Set -Z flags for unstable JSON target specs

Cargo now requires `-Zjson-target-spec` when using a JSON target spec.

Author: ehuss

src/bootstrap/src/core/build_steps/compile.rs

@@ -543,6 +543,9 @@ pub fn std_cargo(
         // `MACOSX_DEPLOYMENT_TARGET`, `IPHONEOS_DEPLOYMENT_TARGET`, etc.
         let mut cmd = builder.rustc_cmd(cargo.compiler());
         cmd.arg("--target").arg(target.rustc_target_arg());
+        if target.filepath().is_some() {
+            cmd.arg("-Zunstable-options");
+        }
         cmd.arg("--print=deployment-target");
         let output = cmd.run_capture_stdout(builder).stdout();
 

src/bootstrap/src/core/builder/cargo.rs

@@ -543,6 +543,9 @@ impl Builder<'_> {
 
         if cmd_kind != Kind::Install {
             cargo.arg("--target").arg(target.rustc_target_arg());
+            if target.filepath().is_some() {
+                cargo.arg("-Zjson-target-spec");
+            }
         } else {
             assert_eq!(target, compiler.host);
         }