Avoid panic by not exceeding bounds of string..

Signed-off-by: James Phillips <jamesdphillips@gmail.com>

language/lexer/lexer.go

@@ -381,6 +381,23 @@ func blockStringValue(in string) string {
 	// Expand a block string's raw value into independent lines.
 	lines := splitLinesRegex.Split(in, -1)
 
+	// Remove leading blank lines.
+	for {
+		if isBlank := lineIsBlank(lines[0]); !isBlank {
+			break
+		}
+		lines = lines[1:]
+	}
+
+	// Remove trailing blank lines.
+	for {
+		i := len(lines) - 1
+		if isBlank := lineIsBlank(lines[i]); !isBlank {
+			break
+		}
+		lines = append(lines[:i], lines[i+1:]...)
+	}
+
 	// Remove common indentation from all lines but first
 	commonIndent := -1
 	for i := 1; i < len(lines); i++ {
@@ -395,27 +412,13 @@ func blockStringValue(in string) string {
 	}
 	if commonIndent > 0 {
 		for i, line := range lines {
+			if commonIndent > len(line) {
+				continue
+			}
 			lines[i] = line[commonIndent:]
 		}
 	}
 
-	// Remove leading blank lines.
-	for {
-		if isBlank := lineIsBlank(lines[0]); !isBlank {
-			break
-		}
-		lines = lines[1:]
-	}
-
-	// Remove trailing blank lines.
-	for {
-		i := len(lines) - 1
-		if isBlank := lineIsBlank(lines[i]); !isBlank {
-			break
-		}
-		lines = append(lines[:i], lines[i+1:]...)
-	}
-
 	// Return a string of the lines joined with U+000A.
 	return strings.Join(lines, "\n")
 }

language/lexer/lexer_test.go

@@ -480,6 +480,22 @@ func TestLexer_LexesBlockStrings(t *testing.T) {
 				Value: "  white space ",
 			},
 		},
+		{
+			Body: `
+				"""
+						my great description
+						spans multiple lines
+
+						with breaks
+				"""
+			`,
+			Expected: Token{
+				Kind:  TokenKind[BLOCK_STRING],
+				Start: 5,
+				End:   89,
+				Value: "my great description\nspans multiple lines\n\nwith breaks",
+			},
+		},
 		{
 			Body: `"""contains " quote"""`,
 			Expected: Token{