Better error diagnostics on targets without std.

Provides a warning if a docker command fails and `rust-std` is not
installed for the target. For example, for `x86_64-unknown-dragonfly`,
the added output would be:

```bash
[cross] warning: rust-std is not installed for x86_64-unknown-dragonfly
[cross] note: you may need to build components for the std library `-Z build-std=<components>`
              the available components are core, std, alloc, and proc_macro
              if no components are provided, cargo will build all components
```

This is done solely if the command fails and if the target is a built-in, so we don't get misleading warnings for custom targets, and there is no overhead if the build succeeds.

Author: Alexhuszagh

src/lib.rs

@@ -360,6 +360,20 @@ impl Serialize for Target {
     }
 }
 
+fn warn_on_failure(target: &Target, toolchain: &str, msg_info: MessageInfo) -> Result<()> {
+    let rust_std = format!("rust-std-{target}");
+    if target.is_builtin() && !rustup::component_is_installed(&rust_std, toolchain, msg_info)? {
+        shell::warn(format!("rust-std is not installed for {target}"), msg_info)?;
+        shell::note(
+            "you may need to build components for the std library `-Z build-std=<components>`
+              the available components are core, std, alloc, and proc_macro
+              if no components are provided, cargo will build all components",
+            msg_info,
+        )?;
+    }
+    Ok(())
+}
+
 pub fn run() -> Result<ExitStatus> {
     let target_list = rustc::target_list(Verbosity::Quiet.into())?;
     let args = cli::parse(&target_list)?;
@@ -524,6 +538,9 @@ pub fn run() -> Result<ExitStatus> {
                     .subcommand
                     .map(|sc| sc.needs_host(is_remote))
                     .unwrap_or(false);
+                if !status.success() {
+                    warn_on_failure(&target, &toolchain, args.msg_info)?;
+                }
                 if !(status.success() && needs_host) {
                     return Ok(status);
                 }