rust: add spack external find support (#24939)

var/spack/repos/builtin/packages/rust/package.py

@@ -2,6 +2,9 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+import re
+
 from six import iteritems
 
 
@@ -459,6 +462,14 @@ class Rust(Package):
                     )
                 )
 
+    executables = ['^rustc$']
+
+    @classmethod
+    def determine_version(csl, exe):
+        output = Executable(exe)('--version', output=str, error=str)
+        match = re.match(r'rustc (\S+)', output)
+        return match.group(1) if match else None
+
     # This routine returns the target architecture we intend to build for.
     def get_rust_target(self):
         if 'platform=linux' in self.spec or 'platform=cray' in self.spec: