Added additional toolchains maintained by cross-rs.

These toolchains are not automatically tested by CI, and must be built
manually by the user. However, they are confirmed to build at least
once, and are likely to work as-is.

Also adds better error handling when it cannot find a suitable Dockerfile
for a given target.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "docker/cross-toolchains"]
+	path = docker/cross-toolchains
+	url = https://github.com/cross-rs/cross-toolchains.git

CHANGELOG.md

@@ -7,6 +7,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 
+- #795 - added images for additional toolchains maintained by cross-rs.
 - #792 - added `CROSS_CONTAINER_IN_CONTAINER` environment variable to replace `CROSS_DOCKER_IN_DOCKER`.
 - #782 - added `build-std` config option, which builds the rust standard library from source if enabled.
 - #775 - forward Cargo exit code to host

README.md

@@ -348,6 +348,8 @@ terminate.
 <!--[6] libc = musl, gcc = emcc. The Docker images for these targets are currently not built automatically
 due to a [compiler bug](https://github.com/rust-lang/rust/issues/85821), you will have to build them yourself for now.-->
 
+Additional Dockerfiles for other targets can be found in [cross-toolchains](https://github.com/cross-rs/cross-toolchains).
+
 ## Debugging
 
 ### QEMU_STRACE (v0.1.9+)

xtask/src/build_docker_image.rs

@@ -48,6 +48,23 @@ pub struct BuildDockerImage {
     targets: Vec<String>,
 }
 
+fn locate_dockerfile(
+    target: String,
+    docker_root: &Path,
+    cross_toolchain_root: &Path,
+) -> cross::Result<(String, String)> {
+    let dockerfile_name = format!("Dockerfile.{target}");
+    let dockerfile_root = if cross_toolchain_root.join(&dockerfile_name).exists() {
+        &cross_toolchain_root
+    } else if docker_root.join(&dockerfile_name).exists() {
+        &docker_root
+    } else {
+        eyre::bail!("unable to find dockerfile for target \"{target}\"");
+    };
+    let dockerfile = dockerfile_root.join(dockerfile_name).display().to_string();
+    Ok((target, dockerfile))
+}
+
 pub fn build_docker_image(
     BuildDockerImage {
         ref_type,
@@ -102,23 +119,28 @@ pub fn build_docker_image(
         }
     }
     let gha = std::env::var("GITHUB_ACTIONS").is_ok();
+    let docker_root = metadata.workspace_root.join("docker");
+    let cross_toolchains_root = docker_root.join("cross-toolchains").join("docker");
+    let targets = targets
+        .into_iter()
+        .map(|t| locate_dockerfile(t, &docker_root, &cross_toolchains_root))
+        .collect::<cross::Result<Vec<_>>>()?;
+
     let mut results = vec![];
-    for target in &targets {
+    for (target, dockerfile) in &targets {
         if gha && targets.len() > 1 {
             println!("::group::Build {target}");
         }
         let mut docker_build = Command::new(engine);
         docker_build.args(&["buildx", "build"]);
-        docker_build.current_dir(metadata.workspace_root.join("docker"));
+        docker_build.current_dir(&docker_root);
 
         if push {
             docker_build.arg("--push");
         } else {
             docker_build.arg("--load");
         }
 
-        let dockerfile = format!("Dockerfile.{target}");
-
         let (target, sub) = if let Some((t, sub)) = target.split_once('.') {
             (t.to_owned(), format!("-{sub}"))
         } else {
@@ -192,7 +214,7 @@ pub fn build_docker_image(
             docker_build.args(&["--label", label]);
         }
 
-        docker_build.args(&["-f", &dockerfile]);
+        docker_build.args(&["-f", dockerfile]);
 
         if gha || progress == "plain" {
             docker_build.args(&["--progress", "plain"]);
@@ -207,7 +229,10 @@ pub fn build_docker_image(
             if gha && targets.len() > 1 {
                 if let Err(e) = &result {
                     // TODO: Determine what instruction errorred, and place warning on that line with appropriate warning
-                    println!("::error file=docker/{dockerfile},title=Build failed::{}", e)
+                    println!(
+                        "::error file=docker/{},title=Build failed::{}",
+                        dockerfile, e
+                    )
                 }
             }
             results.push(