makeRustPlatform: Put back in it's own file.

We expose it on the top level, but I don't think it makes sense to pull
it from a specific version of the rust tools when it is in fact version
agnostic.

This reverts a tiny portion of 912dca193a.

pkgs/development/compilers/rust/1_44.nix

@@ -13,6 +13,7 @@
 , CoreFoundation, Security
 , llvmPackages_5
 , pkgsBuildTarget, pkgsBuildBuild
+, makeRustPlatform
 } @ args:
 
 import ./default.nix {

pkgs/development/compilers/rust/1_45.nix

@@ -13,6 +13,7 @@
 , CoreFoundation, Security
 , llvmPackages_5
 , pkgsBuildTarget, pkgsBuildBuild
+, makeRustPlatform
 } @ args:
 
 import ./default.nix {

pkgs/development/compilers/rust/default.nix

@@ -12,6 +12,7 @@
 , CoreFoundation, Security
 , llvmPackages_5
 , pkgsBuildTarget, pkgsBuildBuild
+, makeRustPlatform
 }: rec {
   toRustTarget = platform: with platform.parsed; let
     cpu_ = {
@@ -22,24 +23,6 @@
   in platform.rustc.config
     or "${cpu_}-${vendor.name}-${kernel.name}${lib.optionalString (abi.name != "unknown") "-${abi.name}"}";
 
-  makeRustPlatform = { rustc, cargo, ... }: rec {
-    rust = {
-      inherit rustc cargo;
-    };
-
-    fetchCargoTarball = buildPackages.callPackage ../../../build-support/rust/fetchCargoTarball.nix {
-      inherit cargo;
-    };
-
-    buildRustPackage = callPackage ../../../build-support/rust {
-      inherit rustc cargo fetchCargoTarball;
-    };
-
-    rustcSrc = callPackage ./rust-src.nix {
-      inherit rustc;
-    };
-  };
-
   # This just contains tools for now. But it would conceivably contain
   # libraries too, say if we picked some default/recommended versions from
   # `cratesIO` to build by Hydra and/or try to prefer/bias in Cargo.lock for

pkgs/development/compilers/rust/make-rust-platform.nix

@@ -0,0 +1,21 @@
+{ buildPackages, callPackage }:
+
+{ rustc, cargo, ... }:
+
+rec {
+  rust = {
+    inherit rustc cargo;
+  };
+
+  fetchCargoTarball = buildPackages.callPackage ../../../build-support/rust/fetchCargoTarball.nix {
+    inherit cargo;
+  };
+
+  buildRustPackage = callPackage ../../../build-support/rust {
+    inherit rustc cargo fetchCargoTarball;
+  };
+
+  rustcSrc = callPackage ./rust-src.nix {
+    inherit rustc;
+  };
+}

pkgs/top-level/all-packages.nix

@@ -9659,7 +9659,8 @@ in
   rustPackages = rustPackages_1_45;
 
   inherit (rustPackages) cargo clippy rustc rustPlatform;
-  inherit (rust) makeRustPlatform;
+
+  makeRustPlatform = callPackage ../development/compilers/rust/make-rust-platform.nix {};
 
   buildRustCrate = callPackage ../build-support/rust/build-rust-crate { };
   buildRustCrateHelpers = callPackage ../build-support/rust/build-rust-crate/helpers.nix { };