Add detection of eval to the parser

While `eval` is still a function, this paves the way for changing that
in the future, and lets the proc/exec functions detect when an eval is
used to allow/disallow certain behaviors and optimizations.

src/complete.cpp

@@ -1465,6 +1465,14 @@ void completer_t::perform() {
                 use_abbr = false;
                 break;
             }
+            case parse_statement_decoration_eval: {
+                use_command = true;
+                use_function = true;
+                use_builtin = true;
+                use_implicit_cd = false;
+                use_abbr = false;
+                break;
+            }
         }
 
         if (cmd_node.location_in_or_at_end_of_source_range(pos)) {

src/exec.cpp

@@ -944,6 +944,7 @@ static bool exec_process_in_job(parser_t &parser, process_t *p, std::shared_ptr<
     // Execute the process.
     p->check_generations_before_launch();
     switch (p->type) {
+        case process_type_t::eval: /* so long as `eval` is a function */
         case process_type_t::function:
         case process_type_t::block_node: {
             // Allow buffering unless this is a deferred run. If deferred, then processes after us

src/parse_constants.h

@@ -97,7 +97,6 @@ const enum_map<parse_token_type_t> token_enum_map[] = {
 //
 // IMPORTANT: These enums must start at zero.
 enum parse_keyword_t {
-    parse_keyword_none = 0,
     parse_keyword_and,
     parse_keyword_begin,
     parse_keyword_builtin,
@@ -105,13 +104,15 @@ enum parse_keyword_t {
     parse_keyword_command,
     parse_keyword_else,
     parse_keyword_end,
+    parse_keyword_eval,
+    parse_keyword_exclam,
     parse_keyword_exec,
     parse_keyword_for,
     parse_keyword_function,
     parse_keyword_if,
     parse_keyword_in,
+    parse_keyword_none,
     parse_keyword_not,
-    parse_keyword_exclam,
     parse_keyword_or,
     parse_keyword_switch,
     parse_keyword_while,
@@ -125,6 +126,7 @@ const enum_map<parse_keyword_t> keyword_enum_map[] = {{parse_keyword_exclam, L"!
                                                       {parse_keyword_command, L"command"},
                                                       {parse_keyword_else, L"else"},
                                                       {parse_keyword_end, L"end"},
+                                                      {parse_keyword_eval, L"eval"},
                                                       {parse_keyword_exec, L"exec"},
                                                       {parse_keyword_for, L"for"},
                                                       {parse_keyword_function, L"function"},
@@ -144,7 +146,8 @@ enum parse_statement_decoration_t {
     parse_statement_decoration_none,
     parse_statement_decoration_command,
     parse_statement_decoration_builtin,
-    parse_statement_decoration_exec
+    parse_statement_decoration_exec,
+    parse_statement_decoration_eval,
 };
 
 // Boolean statement types, stored in node tag.

src/parse_grammar.h

@@ -334,6 +334,8 @@ DEF_ALT(decorated_statement) {
     using cmds = seq<keyword<parse_keyword_command>, plain_statement>;
     using builtins = seq<keyword<parse_keyword_builtin>, plain_statement>;
     using execs = seq<keyword<parse_keyword_exec>, plain_statement>;
+    // using evals = seq<keyword<parse_keyword_eval>, plain_statement>;
+    using evals = single<plain_statement>; /* so long as `eval` is a function */
     ALT_BODY(decorated_statement, plains, cmds, builtins, execs);
 };
 

src/parse_productions.cpp

@@ -314,6 +314,10 @@ RESOLVE(decorated_statement) {
             *out_tag = parse_statement_decoration_exec;
             return production_for<execs>();
         }
+        case parse_keyword_eval: {
+            *out_tag = parse_statement_decoration_eval;
+            return production_for<evals>();
+        }
         default: {
             *out_tag = parse_statement_decoration_none;
             return production_for<plains>();

src/proc.h

@@ -36,6 +36,8 @@ enum class process_type_t {
     block_node,
     /// The exec builtin.
     exec,
+    /// A literal evaluation
+    eval,
 };
 
 enum class job_control_t {