rust: Clean up target configs and test some more

See the new docs for details. The difference is vis-a-vis older versions
of this PR, not master.

doc/languages-frameworks/rust.section.md

@@ -71,14 +71,14 @@ By default, it takes the `stdenv.hostPlatform.config` and replaces components
 where they are known to differ. But there are ways to customize the argument:
 
  - To choose a different target by name, define
-   `stdenv.hostPlatform.rustc.arch.config` as that name (a string), and that
+   `stdenv.hostPlatform.rustc.config` as that name (a string), and that
    name will be used instead.
 
    For example:
    ```nix
    import <nixpkgs> {
      crossSystem = (import <nixpkgs/lib>).systems.examples.armhf-embedded // {
-       rustc.arch.config = "thumbv7em-none-eabi";
+       rustc.config = "thumbv7em-none-eabi";
      };
    }
    ```
@@ -88,18 +88,18 @@ where they are known to differ. But there are ways to customize the argument:
    ```
 
  - To pass a completely custom target, define
-   `stdenv.hostPlatform.rustc.arch.config` with its name, and
+   `stdenv.hostPlatform.rustc.config` with its name, and
-   `stdenv.hostPlatform.rustc.arch.custom` with the value.  The value will be
+   `stdenv.hostPlatform.rustc.platform` with the value.  The value will be
    serialized to JSON in a file called
-   `${stdenv.hostPlatform.rustc.arch.config}.json`, and the path of that file
+   `${stdenv.hostPlatform.rustc.config}.json`, and the path of that file
    will be used instead.
 
    For example:
    ```nix
    import <nixpkgs> {
      crossSystem = (import <nixpkgs/lib>).systems.examples.armhf-embedded // {
-       rustc.arch.config = "thumb-crazy";
+       rustc.config = "thumb-crazy";
-       rustc.arch.custom = { foo = ""; bar = ""; };
+       rustc.platform = { foo = ""; bar = ""; };
      };
    }
    will result in:

pkgs/development/compilers/rust/default.nix

@@ -27,7 +27,7 @@
   # Returns the name of the rust target, even if it is custom. Adjustments are
   # because rust has slightly different naming conventions than we do.
   toRustTarget = platform: with platform.parsed; let
-    cpu_ = platform.rustc.arch or {
+    cpu_ = platform.rustc.platform.arch or {
       "armv7a" = "armv7";
       "armv7l" = "armv7";
       "armv6l" = "arm";
@@ -38,8 +38,8 @@
   # Returns the name of the rust target if it is standard, or the json file
   # containing the custom target spec.
   toRustTargetSpec = platform:
-    if (platform.rustc.arch or {}) ? custom
+    if (platform.rustc or {}) ? platform
-    then builtins.toFile (platform.rustc.config + ".json") (builtins.toJSON platform.rustc.arch.custom)
+    then builtins.toFile (toRustTarget platform + ".json") (builtins.toJSON platform.rustc.platform)
     else toRustTarget platform;
 
   # This just contains tools for now. But it would conceivably contain

pkgs/test/rust-sysroot/default.nix

@@ -1,6 +1,7 @@
-{ lib, rustPlatform, fetchFromGitHub, writeText }:
+{ lib, rust, rustPlatform, fetchFromGitHub }:
 
-rustPlatform.buildRustPackage rec {
+let
+  mkBlogOsTest = target: rustPlatform.buildRustPackage rec {
     name = "blog_os-sysroot-test";
 
     src = fetchFromGitHub {
@@ -12,27 +13,7 @@ rustPlatform.buildRustPackage rec {
 
     cargoSha256 = "1cbcplgz28yxshyrp2krp1jphbrcqdw6wxx3rry91p7hiqyibd30";
 
-    # The book uses rust-lld for linking, but rust-lld is not currently packaged for NixOS.
+    inherit target;
-    # The justification in the book for using rust-lld suggests that gcc can still be used for testing:
-    # > Instead of using the platform's default linker (which might not support Linux targets),
-    # > we use the cross platform LLD linker that is shipped with Rust for linking our kernel.
-    # https://github.com/phil-opp/blog_os/blame/7212ffaa8383122b1eb07fe1854814f99d2e1af4/blog/content/second-edition/posts/02-minimal-rust-kernel/index.md#L157
-    target = writeText "x86_64-blog_os.json" ''
-        {
-            "llvm-target": "x86_64-unknown-none",
-            "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
-            "arch": "x86_64",
-            "target-endian": "little",
-            "target-pointer-width": "64",
-            "target-c-int-width": "32",
-            "os": "none",
-            "executables": true,
-            "linker-flavor": "gcc",
-            "panic-strategy": "abort",
-            "disable-redzone": true,
-            "features": "-mmx,-sse,+soft-float"
-        }
-    '';
 
     RUSTFLAGS = "-C link-arg=-nostartfiles";
 
@@ -42,5 +23,38 @@ rustPlatform.buildRustPackage rec {
     meta = with lib; {
         description = "Test for using custom sysroots with buildRustPackage";
         maintainers = with maintainers; [ aaronjanse ];
+        platforms = lib.platforms.x86_64;
     };
+  };
+
+  # The book uses rust-lld for linking, but rust-lld is not currently packaged for NixOS.
+  # The justification in the book for using rust-lld suggests that gcc can still be used for testing:
+  # > Instead of using the platform's default linker (which might not support Linux targets),
+  # > we use the cross platform LLD linker that is shipped with Rust for linking our kernel.
+  # https://github.com/phil-opp/blog_os/blame/7212ffaa8383122b1eb07fe1854814f99d2e1af4/blog/content/second-edition/posts/02-minimal-rust-kernel/index.md#L157
+  targetContents = {
+    "llvm-target" = "x86_64-unknown-none";
+    "data-layout" = "e-m:e-i64:64-f80:128-n8:16:32:64-S128";
+    "arch" = "x86_64";
+    "target-endian" = "little";
+    "target-pointer-width" = "64";
+    "target-c-int-width" = "32";
+    "os" = "none";
+    "executables" = true;
+    "linker-flavor" = "gcc";
+    "panic-strategy" = "abort";
+    "disable-redzone" = true;
+    "features" = "-mmx,-sse,+soft-float";
+  };
+
+in {
+  blogOS-targetByFile = mkBlogOsTest (builtins.toFile "x86_64-blog_os.json" (builtins.toJSON targetContents));
+  blogOS-targetByNix = let
+    plat = lib.systems.elaborate { config = "x86_64-none"; } // {
+      rustc = {
+        config = "x86_64-blog_os";
+        platform = targetContents;
+      };
+    };
+    in mkBlogOsTest (rust.toRustTargetSpec plat);
 }