Add test to FindRust with Rust_USE_RUSTUP_PROXY

test/find_rust/CMakeLists.txt

@@ -1,2 +1,3 @@
 corrosion_tests_add_test(find_rust "")
+corrosion_tests_add_test(rustup_proxy "")
 

test/find_rust/rustup_proxy/CMakeLists.txt

@@ -0,0 +1,47 @@
+cmake_minimum_required(VERSION 3.15)
+project(RustupProxy LANGUAGES CXX)
+
+set(CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/../../../cmake" ${CMAKE_MODULE_PATH})
+
+function(_assert_is_rustup_proxy executable_path)
+    execute_process(
+        COMMAND
+            ${CMAKE_COMMAND} -E env
+                RUSTUP_FORCE_ARG0=rustup
+            "${executable_path}" --version
+        OUTPUT_VARIABLE _VERSION_RAW
+        ERROR_VARIABLE _VERSION_STDERR
+        RESULT_VARIABLE _VERSION_RESULT
+    )
+
+    if(NOT _VERSION_RESULT EQUAL "0")
+        message(FATAL_ERROR "`${executable_path} --version` failed with ${_VERSION_RESULT}\n"
+            "stderr:\n${_VERSION_STDERR}"
+        )
+    endif()
+
+    if (NOT _VERSION_RAW MATCHES "rustup [0-9\\.]+")
+        message(FATAL_ERROR "`${executable_path} --version` output does not match rustup: ${_VERSION_RAW}\n")
+    endif()
+endfunction()
+
+set(Rust_RESOLVE_RUSTUP_TOOLCHAINS OFF CACHE BOOL "" FORCE)
+find_package(Rust REQUIRED)
+
+if (NOT Rust_FOUND)
+    message(FATAL_ERROR "Rustup not found")
+endif()
+
+get_property(
+    RUSTC_EXECUTABLE
+    TARGET Rust::Rustc PROPERTY IMPORTED_LOCATION
+)
+
+_assert_is_rustup_proxy(${RUSTC_EXECUTABLE})
+
+get_property(
+    CARGO_EXECUTABLE
+    TARGET Rust::Cargo PROPERTY IMPORTED_LOCATION
+)
+
+_assert_is_rustup_proxy(${CARGO_EXECUTABLE})