libexpr: Deprecate URL literals

Closes #437.

Change-Id: I9f67fc965bb4a7e7fd849e5067ac1cb3bab064cd

3 files changed, 16 insertions, 2 deletions

diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index a0ad125fb..36e42fe63 100644
--- a/src/libutil/config.hh
+++ b/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:
 
diff --git a/src/libutil/deprecated-features.cc b/src/libutil/deprecated-features.cc
index 7c59d8598..11b6c42bd 100644
--- a/src/libutil/deprecated-features.cc
+++ b/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(
diff --git a/src/libutil/deprecated-features.hh b/src/libutil/deprecated-features.hh
index 86a9b8a5a..c00a5d7bd 100644
--- a/src/libutil/deprecated-features.hh
+++ b/src/libutil/deprecated-features.hh
@@ -18,6 +18,7 @@ namespace nix {
  */
 enum struct DeprecatedFeature
 {
+    UrlLiterals
 };
 
 /**