flakes: throw an error if `follows`-declaration for an input is invalid

I recently got fairly confused why the following expression didn't have
any effect

    {
      description = "Foobar";
      inputs.sops-nix = {
        url = github:mic92/sops-nix;
        inputs.nixpkgs_22_05.follows = "nixpkgs";
      };
    }

until I found out that the input was called `nixpkgs-22_05` (please note
the dash vs. underscore).

IMHO it's not a good idea to not throw an error in that case and
probably leave end-users rather confused, so I implemented a small check
for that which basically checks whether `follows`-declaration from
overrides actually have corresponding inputs in the transitive flake.

In fact this was done by accident already in our own test-suite where
the removal of a `follows` was apparently forgotten[1].

Since the key of the `std::map` that holds the `overrides` is a vector
and we have to find the last element of each vector (i.e. the override)
this has to be done with a for loop in O(n) complexity with `n` being
the total amount of overrides (which shouldn't be that large though).

Please note that this doesn't work with nested expressions, i.e.

    inputs.fenix.inputs.nixpkgs.follows = "...";

which is a known problem[2].

For the expression demonstrated above, an error like this will be
thrown:

    error: sops-nix has a `follows'-declaration for a non-existant input nixpkgs_22_05!

[1] 2664a216e57169ec57d7f51be1b8383c1be83fd5
[2] https://github.com/NixOS/nix/issues/5790

1 files changed, 23 insertions, 0 deletions

diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 920726b73..b97780a9c 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -373,6 +373,29 @@ LockedFlake lockFlake(
         {
             debug("computing lock file node '%s'", printInputPath(inputPathPrefix));
 
+            auto overrides2 = overrides;
+            for (auto & [inputPath, inputOverride] : overrides2) {
+                auto inputPath2(inputPath);
+                auto follow = inputPath2.back();
+                inputPath2.pop_back();
+                if (inputPath2 == inputPathPrefix
+                    && flakeInputs.find(follow) == flakeInputs.end()
+                ) {
+                    std::string root;
+                    for (auto & element : inputPath2) {
+                        root.append(element);
+                        if (element != inputPath2.back()) {
+                            root.append(".inputs.");
+                        }
+                    }
+                    throw Error(
+                        "%s has a `follows'-declaration for a non-existant input %s!",
+                        root,
+                        follow
+                    );
+                }
+            }
+
             /* Get the overrides (i.e. attributes of the form
                'inputs.nixops.inputs.nixpkgs.url = ...'). */
             for (auto & [id, input] : flakeInputs) {