libexpr: Introduce Deprecated features

They are like experimental features, but opt-in instead of opt-out. They
will allow us to gracefully remove language features. See #437

Change-Id: I9ca04cc48e6926750c4d622c2b229b25cc142c42

src/libexpr/eval.cc

@@ -2710,20 +2710,29 @@ Expr & EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<Sta
 }
 
 
-Expr & EvalState::parseExprFromString(std::string s_, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv, const ExperimentalFeatureSettings & xpSettings)
+Expr & EvalState::parseExprFromString(
+    std::string s_,
+    const SourcePath & basePath,
+    std::shared_ptr<StaticEnv> & staticEnv,
+    const FeatureSettings & featureSettings
+)
 {
     // NOTE this method (and parseStdin) must take care to *fully copy* their input
     // into their respective Pos::Origin until the parser stops overwriting its input
     // data.
     auto s = make_ref<std::string>(s_);
     s_.append("\0\0", 2);
-    return *parse(s_.data(), s_.size(), Pos::String{.source = s}, basePath, staticEnv, xpSettings);
+    return *parse(s_.data(), s_.size(), Pos::String{.source = s}, basePath, staticEnv, featureSettings);
 }
 
 
-Expr & EvalState::parseExprFromString(std::string s, const SourcePath & basePath, const ExperimentalFeatureSettings & xpSettings)
+Expr & EvalState::parseExprFromString(
+    std::string s,
+    const SourcePath & basePath,
+    const FeatureSettings & featureSettings
+)
 {
-    return parseExprFromString(std::move(s), basePath, staticBaseEnv, xpSettings);
+    return parseExprFromString(std::move(s), basePath, staticBaseEnv, featureSettings);
 }
 
 

src/libexpr/eval.hh

@@ -344,8 +344,17 @@ public:
     /**
      * Parse a Nix expression from the specified string.
      */
-    Expr & parseExprFromString(std::string s, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv, const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
-    Expr & parseExprFromString(std::string s, const SourcePath & basePath, const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
+    Expr & parseExprFromString(
+        std::string s,
+        const SourcePath & basePath,
+        std::shared_ptr<StaticEnv> & staticEnv,
+        const FeatureSettings & xpSettings = featureSettings
+    );
+    Expr & parseExprFromString(
+        std::string s,
+        const SourcePath & basePath,
+        const FeatureSettings & xpSettings = featureSettings
+    );
 
     Expr & parseStdin();
 
@@ -569,7 +578,7 @@ private:
         Pos::Origin origin,
         const SourcePath & basePath,
         std::shared_ptr<StaticEnv> & staticEnv,
-        const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
+        const FeatureSettings & xpSettings = featureSettings);
 
     /**
      * Current Nix call stack depth, used with `max-call-depth` setting to throw stack overflow hopefully before we run out of system stack.

src/libexpr/parser/parser.cc

@@ -115,7 +115,7 @@ struct ExprState
 
     std::unique_ptr<Expr> pipe(PosIdx pos, State & state, bool flip = false)
     {
-        if (!state.xpSettings.isEnabled(Xp::PipeOperator))
+        if (!state.featureSettings.isEnabled(Xp::PipeOperator))
             throw ParseError({
                 .msg = HintFmt("Pipe operator is disabled"),
                 .pos = state.positions[pos]
@@ -656,7 +656,7 @@ 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) {
-       bool noURLLiterals = ps.xpSettings.isEnabled(Xp::NoUrlLiterals);
+       bool noURLLiterals = ps.featureSettings.isEnabled(Xp::NoUrlLiterals);
        if (noURLLiterals)
            throw ParseError({
                .msg = HintFmt("URL literals are disabled"),
@@ -858,7 +858,7 @@ Expr * EvalState::parse(
     Pos::Origin origin,
     const SourcePath & basePath,
     std::shared_ptr<StaticEnv> & staticEnv,
-    const ExperimentalFeatureSettings & xpSettings)
+    const FeatureSettings & featureSettings)
 {
     parser::State s = {
         symbols,
@@ -866,7 +866,7 @@ Expr * EvalState::parse(
         basePath,
         positions.addOrigin(origin, length),
         exprSymbols,
-        xpSettings
+        featureSettings,
     };
     parser::ExprState x;
 

src/libexpr/parser/state.hh

@@ -19,7 +19,7 @@ struct State
     SourcePath basePath;
     PosTable::Origin origin;
     const Expr::AstSymbols & s;
-    const ExperimentalFeatureSettings & xpSettings;
+    const FeatureSettings & featureSettings;
 
     void dupAttr(const AttrPath & attrPath, const PosIdx pos, const PosIdx prevPos);
     void dupAttr(Symbol attr, const PosIdx pos, const PosIdx prevPos);