rustPlatform: add buildAndTestSubdir-argument

There are several tarballs (such as the `rust-lang/rust`-source) with a
`Cargo.toml` at root and several sub-packages (with their own Cargo.toml)
without using workspaces[1].

In such a case it's needed to move into a subdir to only build the
specified sub-package (e.g. `rustfmt` or `rsl`), however the artifacts
are at `/target` in the root-dir of the build environment. This breaks
the build since `buildRustPackage` searches for executables in `target`
(which is at the build-env's root) at the end of the `buildPhase`.

With the optional `buildAndTestSubdir`-argument, the builder moves into
the specified subdir using `pushd`/`popd` during `buildPhase` and
`checkPhase`.

Also moved the logic to find executables and libs to the end of the `buildPhase`
from a custom `postBuild`-hook to fix packages with custom `build`/`install`-procedures
such as `uutils-coreutils`.

[1] https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html

pkgs/development/compilers/rust/cargo.nix

@@ -9,8 +9,7 @@ rustPlatform.buildRustPackage {
 
   # the rust source tarball already has all the dependencies vendored, no need to fetch them again
   cargoVendorDir = "vendor";
-  preBuild = "pushd src/tools/cargo";
-  postBuild = "popd";
+  buildAndTestSubdir = "src/tools/cargo";
 
   passthru.rustc = rustc;
 

pkgs/development/compilers/rust/clippy.nix

@@ -5,8 +5,7 @@ rustPlatform.buildRustPackage {
 
   # the rust source tarball already has all the dependencies vendored, no need to fetch them again
   cargoVendorDir = "vendor";
-  preBuild = "pushd src/tools/clippy";
-  postBuild = "popd";
+  buildAndTestSubdir = "src/tools/clippy";
 
   # changes hash of vendor directory otherwise
   dontUpdateAutotoolsGnuConfigScripts = true;

pkgs/development/compilers/rust/rls/default.nix

@@ -10,8 +10,9 @@ rustPlatform.buildRustPackage {
   dontUpdateAutotoolsGnuConfigScripts = true;
 
   cargoVendorDir = "vendor";
+  buildAndTestSubdir = "src/tools/rls";
+
   preBuild = ''
-    pushd src/tools/rls
     # client tests are flaky
     rm tests/client.rs
   '';
@@ -28,8 +29,6 @@ rustPlatform.buildRustPackage {
 
   doCheck = true;
 
-  preInstall = "popd";
-
   doInstallCheck = true;
   installCheckPhase = ''
     $out/bin/rls --version

pkgs/development/compilers/rust/rustfmt.nix

@@ -6,8 +6,7 @@ rustPlatform.buildRustPackage rec {
 
   # the rust source tarball already has all the dependencies vendored, no need to fetch them again
   cargoVendorDir = "vendor";
-  preBuild = "pushd src/tools/rustfmt";
-  preInstall = "popd";
+  buildAndTestSubdir = "src/tools/rustfmt";
 
   # changes hash of vendor directory otherwise
   dontUpdateAutotoolsGnuConfigScripts = true;
@@ -17,12 +16,6 @@ rustPlatform.buildRustPackage rec {
   # As of 1.0.0 and rustc 1.30 rustfmt requires a nightly compiler
   RUSTC_BOOTSTRAP = 1;
 
-  # we run tests in debug mode so tests look for a debug build of
-  # rustfmt. Anyway this adds nearly no compilation time.
-  preCheck = ''
-    cargo build
-  '';
-
   meta = with stdenv.lib; {
     description = "A tool for formatting Rust code according to style guidelines";
     homepage = "https://github.com/rust-lang-nursery/rustfmt";