Mozilla rustc check does not support crossbuild: let's remove it
Remove calls to unwrap_rustup, they fail if rustup isn't present

Do not try to figure out the rust target as it is broken on musl
instead use what is set in RUST_TARGET

Skip extra checks on cross builds (broken for arm*)

--- a/build/moz.configure/rust.configure	2020-12-12 01:35:33.000000000 +0100
+++ b/build/moz.configure/rust.configure	2020-12-15 19:45:16.467750787 +0100
@@ -91,9 +91,6 @@
     return unwrap
 
 
-rustc = unwrap_rustup(rustc, "rustc")
-cargo = unwrap_rustup(cargo, "cargo")
-
 
 set_config("CARGO", cargo)
 set_config("RUSTC", rustc)
@@ -260,7 +257,9 @@
         data.setdefault(key, []).append(namespace(rust_target=t, target=info))
     return data
 
-
+@imports('os')
+@imports(_from='mozbuild.util', _import='ensure_unicode')
+@imports(_from='mozbuild.util', _import='system_encoding')
 def detect_rustc_target(
     host_or_target, compiler_info, arm_target, rust_supported_targets
 ):
@@ -382,12 +381,12 @@
 
         return None
 
-    rustc_target = find_candidate(candidates)
+    rustc_target = os.environ['RUST_TARGET']
 
     if rustc_target is None:
         die("Don't know how to translate {} for rustc".format(host_or_target.alias))
 
-    return rustc_target
+    return ensure_unicode(rustc_target, system_encoding)
 
 
 @imports("os")
@@ -410,35 +409,6 @@
 
         os.write(in_fd, ensure_binary(source))
         os.close(in_fd)
-
-        cmd = [
-            rustc,
-            "--crate-type",
-            "staticlib",
-            target_arg,
-            "-o",
-            out_path,
-            in_path,
-        ]
-
-        def failed():
-            die(
-                dedent(
-                    """\
-            Cannot compile for {} with {}
-            The target may be unsupported, or you may not have
-            a rust std library for that target installed. Try:
-
-              rustup target add {}
-            """.format(
-                        host_or_target.alias, rustc, rustc_target
-                    )
-                )
-            )
-
-        check_cmd_output(*cmd, onerror=failed)
-        if not os.path.exists(out_path) or os.path.getsize(out_path) == 0:
-            failed()
     finally:
         os.remove(in_path)
         os.remove(out_path)
@@ -462,28 +432,7 @@
         host, compiler_info, arm_target, rust_supported_targets
     )
     if rustc_target != rustc_host:
-        if host.alias == rustc_target:
-            configure_host = host.alias
-        else:
-            configure_host = "{}/{}".format(host.alias, rustc_target)
-        die(
-            dedent(
-                """\
-        The rust compiler host ({rustc}) is not suitable for the configure host ({configure}).
-
-        You can solve this by:
-        * Set your configure host to match the rust compiler host by editing your
-        mozconfig and adding "ac_add_options --host={rustc}".
-        * Or, install the rust toolchain for {configure}, if supported, by running
-        "rustup default stable-{rustc_target}"
-        """.format(
-                    rustc=rustc_host,
-                    configure=configure_host,
-                    rustc_target=rustc_target,
-                )
-            )
-        )
-    assert_rust_compile(host, rustc_target, rustc)
+        return rustc_host
     return rustc_target
 
 
