[WIP] devShellTools.buildShellEnv

[roberth]

Description of changes

Note: this is currently draft, because it does not have sufficient tests and documentation for a number of features.
Help wanted :) This is a NixOS/nixpkgs branch, where all maintainers can push, but maybe sync with me first.

Implement build shells and development shells in Nixpkgs for greater flexibility and evolution of the Nix shell concept.

• This is a Nixpkgs-native implementation of POC: devShell interface #206728
• To be used with (TBD) Allow to get rid of nix develop "shell" logic nix#7501
  • or e.g. nix run -f shell.nix --arg nixpkgs ./. devShell (ie like running the output of nix-build shell.nix -A devShell)
• Actually solve Haskell development shells are not very useful #196753
• Actually solve mkShellMinimal: Create an ultra minimal nix-shell #132617 by allowing you to implement the devShell interface yourself.
  • passthru.devShell = something custom;
• Builds upon devShellTools: add environment functions #324789

Terminology cheat sheet

• Nix shell: the general concept of launch a shell with Nix-provided stuff in its environment
• nix shell: nix env shell
• build shell: a stdenv-based environment that's automatically derived from a derivation (see make-derivation.nix), useful for troubleshooting a build
• dev shell: an environment for development. This may be constructed with stdenv-based infra with mkShell, or with other projects like cachix/devenv, numtide/devshell, etc, provided they'll implement the devShell interface

TODO

• more tests
• docs
  • specifically the devShell attribute and the interface of its output
  • example of customizing the devShell
  • (a small part of it will also be a Nix interface: the proposed, extra nix develop and nix print-dev-env behaviors. This is very small and ok to duplicate.)
• structuredAttrs support (maybe, or omit its devShell attr for now)
• fine tune the terminology
• make mkShell return an impure shell by default, because it's for dev shells?
• validate in practice with an implementation of Allow to get rid of nix develop "shell" logic nix#7501

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[roberth]

No NIX_BUILD_TOP. Should we have it? It's not a build...