feat: notation to refer to no attribute search prefix

An attrPath prefix of "." indicates no need to try default attrPath prefixes. For example `nixpkgs#legacyPackages.x86_64-linux.ERROR` searches through

```
trying flake output attribute 'packages.x86_64-linux.legacyPackages.x86_64-linux.ERROR'
using cached attrset attribute ''
trying flake output attribute 'legacyPackages.x86_64-linux.legacyPackages.x86_64-linux.ERROR'
using cached attrset attribute 'legacyPackages.x86_64-linux'
trying flake output attribute 'legacyPackages.x86_64-linux.ERROR'
using cached attrset attribute 'legacyPackages.x86_64-linux'
```

And there is no way to specify that one does not want the automatic
search behavior. Now one can specify
`nixpkgs#.legacyPackages.x86_64-linux.ERROR` to only refer to the rooted
attribute path without any default injection of attribute search path or
system.

Change-Id: Iac1334e1470137b7ce11dcf845513810230638ec
(cherry picked from commit d4aed18883b361133607296fb6cd789c47427a38)

3 files changed, 18 insertions, 2 deletions

diff --git a/src/libcmd/installable-flake.cc b/src/libcmd/installable-flake.cc
index bb75b8ff0..615f70945 100644
--- a/src/libcmd/installable-flake.cc
+++ b/src/libcmd/installable-flake.cc
@@ -28,6 +28,10 @@ namespace nix {
 std::vector<std::string> InstallableFlake::getActualAttrPaths()
 {
     std::vector<std::string> res;
+    if (attrPaths.size() == 1 && attrPaths.front().starts_with(".")){
+        res.push_back(attrPaths.front().substr(1));
+        return res;
+    }
 
     for (auto & prefix : prefixes)
         res.push_back(prefix + *attrPaths.begin());
diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index b40632887..4ffc9af24 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -302,6 +302,11 @@ void completeFlakeRefWithFragment(
             completionType = ctAttrs;
 
             auto fragment = prefix.substr(hash + 1);
+            std::string prefixRoot = "";
+            if (fragment.starts_with(".")){
+                fragment = fragment.substr(1);
+                prefixRoot = ".";
+            }
             auto flakeRefS = std::string(prefix.substr(0, hash));
             auto flakeRef = parseFlakeRef(expandTilde(flakeRefS), absPath("."));
 
@@ -310,6 +315,9 @@ void completeFlakeRefWithFragment(
 
             auto root = evalCache->getRoot();
 
+            if (prefixRoot == "."){
+                attrPathPrefixes.clear();
+            }
             /* Complete 'fragment' relative to all the
                attrpath prefixes as well as the root of the
                flake. */
@@ -334,7 +342,7 @@ void completeFlakeRefWithFragment(
                         auto attrPath2 = (*attr)->getAttrPath(attr2);
                         /* Strip the attrpath prefix. */
                         attrPath2.erase(attrPath2.begin(), attrPath2.begin() + attrPathPrefix.size());
-                        completions->add(flakeRefS + "#" + concatStringsSep(".", evalState->symbols.resolve(attrPath2)));
+                        completions->add(flakeRefS + "#" + prefixRoot + concatStringsSep(".", evalState->symbols.resolve(attrPath2)));
                     }
                 }
             }
@@ -345,7 +353,7 @@ void completeFlakeRefWithFragment(
                 for (auto & attrPath : defaultFlakeAttrPaths) {
                     auto attr = root->findAlongAttrPath(parseAttrPath(*evalState, attrPath));
                     if (!attr) continue;
-                    completions->add(flakeRefS + "#");
+                    completions->add(flakeRefS + "#" + prefixRoot);
                 }
             }
         }
diff --git a/tests/functional/flakes/flakes.sh b/tests/functional/flakes/flakes.sh
index 128f759ea..7781eb51b 100644
--- a/tests/functional/flakes/flakes.sh
+++ b/tests/functional/flakes/flakes.sh
@@ -79,6 +79,10 @@ nix registry add --registry $registry nixpkgs flake1
 nix registry list | grep        '^global'
 nix registry list | grepInverse '^user' # nothing in user registry
 
+# Test fuzzy and exact flake attribute syntax.
+expectStderr 1 nix eval flake1#ERROR | grepQuiet "error:.*does not provide attribute.*or 'ERROR'$"
+expectStderr 1 nix eval flake1#.ERROR | grepQuiet "error:.*does not provide attribute 'ERROR'$"
+
 # Test 'nix flake metadata'.
 nix flake metadata flake1
 nix flake metadata flake1 | grepQuiet 'Locked URL:.*flake1.*'