Make C++ documentation generation an xtask

Tear it out of the CMakeLists.txt and instead run it via

    cargo xtask cppdocs

This allows the build_and_test step in the CI to only run cmake for the
library/header related bits and the docs_and_demos step to only generate
docs and not require a full host build of the library (as cargo xtask
cmake would otherwise do).

xtask/src/cppdocs.rs

@@ -0,0 +1,107 @@
+/* LICENSE BEGIN
+    This file is part of the SixtyFPS Project -- https://sixtyfps.io
+    Copyright (c) 2020 Olivier Goffart <olivier.goffart@sixtyfps.io>
+    Copyright (c) 2020 Simon Hausmann <simon.hausmann@sixtyfps.io>
+
+    SPDX-License-Identifier: GPL-3.0-only
+    This file is also available under commercial licensing terms.
+    Please contact info@sixtyfps.io for more information.
+LICENSE END */
+use anyhow::{Context, Result};
+use std::ffi::OsString;
+use std::path::{Path, PathBuf};
+
+fn symlink_file<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> Result<()> {
+    if dst.as_ref().exists() {
+        std::fs::remove_file(dst.as_ref()).context("Error removing old symlink")?;
+    }
+    #[cfg(target_os = "windows")]
+    return std::os::windows::fs::symlink_file(&src, &dst).context("Error creating symlink");
+    #[cfg(not(target_os = "windows"))]
+    return std::os::unix::fs::symlink(&src, &dst).context(format!(
+        "Error creating symlink from {} to {}",
+        src.as_ref().display(),
+        dst.as_ref().display()
+    ));
+}
+
+fn symlink_dir<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> Result<()> {
+    if dst.as_ref().exists() {
+        std::fs::remove_file(dst.as_ref()).context("Error removing old symlink")?;
+    }
+    #[cfg(target_os = "windows")]
+    return std::os::windows::fs::symlink_dir(&src, &dst).context("Error creating symlink");
+    #[cfg(not(target_os = "windows"))]
+    return std::os::unix::fs::symlink(&src, &dst).context(format!(
+        "Error creating symlink from {} to {}",
+        src.as_ref().display(),
+        dst.as_ref().display()
+    ));
+}
+
+fn symlink_files_in_dir<S: AsRef<Path>, T: AsRef<Path>, TS: AsRef<Path>>(
+    src: S,
+    target: T,
+    target_to_source: TS,
+) -> Result<()> {
+    for entry in std::fs::read_dir(src.as_ref()).context("Error reading docs source directory")? {
+        let entry = entry.context("Error reading directory entry")?;
+        let path = entry.path();
+        if path.is_file() {
+            let file_name = path.file_name().unwrap();
+            let symlink_source = target_to_source.as_ref().to_path_buf().join(&file_name);
+            let symlink_target = target.as_ref().to_path_buf().join(path.file_name().unwrap());
+            symlink_file(symlink_source, symlink_target)?;
+        }
+    }
+    Ok(())
+}
+
+pub fn generate() -> Result<(), Box<dyn std::error::Error>> {
+    let root = super::root_dir()?;
+
+    let docs_source_dir = root.join("api/sixtyfps-cpp");
+    let docs_build_dir = root.join("target/cppdocs");
+
+    std::fs::create_dir_all(docs_build_dir.as_path()).context("Error creating docs build dir")?;
+
+    symlink_files_in_dir(
+        docs_source_dir.join("docs"),
+        &docs_build_dir,
+        ["..", "..", "api", "sixtyfps-cpp", "docs"].iter().collect::<PathBuf>(),
+    )
+    .context("Error symlinking files from docs source to docs build dir")?;
+
+    symlink_dir(["..", "..", "docs"].iter().collect::<PathBuf>(), docs_build_dir.join("markdown"))?;
+
+    symlink_file(
+        ["..", "..", "api", "sixtyfps-cpp", "README.md"].iter().collect::<PathBuf>(),
+        docs_build_dir.join("README.md"),
+    )?;
+
+    let pip_env =
+        vec![(OsString::from("PIPENV_PIPFILE"), docs_source_dir.join("docs/Pipfile").to_owned())];
+
+    println!("Running pipenv install");
+
+    super::run_command("pipenv", &["install"], pip_env.clone())
+        .context("Error running pipenv install")?;
+
+    println!("Running sphinx-build");
+
+    let output = super::run_command(
+        "pipenv",
+        &[
+            "run",
+            "sphinx-build",
+            docs_build_dir.to_str().unwrap(),
+            docs_build_dir.join("html").to_str().unwrap(),
+        ],
+        pip_env.clone(),
+    )
+    .context("Error running pipenv install")?;
+
+    println!("{}", String::from_utf8_lossy(&output));
+
+    Ok(())
+}