CMake+Rust: Fix offline builds w. vendored git repos

Some builds using the tarball with the vendored Rust dependencies are
failing.

The onenote-rs dependency is presently tied to a git branch from github
rather than using a release from crates.io. This is the differing factor
though I'm unsure why it is causing the build to try to update the repo
rather than just building the vendored source.

This commit adds a `--offline` parameter to the build options if the
vendored source is detected, in an attempt to force Cargo to use what it
has and stay offline.

cmake/FindRust.cmake

@@ -184,28 +184,28 @@ function(cargo_vendor)
     set(oneValueArgs TARGET SOURCE_DIRECTORY BINARY_DIRECTORY)
     cmake_parse_arguments(ARGS "${options}" "${oneValueArgs}" "${multiValueArgs}" ${ARGN})
 
-    if(NOT EXISTS ${ARGS_SOURCE_DIRECTORY}/.cargo/config.toml)
-        # Vendor the dependencies and create .cargo/config.toml
-        # Vendored dependencies will be used during the build.
-        # This will allow us to package vendored dependencies in source tarballs
-        # for online builds when we run `cpack --config CPackSourceConfig.cmake`
-        message(STATUS "Running `cargo vendor` to collect dependencies for ${ARGS_TARGET}. This may take a while if the local crates.io index needs to be updated ...")
-        make_directory(${ARGS_SOURCE_DIRECTORY}/.cargo)
-        execute_process(
-            COMMAND ${CMAKE_COMMAND} -E env "CARGO_TARGET_DIR=${ARGS_BINARY_DIRECTORY}" ${cargo_EXECUTABLE} vendor ".cargo/vendor"
-            WORKING_DIRECTORY "${ARGS_SOURCE_DIRECTORY}"
-            OUTPUT_VARIABLE CARGO_VENDOR_OUTPUT
-            ERROR_VARIABLE CARGO_VENDOR_ERROR
-            RESULT_VARIABLE CARGO_VENDOR_RESULT
-        )
+    # Vendor the dependencies and create .cargo/config.toml
+    # Vendored dependencies will be used during the build.
+    # This will allow us to package vendored dependencies in source tarballs
+    # for online builds when we run `cpack --config CPackSourceConfig.cmake`
+    message(STATUS "Running `cargo vendor` to collect dependencies for ${ARGS_TARGET}. This may take a while if the local crates.io index needs to be updated ...")
+    make_directory(${CMAKE_SOURCE_DIR}/.cargo)
+    execute_process(
+        COMMAND ${CMAKE_COMMAND} -E env "CARGO_TARGET_DIR=${ARGS_BINARY_DIRECTORY}" ${cargo_EXECUTABLE} vendor "${CMAKE_SOURCE_DIR}/.cargo/vendor"
+        WORKING_DIRECTORY "${ARGS_SOURCE_DIRECTORY}"
+        OUTPUT_VARIABLE CARGO_VENDOR_OUTPUT
+        ERROR_VARIABLE CARGO_VENDOR_ERROR
+        RESULT_VARIABLE CARGO_VENDOR_RESULT
+    )
 
-        if(NOT ${CARGO_VENDOR_RESULT} EQUAL 0)
-            message(FATAL_ERROR "Failed!\n${CARGO_VENDOR_ERROR}")
-        else()
-            message("Success!")
-        endif()
+    if(NOT ${CARGO_VENDOR_RESULT} EQUAL 0)
+        message(FATAL_ERROR "Failed!\n${CARGO_VENDOR_ERROR}")
+    else()
+        message("Success!")
+    endif()
 
-        write_file(${ARGS_SOURCE_DIRECTORY}/.cargo/config.toml "
+    if(NOT EXISTS ${CMAKE_SOURCE_DIR}/.cargo/config.toml)
+        write_file(${CMAKE_SOURCE_DIR}/.cargo/config.toml "
 [source.crates-io]
 replace-with = \"vendored-sources\"
 
@@ -460,6 +460,10 @@ endif()
 
 set(CARGO_ARGS "build")
 
+if(EXISTS "${CMAKE_SOURCE_DIR}/.cargo/vendor")
+    list(APPEND CARGO_ARGS "--offline")
+endif()
+
 if(NOT "${RUST_COMPILER_TARGET}" MATCHES "^universal-apple-darwin$")
     # Don't specify the target for macOS universal builds, we'll do that manually for each build.
     list(APPEND CARGO_ARGS "--target" ${RUST_COMPILER_TARGET})