8 files changed, 44 insertions, 5 deletions
diff --git a/doc/manual/change-authors.yml b/doc/manual/change-authors.yml
index 5894ce821..28a20b25f 100644
--- a/doc/manual/change-authors.yml
+++ b/doc/manual/change-authors.yml
@@ -44,6 +44,11 @@ cole-h:
   display_name: Cole Helbling
   github: cole-h
 
+delan:
+  display_name: delan
+  forgejo: delan
+  github: delan
+
 edolstra:
   display_name: Eelco Dolstra
   github: edolstra
diff --git a/doc/manual/rl-next/registry-add-shorthand-only.md b/doc/manual/rl-next/registry-add-shorthand-only.md
new file mode 100644
index 000000000..72f972b0a
--- /dev/null
+++ b/doc/manual/rl-next/registry-add-shorthand-only.md
@@ -0,0 +1,10 @@
+---
+synopsis: "`nix registry add` now requires a shorthand flakeref on the 'from' side"
+cls: 1494
+credits: delan
+category: Improvements
+---
+
+The 'from' argument must now be a shorthand flakeref like `nixpkgs` or `nixpkgs/nixos-20.03`, making it harder to accidentally swap the 'from' and 'to' arguments.
+
+Registry entries that map from other flake URLs can still be specified in registry.json, the `nix.registry` option in NixOS, or the `--override-flake` option in the CLI, but they are not guaranteed to work correctly.
diff --git a/src/libexpr/flake/flakeref.cc b/src/libexpr/flake/flakeref.cc
index 1c90bfc43..a95df04ba 100644
--- a/src/libexpr/flake/flakeref.cc
+++ b/src/libexpr/flake/flakeref.cc
@@ -85,8 +85,8 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
         + "(?:#(" + queryRegex + "))?",
         std::regex::ECMAScript);
 
-    static std::regex flakeRegex(
-        "((" + flakeIdRegexS + ")(?:/(?:" + refAndOrRevRegex + "))?)"
+    static std::regex flakeShorthandRegex(
+        flakeShorthandRegexS
         + "(?:#(" + queryRegex + "))?",
         std::regex::ECMAScript);
 
@@ -95,7 +95,7 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
     /* Check if 'url' is a flake ID. This is an abbreviated syntax for
        'flake:<flake-id>?ref=<ref>&rev=<rev>'. */
 
-    if (std::regex_match(url, match, flakeRegex)) {
+    if (std::regex_match(url, match, flakeShorthandRegex)) {
         auto parsedURL = ParsedURL{
             .url = url,
             .base = "flake:" + match.str(1),
diff --git a/src/libutil/url-parts.hh b/src/libutil/url-parts.hh
index 6efcc7e50..1f570567c 100644
--- a/src/libutil/url-parts.hh
+++ b/src/libutil/url-parts.hh
@@ -46,4 +46,7 @@ const static std::string refAndOrRevRegex = "(?:(" + revRegexS + ")|(?:(" + refR
 const static std::string flakeIdRegexS = "[a-zA-Z][a-zA-Z0-9_-]*";
 extern std::regex flakeIdRegex;
 
+const static std::string flakeShorthandRegexS = "((" + flakeIdRegexS + ")(?:/(?:" + refAndOrRevRegex + "))?)";
+extern std::regex flakeShorthandRegex;
+
 }
diff --git a/src/libutil/url.cc b/src/libutil/url.cc
index 46688cef5..87146ca56 100644
--- a/src/libutil/url.cc
+++ b/src/libutil/url.cc
@@ -9,6 +9,7 @@ std::regex refRegex(refRegexS, std::regex::ECMAScript);
 std::regex badGitRefRegex(badGitRefRegexS, std::regex::ECMAScript);
 std::regex revRegex(revRegexS, std::regex::ECMAScript);
 std::regex flakeIdRegex(flakeIdRegexS, std::regex::ECMAScript);
+std::regex flakeShorthandRegex(flakeShorthandRegexS, std::regex::ECMAScript);
 
 ParsedURL parseURL(const std::string & url)
 {
diff --git a/src/nix/registry-add.md b/src/nix/registry-add.md
index a947fa0b3..90b54a596 100644
--- a/src/nix/registry-add.md
+++ b/src/nix/registry-add.md
@@ -31,8 +31,9 @@ R""(
 # Description
 
 This command adds an entry to the user registry that maps flake
-reference *from-url* to flake reference *to-url*. If an entry for
-*from-url* already exists, it is overwritten.
+reference *from-url* to flake reference *to-url*, where *from-url*
+must be a shorthand like 'nixpkgs' or 'nixpkgs/nixos-20.03'. If an
+entry for *from-url* already exists, it is overwritten.
 
 Entries can be removed using [`nix registry
 remove`](./nix3-registry-remove.md).
diff --git a/src/nix/registry.cc b/src/nix/registry.cc
index f509ccae8..9619b85d4 100644
--- a/src/nix/registry.cc
+++ b/src/nix/registry.cc
@@ -5,6 +5,7 @@
 #include "flake/flake.hh"
 #include "store-api.hh"
 #include "fetchers.hh"
+#include "url-parts.hh"
 #include "registry.hh"
 
 using namespace nix;
@@ -109,7 +110,14 @@ struct CmdRegistryAdd : MixEvalArgs, Command, RegistryCommand
 
     void run() override
     {
+        std::smatch match;
+        if (!std::regex_match(fromUrl, match, flakeShorthandRegex)) {
+            throw UsageError("'from-url' argument must be a shorthand like 'nixpkgs' or 'nixpkgs/nixos-20.03'");
+        }
         auto fromRef = parseFlakeRef(fromUrl);
+        if (fromRef.input.direct) {
+            throw UsageError("'from-url' argument must be an indirect flakeref like 'nixpkgs' or 'flake:nixpkgs'");
+        }
         auto toRef = parseFlakeRef(toUrl);
         auto registry = getRegistry();
         fetchers::Attrs extraAttrs;
diff --git a/tests/functional/flakes/flakes.sh b/tests/functional/flakes/flakes.sh
index 68a2fd2ce..97dc6d818 100644
--- a/tests/functional/flakes/flakes.sh
+++ b/tests/functional/flakes/flakes.sh
@@ -366,6 +366,17 @@ nix registry pin flake1 flake3
 nix registry remove flake1
 [[ $(nix registry list | wc -l) == 5 ]]
 
+# 'nix registry add' should accept flake shorthands (with or without branch or rev)
+# in the from argument, but reject fully-qualified from-urls (direct or indirect).
+nix registry add nixpkgz github:NixOS/nixpkgz
+nix registry remove nixpkgz
+nix registry add nixpkgz/branch github:NixOS/nixpkgz
+nix registry remove nixpkgz/branch
+nix registry add nixpkgz/branch/1db42b7fe3878f3f5f7a4f2dc210772fd080e205 github:NixOS/nixpkgz
+nix registry remove nixpkgz/branch/1db42b7fe3878f3f5f7a4f2dc210772fd080e205
+! nix registry add flake:nixpkgz github:NixOS/nixpkgz
+! nix registry add github:NixOS/nixpkgz github:NixOS/nixpkgz
+
 # Test 'nix registry list' with a disabled global registry.
 nix registry add user-flake1 git+file://$flake1Dir
 nix registry add user-flake2 git+file://$flake2Dir