invalidate cross image if rust version has changed (but nothing else)

docker/Dockerfile.cross

@@ -1,14 +1,13 @@
 FROM ubuntu:20.04 as rust
 ARG DEBIAN_FRONTEND=noninteractive
+ARG RUST_STABLE
 COPY docker/lib.sh docker/cross.sh /
 COPY ./ /project
-RUN /cross.sh /project
+RUN /cross.sh /project $RUST_STABLE
 
 # we build our images in 2 steps, to ensure we have a compact
 # image, since we want to add our current subdirectory
 FROM ubuntu:20.04
-COPY --from=rust /root/.cargo /root/.cargo
-COPY --from=rust /root/.rustup /root/.rustup
 
 # need some basic devtools, and requirements for docker
 RUN apt-get update && apt-get install --assume-yes --no-install-recommends \
@@ -18,5 +17,8 @@ RUN apt-get update && apt-get install --assume-yes --no-install-recommends \
 
 RUN curl -fsSL https://get.docker.com | sh
 
+COPY --from=rust /root/.cargo /root/.cargo
+COPY --from=rust /root/.rustup /root/.rustup
+
 ENV CROSS_CONTAINER_IN_CONTAINER=1 \
     PATH=/root/.cargo/bin:$PATH

src/lib.rs

@@ -28,7 +28,7 @@ mod file;
 mod id;
 mod interpreter;
 pub mod rustc;
-mod rustup;
+pub mod rustup;
 pub mod shell;
 pub mod temp;
 

src/rustup.rs

@@ -27,7 +27,7 @@ impl AvailableTargets {
     }
 }
 
-fn rustup_command(msg_info: &mut MessageInfo, no_flags: bool) -> Command {
+pub fn rustup_command(msg_info: &mut MessageInfo, no_flags: bool) -> Command {
     let mut cmd = Command::new("rustup");
     if no_flags {
         return cmd;
@@ -127,6 +127,7 @@ pub fn install_component(
         .wrap_err_with(|| format!("couldn't install the `{component}` component"))
 }
 
+#[derive(Debug)]
 pub enum Component<'a> {
     Installed(&'a str),
     Available(&'a str),

xtask/src/build_docker_image.rs

@@ -238,6 +238,29 @@ pub fn build_docker_image(
         if verbose > 1 {
             docker_build.args(&["--build-arg", "VERBOSE=1"]);
         }
+        // FIXME: Move to matrix?
+        if target.triplet == "cross" {
+            let rustup = cross::rustup::rustup_command(msg_info, false)
+                .arg("check")
+                .run_and_get_stdout(msg_info)?;
+            let version = rustup
+                .lines()
+                .find(|line| line.starts_with("stable"))
+                .and_then(|stable| {
+                    // Check is formatted as
+                    // `Update available : 1.61.0 (fe5b13d68 2022-05-18) -> 1.62.0 (a8314ef7d 2022-06-27)`
+                    // or
+                    // `Up to date : 1.62.0 (a8314ef7d 2022-06-27)`
+                    stable
+                        .split_once(" -> ")
+                        .or_else(|| stable.split_once(" : "))
+                })
+                .and_then(|(_, v)| v.split_once(' '))
+                .map(|(v, _)| v)
+                .unwrap_or("stable");
+            dbg!(&version);
+            docker_build.args(&["--build-arg", &format!("RUST_STABLE={version}")]);
+        }
 
         if target.needs_workspace_root_context() {
             docker_build.arg(&root);