makeRustPlatform prevents cargo*Hook from being spliced

[ghost]

splice.nix walks the packageset attribute tree, but it can't see through invocations of makeRustPlatform (unless they get inherited into the top-level packageset).

To see that this is true, move cargoSetupHook from nativeBuildInputs to buildInputs in build-rust-package/default.nix and run nix-diff on something like pkgsMusl.fd. You'll see that the derivation moves from buildInputs to nativeBuildInputs, but the script is exactly the same, in spite of the fact that cargoSetupHook writes stdenv.{build,host,target}Platform into its output!

The fact that our rust hooks are unspliced has a few consequences which we will simply need to keep in mind until this gets fixed (likely by eliminating rustPlatform):

1. Unlike all other hooks, the hooks in rust/hooks/default.nix are NOT phase-shifted back one phase. So the changes in rustPlatform.cargoSetupHook: fix platform check #260068 are correct, but as soon as we fix the unspliced hooks, we must undo those changes.
2. A lot of rust.lib.envVars appears unnecessary, because of the lack of splicing. e.g. rust.lib.envVars: clean up unnecessary variables #260167

[ghost]

Proof that the rust hooks are not spliced: #263082 (comment)

[kjeremy]

@amjoseph-nixpkgs Is there a plan to solve this? I can't seem to work around the python.cryptography issue.

[ghost]

@amjoseph-nixpkgs Is there a plan to solve this?

Yes.

The plan is:

1. After 23.11 branches off we will merge make-derivation.nix: lib.warn if drv.__spliced missing #263082. It emits lib.warn lines for unspliced dependencies.
2. Fix all of the lib.warn lines emitted by nix-instantiate . -A nix-instantiate . -A pkgsCross.armv7l-hf-multiplatform.python3Packages.cryptography.

I believe that once 2 is done, the python3Packages.cryptography issue will either be fixed (most likely), or some other issue will be revealed (possible but unlikely).

If you have spare time you can start on 2 without waiting for 1. Here are the warnings:

trace: warning: derivation ncurses:
                     unspliced build-host dependency ncurses
                       build=x86_64-unknown-linux-gnu
                       host=x86_64-unknown-linux-gnu
                       target=armv7l-unknown-linux-gnueabihf
                       For advice on fixing this, read the comment above the lib.warn that produced this message.

trace: warning: derivation python3:
                     unspliced build-host dependency python3
                       build=x86_64-unknown-linux-gnu
                       host=x86_64-unknown-linux-gnu
                       target=armv7l-unknown-linux-gnueabihf
                       For advice on fixing this, read the comment above the lib.warn that produced this message.

trace: warning: derivation !!no pname!!:
                     unspliced build-host dependency cargo-vendor-normalise
                       build=x86_64-unknown-linux-gnu
                       host=x86_64-unknown-linux-gnu
                       target=armv7l-unknown-linux-gnueabihf
                       For advice on fixing this, read the comment above the lib.warn that produced this message.

trace: warning: derivation !!no pname!!:
                     unspliced build-host dependency armv7l-unknown-linux-gnueabihf-cargo-1.72.1
                       build=x86_64-unknown-linux-gnu
                       host=x86_64-unknown-linux-gnu
                       target=armv7l-unknown-linux-gnueabihf
                       For advice on fixing this, read the comment above the lib.warn that produced this message.

trace: warning: derivation python3:
                     unspliced build-host dependency python3
                       build=x86_64-unknown-linux-gnu
                       host=x86_64-unknown-linux-gnu
                       target=armv7l-unknown-linux-gnueabihf
                       For advice on fixing this, read the comment above the lib.warn that produced this message.

trace: warning: derivation cryptography:
                     unspliced build-host dependency setuptools-rust-setup-hook
                       build=x86_64-unknown-linux-gnu
                       host=armv7l-unknown-linux-gnueabihf
                       target=armv7l-unknown-linux-gnueabihf
                       For advice on fixing this, read the comment above the lib.warn that produced this message.

trace: warning: derivation python3:
                     unspliced build-host dependency python3
                       build=x86_64-unknown-linux-gnu
                       host=x86_64-unknown-linux-gnu
                       target=armv7l-unknown-linux-gnueabihf
                       For advice on fixing this, read the comment above the lib.warn that produced this message.

[kjeremy]

I submitted #265060 for ncurses but I think the other ones are beyond me.