libexpr/parser: Test experimental features

Currently, the parser relies on the global experimental feature flags.
In order to properly test conditional language features, we instead need
to pass it around in the parser::State.

This means that the parser cannot cache the result of isEnabled anymore,
which wouldn't necessarily hurt performance if the function didn't
perform a linear search on the list of enabled features on every single
call. While we could simply evaluate once at the start of parsing and
cache the result in the parser state, the more sustainable solution
would be to fix `isEnabled` such that all callers may profit from the
performance improvement.

Change-Id: Ic9b9c5d882b6270e1114988b63e6064d36c25cf2

src/libexpr/parser/parser.cc

@@ -631,7 +631,7 @@ template<> struct BuildAST<grammar::expr::path> : p::maybe_nothing {};
 
 template<> struct BuildAST<grammar::expr::uri> {
     static void apply(const auto & in, ExprState & s, State & ps) {
-       static bool noURLLiterals = experimentalFeatureSettings.isEnabled(Xp::NoUrlLiterals);
+       bool noURLLiterals = ps.xpSettings.isEnabled(Xp::NoUrlLiterals);
        if (noURLLiterals)
            throw ParseError({
                .msg = HintFmt("URL literals are disabled"),
@@ -832,7 +832,8 @@ Expr * EvalState::parse(
     size_t length,
     Pos::Origin origin,
     const SourcePath & basePath,
-    std::shared_ptr<StaticEnv> & staticEnv)
+    std::shared_ptr<StaticEnv> & staticEnv,
+    const ExperimentalFeatureSettings & xpSettings)
 {
     parser::State s = {
         symbols,
@@ -840,6 +841,7 @@ Expr * EvalState::parse(
         basePath,
         positions.addOrigin(origin, length),
         exprSymbols,
+        xpSettings
     };
     parser::ExprState x;