Teach the tokenizer to report escaped newlines

Add fields and flags so that escaped newlines can be reported, for the
benefit of fish_indent.

src/parse_tree.h

@@ -28,15 +28,18 @@ constexpr source_offset_t SOURCE_OFFSET_INVALID = static_cast<source_offset_t>(-
 /// A struct representing the token type that we use internally.
 struct parse_token_t {
     enum parse_token_type_t type;  // The type of the token as represented by the parser
-    enum parse_keyword_t keyword;  // Any keyword represented by this token
-    bool has_dash_prefix;          // Hackish: whether the source contains a dash prefix
-    bool is_help_argument;         // Hackish: whether the source looks like '-h' or '--help'
-    bool is_newline;               // Hackish: if TOK_END, whether the source is a newline.
-    source_offset_t source_start;
-    source_offset_t source_length;
+    enum parse_keyword_t keyword{parse_keyword_none};  // Any keyword represented by this token
+    bool has_dash_prefix{false};          // Hackish: whether the source contains a dash prefix
+    bool is_help_argument{false};         // Hackish: whether the source looks like '-h' or '--help'
+    bool is_newline{false};               // Hackish: if TOK_END, whether the source is a newline.
+    bool preceding_escaped_nl{false};     // Whether there was an escaped newline preceding this token.
+    source_offset_t source_start{SOURCE_OFFSET_INVALID};
+    source_offset_t source_length{0};
 
     wcstring describe() const;
     wcstring user_presentable_description() const;
+
+    constexpr parse_token_t(parse_token_type_t type) : type(type) {}
 };
 
 enum {
@@ -66,6 +69,11 @@ const wchar_t *keyword_description(parse_keyword_t type);
 enum {
     /// Flag indicating that the node has associated comment nodes.
     parse_node_flag_has_comments = 1 << 0,
+
+    /// Flag indicating that the token was preceded by an escaped newline, e.g.
+    ///   echo abc | \
+    ///      cat
+    parse_node_flag_preceding_escaped_nl = 1 << 1,
 };
 typedef uint8_t parse_node_flags_t;
 
@@ -123,7 +131,12 @@ class parse_node_t {
 
     /// Indicate if the node has comment nodes.
     bool has_comments() const {
-        return static_cast<bool>(this->flags & parse_node_flag_has_comments);
+        return this->flags & parse_node_flag_has_comments;
+    }
+
+    /// Indicates if we have a preceding escaped newline.
+    bool has_preceding_escaped_newline() const {
+        return this->flags & parse_node_flag_preceding_escaped_nl;
     }
 
     /// Gets source for the node, or the empty string if it has no source.