libexpr: Deprecate URL literals

Closes #437.

Change-Id: I9f67fc965bb4a7e7fd849e5067ac1cb3bab064cd

src/libexpr/parser/parser.cc

@@ -656,10 +656,10 @@ 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.featureSettings.isEnabled(Xp::NoUrlLiterals);
-       if (noURLLiterals)
+       bool URLLiterals = ps.featureSettings.isEnabled(Dep::UrlLiterals);
+       if (!URLLiterals)
            throw ParseError({
-               .msg = HintFmt("URL literals are disabled"),
+               .msg = HintFmt("URL literals are deprecated, allow using them with --extra-deprecated-features=url-literals"),
                .pos = ps.positions[ps.at(in)]
            });
        s.pushExpr<ExprString>(ps.at(in), in.string());

src/libutil/config.hh

@@ -488,7 +488,13 @@ struct FeatureSettings : Config {
     Setting<std::set<DeprecatedFeature>> deprecatedFeatures{
         this, {}, "deprecated-features",
         R"(
-          Deprecated features that are allowed. (Currently there are none.)
+          Deprecated features that are allowed.
+
+          Example:
+
+          ```
+          deprecated-features = url-literals
+          ```
 
           The following deprecated feature features can be re-activated:
 

src/libutil/deprecated-features.cc

@@ -24,9 +24,16 @@ struct DeprecatedFeatureDetails
  * feature, we either have no issue at all if few features are not added
  * at the end of the list, or a proper merge conflict if they are.
  */
-constexpr size_t numDepFeatures = 0;
+constexpr size_t numDepFeatures = 1 + static_cast<size_t>(Dep::UrlLiterals);
 
 constexpr std::array<DeprecatedFeatureDetails, numDepFeatures> depFeatureDetails = {{
+    {
+        .tag = Dep::UrlLiterals,
+        .name = "url-literals",
+        .description = R"(
+            Allow unquoted URLs as part of the Nix language syntax.
+        )",
+    },
 }};
 
 static_assert(

src/libutil/deprecated-features.hh

@@ -18,6 +18,7 @@ namespace nix {
  */
 enum struct DeprecatedFeature
 {
+    UrlLiterals
 };
 
 /**