Add warning when mixing -Zbuild-std with --build-plan.

Author: ehuss

src/cargo/ops/cargo_compile.rs

@@ -309,6 +309,11 @@ pub fn compile_ws<'a>(
     let (mut packages, resolve_with_overrides) = resolve;
 
     let std_resolve = if let Some(crates) = &config.cli_unstable().build_std {
+        if build_config.build_plan {
+            config
+                .shell()
+                .warn("-Zbuild-std does not currently fully support --build-plan")?;
+        }
         if build_config.requested_target.is_none() {
             // TODO: This should eventually be fixed. Unfortunately it is not
             // easy to get the host triple in BuildConfig. Consider changing