Revert "parser: try to avoid some strings being copied"

This reverts commit 7a74198aa3.

Believe it or not this commit actually increased copying. When accepting
a value you know you're going to take ownership of, just accept it by
value; then temporaries can invoke the move ctor and blah blah blah.

We really need a lightweight refcounted pass-by-value string to make this
less error prone.

src/parse_tree.cpp

@@ -1135,7 +1135,7 @@ const parse_node_t *parse_node_tree_t::get_child(const parse_node_t &parent, nod
     return result;
 }
 
-parsed_source_ref_t parse_source(const wcstring &src, parse_tree_flags_t flags, parse_error_list_t *errors,
+parsed_source_ref_t parse_source(wcstring src, parse_tree_flags_t flags, parse_error_list_t *errors,
                                  parse_token_type_t goal) {
     parse_node_tree_t tree;
     if (!parse_tree_from_string(src, flags, &tree, errors, goal)) return {};

src/parse_tree.h

@@ -233,7 +233,7 @@ struct parsed_source_t {
 };
 /// Return a shared pointer to parsed_source_t, or null on failure.
 using parsed_source_ref_t = std::shared_ptr<const parsed_source_t>;
-parsed_source_ref_t parse_source(const wcstring &src, parse_tree_flags_t flags, parse_error_list_t *errors,
+parsed_source_ref_t parse_source(wcstring src, parse_tree_flags_t flags, parse_error_list_t *errors,
                                  parse_token_type_t goal = symbol_job_list);
 
 #endif

src/parser.cpp

@@ -618,7 +618,7 @@ profile_item_t *parser_t::create_profile_item() {
     return result;
 }
 
-int parser_t::eval(const wcstring &cmd, const io_chain_t &io, enum block_type_t block_type) {
+int parser_t::eval(wcstring cmd, const io_chain_t &io, enum block_type_t block_type) {
     // Parse the source into a tree, if we can.
     parse_error_list_t error_list;
     parsed_source_ref_t ps = parse_source(cmd, parse_flag_none, &error_list);

src/parser.h

@@ -222,7 +222,7 @@ class parser_t : public std::enable_shared_from_this<parser_t> {
     /// \param block_type The type of block to push on the block stack
     ///
     /// \return 0 on success, 1 on a parse error.
-    int eval(const wcstring &cmd, const io_chain_t &io, enum block_type_t block_type);
+    int eval(wcstring cmd, const io_chain_t &io, enum block_type_t block_type);
 
     /// Evaluate the parsed source ps.
     void eval(parsed_source_ref_t ps, const io_chain_t &io, enum block_type_t block_type);