Fix channel detection for rust-toolchain.toml

[scholzp]

Tl;dr

The current implementation assumes the presence of a key date which is not part of the toolchain.toml format. Instead, we need to parse the channel's value to extract the date and channel from it to form a valid download URL.

Problem

The overlay allows to specifiy a the path to rust-toolchain.toml to download the correct Rust toolchain. For this, the overlay uses the Nix builtin fromTOML to read the respective file. This function makes the keys=value of the toolchain section available. The current code assumes the presence of a key named date in the toolchain section. Rust toolchain files do not contain a key named date. Instead the date is part of the channel settings. As a result, the code evaluates to NULL for the variable date in the Nix code. For example, a toolchain.toml looking like this:

[toolchain]
channel = "nightly-2024-01-24"
components = [ "rust-std", "rust-src", "rustc", "cargo", "clippy", "rustfmt" ]

Leads to the generation of a invalid toolchain download URL, similar to:

trying https://static.rust-lang.org/dist/channel-rust-nightly-2024-01-24.toml
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0   312    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (22) The requested URL returned error: 404
error: cannot download channel-rust-nightly-2024-01-24.toml from any mirror
error:
       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:37:12:
           36|
           37|   strict = derivationStrict drvAttrs;
             |            ^
           38|

       … while evaluating derivation 'rust-workspace-unknown'
         whose name attribute is located at /nix/store/pp307nbzkgsd6393zl2i9j4j86z5nz9b-nixpkgs-src/pkgs/stdenv/generic/make-derivation.nix:302:7

       … while evaluating attribute 'RUSTC' of derivation 'rust-workspace-unknown'
         at /nix/store/nn3wrsxl9x4pjp4ivh3bka93jrm021kn-naersk-src/build.nix:144:5:
          143|
          144|     RUSTC = "${rustc}/bin/rustc";
             |     ^
          145|     cargo_release = lib.optionalString release "--release";

       … from call site
         at /home/xxxx/yyyy/cccc/nix/test.nix:13:16:
           12|
           13|   toolchain = (pkgs.rustChannelOf {
             |                ^
           14|     rustToolchain = ../implementation/rust-toolchain.toml;

       … while calling 'rustChannelOf'
         at /nix/store/b1gwcwpnssjyz76q7z090wya6b007v6k-nixpkgs-mozilla-src/rust-overlay.nix:351:19:
          350|
          351|   rustChannelOf = { sha256 ? null, ... } @ manifest_args: fromManifest
             |                   ^
          352|     sha256 (manifest_v2_url manifest_args)

       … from call site
         at /nix/store/b1gwcwpnssjyz76q7z090wya6b007v6k-nixpkgs-mozilla-src/rust-overlay.nix:351:59:
          350|
          351|   rustChannelOf = { sha256 ? null, ... } @ manifest_args: fromManifest
             |                                                           ^
          352|     sha256 (manifest_v2_url manifest_args)

       … while calling 'fromManifest'
         at /nix/store/b1gwcwpnssjyz76q7z090wya6b007v6k-nixpkgs-mozilla-src/rust-overlay.nix:337:36:
          336|
          337|   fromManifest = sha256: manifest: { stdenv, lib, fetchurl, patchelf }:
             |                                    ^
          338|     let manifestFile = if sha256 == null then builtins.fetchurl manifest else fetchurl { url = manifest; inherit sha256; };

       … from call site
         at /nix/store/b1gwcwpnssjyz76q7z090wya6b007v6k-nixpkgs-mozilla-src/rust-overlay.nix:339:8:
          338|     let manifestFile = if sha256 == null then builtins.fetchurl manifest else fetchurl { url = manifest; inherit sha256; };
          339|     in fromManifestFile manifestFile { inherit stdenv lib fetchurl patchelf; };
             |        ^
          340|

       … while calling 'fromManifestFile'
         at /nix/store/b1gwcwpnssjyz76q7z090wya6b007v6k-nixpkgs-mozilla-src/rust-overlay.nix:270:32:
          269|   #                       you will need to specify this extension in the extensions options or it will not be installed!
          270|   fromManifestFile = manifest: { stdenv, lib, fetchurl, patchelf }:
             |                                ^
          271|     let

       … from call site
         at /nix/store/b1gwcwpnssjyz76q7z090wya6b007v6k-nixpkgs-mozilla-src/rust-overlay.nix:277:5:
          276|     in
          277|     flip mapAttrs pkgs.pkg (name: pkg:
             |     ^
          278|       makeOverridable ({extensions, targets, targetExtensions}:

       … while calling 'flip'
         at /nix/store/pp307nbzkgsd6393zl2i9j4j86z5nz9b-nixpkgs-src/lib/trivial.nix:138:16:
          137|   */
          138|   flip = f: a: b: f b a;
             |                ^
          139|

       … while calling the 'mapAttrs' builtin
         at /nix/store/pp307nbzkgsd6393zl2i9j4j86z5nz9b-nixpkgs-src/lib/trivial.nix:138:19:
          137|   */
          138|   flip = f: a: b: f b a;
             |                   ^
          139|

       … while calling the 'fromTOML' builtin
         at /nix/store/b1gwcwpnssjyz76q7z090wya6b007v6k-nixpkgs-mozilla-src/rust-overlay.nix:275:14:
          274|       inherit (super.lib) flip mapAttrs;
          275|       pkgs = fromTOML (builtins.readFile manifest);
             |              ^
          276|     in

       … while evaluating the argument passed to builtins.fromTOML

       … while calling the 'readFile' builtin
         at /nix/store/b1gwcwpnssjyz76q7z090wya6b007v6k-nixpkgs-mozilla-src/rust-overlay.nix:275:24:
          274|       inherit (super.lib) flip mapAttrs;
          275|       pkgs = fromTOML (builtins.readFile manifest);
             |                        ^
          276|     in

       … while realising the context of path '/nix/store/piscm1b6wh0xixx52b33wm6xkaq4bmb7-channel-rust-nightly-2024-01-24.toml'

In this case, the correct URL would be: https://static.rust-lang.org/dist/2024-01-24/channel-rust-nightly.toml

Proposed solution

Parse the channel key's value to obtain information about the channel (e.g. nightly) and the date.

What's still missing?

My solution at this point only supports channel values of stable, beta and nightly. It needs to be extended to support versioned channels, but I currently lack the time to do so. Nevertheless, this should fix #287 and similar problems. That's why I though I should leave the fix here. Let me know if anything is missing, then I'll might look into it again in June.

Have a nice week!