Reintroducing mkRustCrate

doc/languages-frameworks/rust.md

@@ -19,7 +19,7 @@ For daily builds (beta and nightly) use either rustup from
 nixpkgs or use the [Rust nightlies
 overlay](#using-the-rust-nightlies-overlay).
 
-## Packaging Rust applications
+## Compiling Rust applications with Cargo
 
 Rust applications are packaged by using the `buildRustPackage` helper from `rustPlatform`:
 
@@ -57,6 +57,173 @@ checksum can be then take from the failed build.
 To install crates with nix there is also an experimental project called
 [nixcrates](https://github.com/fractalide/nixcrates).
 
+## Compiling Rust crates using Nix instead of Cargo
+
+When run, `cargo build` produces a file called `Cargo.lock`,
+containing pinned versions of all dependencies. Nixpkgs contains a
+tool called `carnix` (`nix-env -iA nixos.carnix`), which can be used
+to turn a `Cargo.lock` into a Nix expression.
+
+That Nix expression calls `rustc` directly (hence bypassing Cargo),
+and can be used to compile a crate and all its dependencies. Here is
+an example for a minimal `hello` crate:
+
+
+    $ cargo new hello
+    $ cd hello
+    $ cargo build
+     Compiling hello v0.1.0 (file:///tmp/hello)
+      Finished dev [unoptimized + debuginfo] target(s) in 0.20 secs
+    $ carnix -o hello.nix --src ./. Cargo.lock --standalone
+    $ nix-build hello.nix
+
+Now, the file produced by the call to `carnix`, called `hello.nix`, looks like:
+
+```
+with import <nixpkgs> {};
+let release = true;
+    verbose = true;
+    hello_0_1_0_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
+      crateName = "hello";
+      version = "0.1.0";
+      src = ./.;
+      inherit dependencies buildDependencies features release verbose;
+    };
+
+in
+rec {
+  hello_0_1_0 = hello_0_1_0_ {};
+}
+```
+
+In particular, note that the argument given as `--src` is copied
+verbatim to the source.  If we look at a more complicated
+dependencies, for instance by adding a single line `libc="*"` to our
+`Cargo.toml`, we first need to run `cargo build` to update the
+`Cargo.lock`. Then, `carnix` needs to be run again, and produces the
+following nix file:
+
+```
+with import <nixpkgs> {};
+let release = true;
+    verbose = true;
+    hello_0_1_0_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
+      crateName = "hello";
+      version = "0.1.0";
+      src = ./.;
+      inherit dependencies buildDependencies features release verbose;
+    };
+    libc_0_2_33_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
+      crateName = "libc";
+      version = "0.2.33";
+      sha256 = "1l7synziccnvarsq2kk22vps720ih6chmn016bhr2bq54hblbnl1";
+      inherit dependencies buildDependencies features release verbose;
+    };
+
+in
+rec {
+  hello_0_1_0 = hello_0_1_0_ {
+    dependencies = [ libc_0_2_33 ];
+  };
+  libc_0_2_33 = libc_0_2_33_ {
+    features = [ "use_std" ];
+  };
+}
+```
+
+Here, the `libc` crate has no `src` attribute, so `buildRustCrate`
+will fetch it from [crates.io](https://crates.io). A `sha256`
+attribute is still needed for Nix purity.
+
+Some crates require external libraries. For crates from
+[crates.io](https://crates.io), such libraries can be specified in
+`<nixpkgs/pkgs/build-support/rust/defaultCrateOverrides.nix>`.
+
+Starting from that file, one can add more overrides, to add features
+or build inputs, as follows:
+
+```
+with import <nixpkgs> {};
+let kernel = buildPlatform.parsed.kernel.name;
+    abi = buildPlatform.parsed.abi.name;
+    helo_0_1_0_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
+      crateName = "helo";
+      version = "0.1.0";
+      authors = [ "pe@pijul.org <pe@pijul.org>" ];
+      src = ./.;
+      inherit dependencies buildDependencies features;
+    };
+in
+rec {
+  helo_0_1_0 = (helo_0_1_0_ {}).override {
+    crateOverrides = defaultCrateOverrides // {
+      helo = attrs: { buildInputs = [ openssl ]; };
+    };
+  };
+}
+```
+
+Here, `crateOverrides` is expected to be a attribute set, where the
+key is the crate name without version number and the value a function.
+The function gets all attributes passed to `buildRustCrate` as first
+argument and returns a set that contains all attribute that should be
+overwritten.
+
+For more complicated cases, such as when parts of the crate's
+derivation depend on the the crate's version, the `attrs` argument of
+the override above can be read, as in the following example, which
+patches the derivation:
+
+```
+with import <nixpkgs> {};
+let kernel = buildPlatform.parsed.kernel.name;
+    abi = buildPlatform.parsed.abi.name;
+    helo_0_1_0_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
+      crateName = "helo";
+      version = "0.1.0";
+      authors = [ "pe@pijul.org <pe@pijul.org>" ];
+      src = ./.;
+      inherit dependencies buildDependencies features;
+    };
+in
+rec {
+  helo_0_1_0 = (helo_0_1_0_ {}).override {
+    crateOverrides = defaultCrateOverrides // {
+      helo = attrs: lib.optionalAttrs (lib.versionAtLeast attrs.version "1.0")  {
+        postPatch = ''
+          substituteInPlace lib/zoneinfo.rs \
+            --replace "/usr/share/zoneinfo" "${tzdata}/share/zoneinfo"
+        '';
+      };
+    };
+  };
+}
+```
+
+
+Three more parameters can be overridden:
+
+- The version of rustc used to compile the crate:
+
+  ```
+  hello_0_1_0.override { rust = pkgs.rust; };
+  ```
+
+- Whether to build in release mode or debug mode (release mode by
+  default):
+
+  ```
+  hello_0_1_0.override { release = false; };
+  ```
+
+- Whether to print the commands sent to rustc when building
+  (equivalent to `--verbose` in cargo:
+
+  ```
+  hello_0_1_0.override { verbose = false; };
+  ```
+
+
 ## Using the Rust nightlies overlay
 
 Mozilla provides an overlay for nixpkgs to bring a nightly version of Rust into scope.

lib/maintainers.nix

@@ -493,6 +493,7 @@
   plcplc = "Philip Lykke Carlsen <plcplc@gmail.com>";
   plumps = "Maksim Bronsky <maks.bronsky@web.de";
   pmahoney = "Patrick Mahoney <pat@polycrystal.org>";
+  pmeunier = "Pierre-Étienne Meunier <pierre-etienne.meunier@inria.fr>";
   pmiddend = "Philipp Middendorf <pmidden@secure.mailbox.org>";
   polyrod = "Maurizio Di Pietro <dc1mdp@gmail.com>";
   pradeepchhetri = "Pradeep Chhetri <pradeep.chhetri89@gmail.com>";