Describe the bug

If I have a flake input with a transitive input constrained via follows, then I remove that follows constraint, the new transitive input version in my flake.lock ignores that dependency's original flake.lock.

Steps To Reproduce

1. Create a flake.nix with oxalica/rust-overlay@1d1c8a0 as the only input:

   {
     inputs = {
       rust-overlay.url = "github:oxalica/rust-overlay/1d1c8a009adeb4c0561bb1feea8ec7b97749946b";
     };
     outputs = { ... }: { };
   }

2. Run nix flake lock, observing that the transitive input is NixOS/nixpkgs@18dd725, the same commit as in rust-overlay's flake.lock:

   $ nix flake lock
   • Added input 'rust-overlay':
       'github:oxalica/rust-overlay/1d1c8a009adeb4c0561bb1feea8ec7b97749946b?narHash=sha256-mylLEdzqx4M9TrwFIPc%2Bo0dqabHvFGbNJZGRgE43Ya0%3D' (2025-10-23)
   • Added input 'rust-overlay/nixpkgs':
       'github:NixOS/nixpkgs/18dd725c29603f582cf1900e0d25f9f1063dbf11?narHash=sha256-awS2zRgF4uTwrOKwwiJcByDzDOdo3Q1rPZbiHQg/N38%3D' (2025-04-13)
   

3. Add a separate input for NixOS/nixpkgs@87848bf:

   {
     inputs = {
       nixpkgs.url = "github:NixOS/nixpkgs/87848bf0cc4f87717fc813a4575f07330c3e743c";
       rust-overlay.url = "github:oxalica/rust-overlay/1d1c8a009adeb4c0561bb1feea8ec7b97749946b";
     };
     outputs = { ... }: { };
   }

4. Run nix flake lock again:

   $ nix flake lock
   • Added input 'nixpkgs':
       'github:NixOS/nixpkgs/87848bf0cc4f87717fc813a4575f07330c3e743c?narHash=sha256-/oUYsC0lUCBory65VK%2BUHqCCsCspbL1Vgfcf1KUYqVw%3D' (2025-10-20)
   

5. Constrain rust-overlay's transitive Nixpkgs input to follow our direct Nixpkgs input:

   {
     inputs = {
       nixpkgs.url = "github:NixOS/nixpkgs/87848bf0cc4f87717fc813a4575f07330c3e743c";
       rust-overlay = {
         url = "github:oxalica/rust-overlay/1d1c8a009adeb4c0561bb1feea8ec7b97749946b";
         inputs.nixpkgs.follows = "nixpkgs";
       };
     };
     outputs = { ... }: { };
   }

6. Run nix flake lock again:

   $ nix flake lock
   • Updated input 'rust-overlay/nixpkgs':
       'github:NixOS/nixpkgs/18dd725c29603f582cf1900e0d25f9f1063dbf11?narHash=sha256-awS2zRgF4uTwrOKwwiJcByDzDOdo3Q1rPZbiHQg/N38%3D' (2025-04-13)
     → follows 'nixpkgs'
   

7. Remove that constraint:

   {
     inputs = {
       nixpkgs.url = "github:NixOS/nixpkgs/87848bf0cc4f87717fc813a4575f07330c3e743c";
       rust-overlay.url = "github:oxalica/rust-overlay/1d1c8a009adeb4c0561bb1feea8ec7b97749946b";
     };
     outputs = { ... }: { };
   }

8. Run nix flake lock a final time, observing that rust-overlay's transitive Nixpkgs input is now neither the NixOS/nixpkgs@18dd725 from rust-overlay's flake.lock, nor NixOS/nixpkgs@87848bf from the earlier inputs.nixpkgs.follows, but is instead whatever is the latest nixpkgs-unstable commit at the time the command is run:

   $ nix flake lock
   • Updated input 'rust-overlay/nixpkgs':
       follows 'nixpkgs'
     → 'github:NixOS/nixpkgs/cb82756ecc37fa623f8cf3e88854f9bf7f64af93?narHash=sha256-0JDOal5P7xzzAibvD0yTE3ptyvoVOAL0rcELmDdtSKg%3D' (2025-10-20)
   

Expected behavior

I expected rust-overlay's Nixpkgs input to revert back to NixOS/nixpkgs@18dd725 as specified in its flake.lock.

Metadata

I tested this with the current stable Nix on x86_64-linux:

$ nix-env --version
nix-env (Nix) 2.28.5

I also tested this with the current latest Nix on aarch64-darwin:

$ nix-env --version
nix-env (Determinate Nix 3.11.3) 2.31.2

Additional context

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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