Describe the bug

I'm trying to have styles included in a CSS module via composes processed by PostCSS, but it's not happening. All other styles are processed by PostCSS correctly.

Take the following CSS modules and PostCSS config:

{
  "plugins": {
    "postcss-preset-env": {
      "browsers": "defaults, ie >= 11",
      "importFrom": [
        "./src/variables.css"
      ]
    },
    "postcss-nested": { }
  }
}

/* variables.css */
:root {
  --color-red: red;
  --color-blue: blue;
}

/* component.module.css */
.component {
  color: var(--color-red);

  .nested {
    width: 100%;
  }
}

/* main.module.css */
.main {
  composes: component from "./component.module.css";
  color: var(--color-blue);

  .nested {
    display: block;
  }
}

Expected result

I exect to see the styles I include with .main via `composes: component from "./component.module.css" to be valid (with PostCSS plugins applied). Expected result:

 ._component_xs2hm_5 {
  color: red;
  color: var(--color-red);
 }

 ._component_xs2hm_5 ._nested_xs2hm_13 {
  width: 100%;
}

._foo_1mrk0_1 {
  color: blue;
  color: var(--color-blue);
}

._foo_1mrk0_1 ._nested_1mrk0_7 {
  width: 100%;
}

Actual result

The styles included via composes are not processed by PostCSS. They are included in the <style> tag as-is, resulting in invalid CSS.

 ._component_xs2hm_5 {
  color: var(--color-red);

  ._nested_xs2hm_13 {
    width: 100%;
  }
}

._foo_1mrk0_1 {
  color: blue;
  color: var(--color-blue);
}

._foo_1mrk0_1 ._nested-bar_1mrk0_7 {
  width: 100%;
}

The same bug can be observed in the production build.

There was a similar issue logged and fixed in vue-loader: vuejs/vue-loader#959

Reproduction

https://github.com/elisehein/vite-postcss-compose-repro

System Info

System:
    OS: macOS 12.4
    CPU: (10) arm64 Apple M1 Pro
    Memory: 721.78 MB / 32.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 16.16.0 - ~/.asdf/installs/nodejs/16.16.0/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 8.15.0 - ~/.asdf/plugins/nodejs/shims/npm
    Watchman: 2022.05.16.00 - /usr/local/bin/watchman
  Browsers:
    Firefox: 103.0.2
    Safari: 15.5
    Safari Technology Preview: 15.4
  npmPackages:
    @vitejs/plugin-legacy: ^1.8.2 => 1.8.2 
    @vitejs/plugin-react: ^1.0.0 => 1.3.2 
    vite: ^3.0.2 => 3.1.0

Used Package Manager

npm

Logs

No response

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• The provided reproduction is a minimal reproducible example of the bug.