nix build creates a result-${outName} symlink unexpectedly

[roberth]

Describe the bug

nix build's result symlink filename depends on how the output was created, which is quite unexpected and catches me off guard whenever I build a single output.

Steps To Reproduce

1. nix build nix#nix.doc
2. xdg-open result/share/doc/nix/manual/index.html
3. 💥 didn't build the docs? no, the result symlink was wrong.

Alternatively, try to build and then run a package that has a bin outputs, such as those from Nixpkgs Haskell enableSeparateBinOutput.

Expected behavior

nix build always produces a build or build-${n} symlink regardless of how a package implements its package outputs - whether that's through multi-output derivations or multiple derivations. Store-level details such as the output name are insignificant and don't lead to different behaviors on the CLI.

nix-env --version output

Additional context

• A package attrset should be composable from multiple derivations, as a matter of good layering (putting the language in charge, not reaching into the store layer), and as a consequence of RFC 92. The language treats outputs equally, and therefore, so should the CLI.
  • Redefine outputs in terms of language-level "package", not necessarily store-level derivation (RFC-92, and multi-drv packages, docs) #6507

Priorities

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