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)

1 files changed, 4 insertions, 0 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());