Maximilian Bosch
98946e2d9c
Basically an attempt to resume fixing #5543 for a breakage introduced earlier[1]. Basically, when evaluating an older `nixpkgs` with `nix-shell` the following error occurs: λ ma27 [~] → nix-shell -I nixpkgs=channel:nixos-18.03 -p nix error: anonymous function at /nix/store/zakqwc529rb6xcj8pwixjsxscvlx9fbi-source/pkgs/top-level/default.nix:20:1 called with unexpected argument 'inNixShell' at /nix/store/zakqwc529rb6xcj8pwixjsxscvlx9fbi-source/pkgs/top-level/impure.nix:82:1: 81| 82| import ./. (builtins.removeAttrs args [ "system" "platform" ] // { | ^ 83| inherit config overlays crossSystem; This is a problem because one of the main selling points of Nix is that you can evaluate any old Nix expression and still get the same result (which also means that it *still evaluates*). In fact we're deprecating, but not removing a lot of stuff for that reason such as unquoted URLs[2] or `builtins.toPath`. However this property was essentially thrown away here. The change is rather simple: check if `inNixShell` is specified in the formals of an auto-called function. This means that { inNixShell ? false }: builtins.trace inNixShell (with import <nixpkgs> { }; makeShell { name = "foo"; }) will show `trace: true` while args@{ ... }: builtins.trace args.inNixShell (with import <nixpkgs> { }; makeShell { name = "foo"; }) will throw the following error: error: attribute 'inNixShell' missing This is explicitly needed because the function in `pkgs/top-level/impure.nix` of e.g. NixOS 18.03 has an ellipsis[3], but passes the attribute-set on to another lambda with formals that doesn't have an ellipsis anymore (hence the error from above). This was perhaps a mistake, but we can't fix it anymore. This also means that there's AFAICS no proper way to check if the attr-set that's passed to the Nix code via `EvalState::autoCallFunction` is eventually passed to a lambda with formals where `inNixShell` is missing. However, this fix comes with a certain price. Essentially every `shell.nix` that assumes `inNixShell` to be passed to the formals even without explicitly specifying it would break with this[4]. However I think that this is ugly, but preferable: * Nix 2.3 was declared stable by NixOS up until recently (well, it still is as long as 21.11 is alive), so most people might not have even noticed that feature. * We're talking about a way shorter time-span with this change being in the wild, so the fallout should be smaller IMHO. [1] https://github.com/NixOS/nix/commit/9d612c393abc3a73590650d24bcfe2ee57792872 [2] https://github.com/NixOS/rfcs/pull/45#issuecomment-488232537 [3] https://github.com/NixOS/nixpkgs/blob/release-18.03/pkgs/top-level/impure.nix#L75 [4] See e.g. the second expression in this commit-message or the changes for `tests/ca/nix-shell.sh`.
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