Code we returned, but then we
+					// force it to zero so that the result will still be
+					// in range for a byte-oriented slice of Code.
+					HighlightStartOffset: 0,
+					HighlightEndOffset:   1,
+				},
+			},
+		},
 		"error with source code subject and known expression": {
 			&hcl.Diagnostic{
 				Severity: hcl.DiagError,

internal/command/views/json/testdata/diagnostic/error-whose-range-starts-at-a-newline.json

@@ -0,0 +1,26 @@
+{
+  "severity": "error",
+  "summary": "Invalid newline",
+  "detail": "How awkward!",
+  "range": {
+    "filename": "odd-comment.tf",
+    "start": {
+      "line": 2,
+      "column": 5,
+      "byte": 4
+    },
+    "end": {
+      "line": 3,
+      "column": 1,
+      "byte": 6
+    }
+  },
+  "snippet": {
+    "context": null,
+    "code": "#",
+    "start_line": 2,
+    "highlight_start_offset": 0,
+    "highlight_end_offset": 1,
+    "values": []
+  }
+}