nix-shell: restore backwards-compat with old nixpkgs

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] 9d612c393a
[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`.

src/nix-build/nix-build.cc

@@ -257,11 +257,12 @@ static void main_nix_build(int argc, char * * argv)
 
     auto autoArgs = myArgs.getAutoArgs(*state);
 
+    auto autoArgsWithInNixShell = autoArgs;
     if (runEnv) {
-        auto newArgs = state->buildBindings(autoArgs->size() + 1);
+        auto newArgs = state->buildBindings(autoArgsWithInNixShell->size() + 1);
         newArgs.alloc("inNixShell").mkBool(true);
         for (auto & i : *autoArgs) newArgs.insert(i);
-        autoArgs = newArgs.finish();
+        autoArgsWithInNixShell = newArgs.finish();
     }
 
     if (packages) {
@@ -316,10 +317,39 @@ static void main_nix_build(int argc, char * * argv)
         Value vRoot;
         state->eval(e, vRoot);
 
+        std::function<bool(const Value & v)> takesNixShellAttr;
+        takesNixShellAttr = [&](const Value & v) {
+            if (!runEnv) {
+                return false;
+            }
+            bool add = false;
+            if (v.type() == nFunction && v.lambda.fun->hasFormals()) {
+                for (auto & i : v.lambda.fun->formals->formals) {
+                    if (state->symbols[i.name] == "inNixShell") {
+                        add = true;
+                        break;
+                    }
+                }
+            }
+            return add;
+        };
+
         for (auto & i : attrPaths) {
-            Value & v(*findAlongAttrPath(*state, i, *autoArgs, vRoot).first);
+            Value & v(*findAlongAttrPath(
+                *state,
+                i,
+                takesNixShellAttr(vRoot) ? *autoArgsWithInNixShell : *autoArgs,
+                vRoot
+            ).first);
             state->forceValue(v, [&]() { return v.determinePos(noPos); });
-            getDerivations(*state, v, "", *autoArgs, drvs, false);
+            getDerivations(
+                *state,
+                v,
+                "",
+                takesNixShellAttr(v) ? *autoArgsWithInNixShell : *autoArgs,
+                drvs,
+                false
+            );
         }
     }
 

tests/ca-shell.nix

@@ -1 +1 @@
-{ ... }@args: import ./shell.nix (args // { contentAddressed = true; })
+{ inNixShell ? false, ... }@args: import ./shell.nix (args // { contentAddressed = true; })

tests/nix-shell.sh

@@ -102,3 +102,10 @@ source <(nix print-dev-env -f "$shellDotNix" shellDrv)
 [[ ${arr2[1]} = $'\n' ]]
 [[ ${arr2[2]} = $'x\ny' ]]
 [[ $(fun) = blabla ]]
+
+# Test nix-shell with ellipsis and no `inNixShell` argument (for backwards compat with old nixpkgs)
+cat >$TEST_ROOT/shell-ellipsis.nix <<EOF
+{ system ? "x86_64-linux", ... }:
+(import $shellDotNix { }).shellDrv
+EOF
+nix-shell $TEST_ROOT/shell-ellipsis.nix --run "true"