Revert "carnix,cratesIO: remove" in doc/languages-frameworks/rust.section.md

This reverts the part of commit
82fe76d1cd that affected
doc/languages-frameworks/rust.section.md

doc/languages-frameworks/rust.section.md

@@ -642,6 +642,227 @@ buildPythonPackage rec {
 }
 ```
 
+## `buildRustCrate`: Compiling Rust crates using Nix instead of Cargo {#compiling-rust-crates-using-nix-instead-of-cargo}
+
+### Simple operation {#simple-operation}
+
+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:
+
+```ShellSession
+$ 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 -A hello_0_1_0
+```
+
+Now, the file produced by the call to `carnix`, called `hello.nix`, looks like:
+
+```nix
+# Generated by carnix 0.6.5: carnix -o hello.nix --src ./. Cargo.lock --standalone
+{ stdenv, buildRustCrate, fetchgit }:
+let kernel = stdenv.buildPlatform.parsed.kernel.name;
+    # ... (content skipped)
+in
+rec {
+  hello = f: hello_0_1_0 { features = hello_0_1_0_features { hello_0_1_0 = f; }; };
+  hello_0_1_0_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
+    crateName = "hello";
+    version = "0.1.0";
+    authors = [ "pe@pijul.org <pe@pijul.org>" ];
+    src = ./.;
+    inherit dependencies buildDependencies features;
+  };
+  hello_0_1_0 = { features?(hello_0_1_0_features {}) }: hello_0_1_0_ {};
+  hello_0_1_0_features = f: updateFeatures f (rec {
+        hello_0_1_0.default = (f.hello_0_1_0.default or true);
+    }) [ ];
+}
+```
+
+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:
+
+```nix
+# Generated by carnix 0.6.5: carnix -o hello.nix --src ./. Cargo.lock --standalone
+{ stdenv, buildRustCrate, fetchgit }:
+let kernel = stdenv.buildPlatform.parsed.kernel.name;
+    # ... (content skipped)
+in
+rec {
+  hello = f: hello_0_1_0 { features = hello_0_1_0_features { hello_0_1_0 = f; }; };
+  hello_0_1_0_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
+    crateName = "hello";
+    version = "0.1.0";
+    authors = [ "pe@pijul.org <pe@pijul.org>" ];
+    src = ./.;
+    inherit dependencies buildDependencies features;
+  };
+  libc_0_2_36_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
+    crateName = "libc";
+    version = "0.2.36";
+    authors = [ "The Rust Project Developers" ];
+    sha256 = "01633h4yfqm0s302fm0dlba469bx8y6cs4nqc8bqrmjqxfxn515l";
+    inherit dependencies buildDependencies features;
+  };
+  hello_0_1_0 = { features?(hello_0_1_0_features {}) }: hello_0_1_0_ {
+    dependencies = mapFeatures features ([ libc_0_2_36 ]);
+  };
+  hello_0_1_0_features = f: updateFeatures f (rec {
+    hello_0_1_0.default = (f.hello_0_1_0.default or true);
+    libc_0_2_36.default = true;
+  }) [ libc_0_2_36_features ];
+  libc_0_2_36 = { features?(libc_0_2_36_features {}) }: libc_0_2_36_ {
+    features = mkFeatures (features.libc_0_2_36 or {});
+  };
+  libc_0_2_36_features = f: updateFeatures f (rec {
+    libc_0_2_36.default = (f.libc_0_2_36.default or true);
+    libc_0_2_36.use_std =
+      (f.libc_0_2_36.use_std or false) ||
+      (f.libc_0_2_36.default or false) ||
+      (libc_0_2_36.default or false);
+  }) [];
+}
+```
+
+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.
+
+### Handling external dependencies {#handling-external-dependencies}
+
+Some crates require external libraries. For crates from
+[crates.io](https://crates.io), such libraries can be specified in
+`defaultCrateOverrides` package in nixpkgs itself.
+
+Starting from that file, one can add more overrides, to add features
+or build inputs by overriding the hello crate in a separate file.
+
+```nix
+with import <nixpkgs> {};
+((import ./hello.nix).hello {}).override {
+  crateOverrides = defaultCrateOverrides // {
+    hello = 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 crate's version, the `attrs` argument of
+the override above can be read, as in the following example, which
+patches the derivation:
+
+```nix
+with import <nixpkgs> {};
+((import ./hello.nix).hello {}).override {
+  crateOverrides = defaultCrateOverrides // {
+    hello = attrs: lib.optionalAttrs (lib.versionAtLeast attrs.version "1.0")  {
+      postPatch = ''
+        substituteInPlace lib/zoneinfo.rs \
+          --replace "/usr/share/zoneinfo" "${tzdata}/share/zoneinfo"
+      '';
+    };
+  };
+}
+```
+
+Another situation is when we want to override a nested
+dependency. This actually works in the exact same way, since the
+`crateOverrides` parameter is forwarded to the crate's
+dependencies. For instance, to override the build inputs for crate
+`libc` in the example above, where `libc` is a dependency of the main
+crate, we could do:
+
+```nix
+with import <nixpkgs> {};
+((import hello.nix).hello {}).override {
+  crateOverrides = defaultCrateOverrides // {
+    libc = attrs: { buildInputs = []; };
+  };
+}
+```
+
+### Options and phases configuration {#options-and-phases-configuration}
+
+Actually, the overrides introduced in the previous section are more
+general. A number of other parameters can be overridden:
+
+- The version of `rustc` used to compile the crate:
+
+  ```nix
+  (hello {}).override { rust = pkgs.rust; };
+  ```
+
+- Whether to build in release mode or debug mode (release mode by
+  default):
+
+  ```nix
+  (hello {}).override { release = false; };
+  ```
+
+- Whether to print the commands sent to `rustc` when building
+  (equivalent to `--verbose` in cargo:
+
+  ```nix
+  (hello {}).override { verbose = false; };
+  ```
+
+- Extra arguments to be passed to `rustc`:
+
+  ```nix
+  (hello {}).override { extraRustcOpts = "-Z debuginfo=2"; };
+  ```
+
+- Phases, just like in any other derivation, can be specified using
+  the following attributes: `preUnpack`, `postUnpack`, `prePatch`,
+  `patches`, `postPatch`, `preConfigure` (in the case of a Rust crate,
+  this is run before calling the "build" script), `postConfigure`
+  (after the "build" script),`preBuild`, `postBuild`, `preInstall` and
+  `postInstall`. As an example, here is how to create a new module
+  before running the build script:
+
+  ```nix
+  (hello {}).override {
+    preConfigure = ''
+       echo "pub const PATH=\"${hi.out}\";" >> src/path.rs"
+    '';
+  };
+  ```
+
+### Features {#features}
+
+One can also supply features switches. For example, if we want to
+compile `diesel_cli` only with the `postgres` feature, and no default
+features, we would write:
+
+```nix
+(callPackage ./diesel.nix {}).diesel {
+  default = false;
+  postgres = true;
+}
+```
+
+Where `diesel.nix` is the file generated by Carnix, as explained above.
+
 ## Setting Up `nix-shell` {#setting-up-nix-shell}
 
 Oftentimes you want to develop code from within `nix-shell`. Unfortunately