Add an FFI test facility

This allow testing Rust functions (from fish_tests.cpp) which need to
cross the FFI. See the example in smoke.rs.

cmake/Rust.cmake

@@ -17,6 +17,7 @@ set(fish_autocxx_gen_dir "${CMAKE_BINARY_DIR}/fish-autocxx-gen/")
 
 corrosion_import_crate(
     MANIFEST_PATH "${CMAKE_SOURCE_DIR}/fish-rust/Cargo.toml"
+    FEATURES "fish-ffi-tests"
 )
 
 # We need the build dir because cxx puts our headers in there.

fish-rust/Cargo.lock

@@ -232,6 +232,16 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "ctor"
+version = "0.1.26"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6d2301688392eb071b0bf1a37be05c469d3cc4dbbd95df672fe28ab021e6a096"
+dependencies = [
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "cxx"
 version = "1.0.81"
@@ -343,6 +353,7 @@ dependencies = [
  "cxx-build",
  "cxx-gen",
  "errno",
+ "inventory",
  "lazy_static",
  "libc",
  "miette",
@@ -364,6 +375,17 @@ dependencies = [
  "wasi",
 ]
 
+[[package]]
+name = "ghost"
+version = "0.1.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "41973d4c45f7a35af8753ba3457cc99d406d863941fd7f52663cff54a5ab99b3"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "gimli"
 version = "0.27.0"
@@ -432,6 +454,16 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "inventory"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "16fe3b35d64bd1f72917f06425e7573a2f63f74f42c8f56e53ea6826dde3a2b5"
+dependencies = [
+ "ctor",
+ "ghost",
+]
+
 [[package]]
 name = "is_ci"
 version = "1.1.1"

fish-rust/Cargo.toml

@@ -10,6 +10,7 @@ widestring-suffix = { path = "./widestring-suffix/" }
 autocxx = "0.23.1"
 cxx = "1.0"
 errno = "0.2.8"
+inventory = { version = "0.3.3", optional = true}
 lazy_static = "1.4.0"
 libc = "0.2.137"
 nix = "0.25.0"
@@ -26,6 +27,12 @@ miette = { version = "5", features = ["fancy"] }
 [lib]
 crate-type=["staticlib"]
 
+[features]
+# The fish-ffi-tests feature causes tests to be built which need to use the FFI.
+# These tests are run by fish_tests().
+default = ["fish-ffi-tests"]
+fish-ffi-tests = ["inventory"]
+
 [patch.crates-io]
 cxx = { git = "https://github.com/ridiculousfish/cxx", branch = "fish" }
 cxx-gen = { git = "https://github.com/ridiculousfish/cxx", branch = "fish" }

fish-rust/build.rs

@@ -21,6 +21,7 @@ fn main() -> miette::Result<()> {
     let source_files = vec![
         "src/fd_readable_set.rs",
         "src/ffi_init.rs",
+        "src/ffi_tests.rs",
         "src/smoke.rs",
         "src/topic_monitor.rs",
     ];

fish-rust/src/ffi_tests.rs

@@ -0,0 +1,63 @@
+/// Support for tests which need to cross the FFI.
+/// Because the C++ is not compiled by `cargo test` and there is no natural way to
+/// do it, use the following facilities for tests which need to use C++ types.
+/// This uses the inventory crate to build a custom-test harness
+/// as described at https://www.infinyon.com/blog/2021/04/rust-custom-test-harness/
+/// See smoke.rs add_test for an example of how to use this.
+
+#[cfg(all(feature = "fish-ffi-tests", not(test)))]
+mod ffi_tests_impl {
+    use inventory;
+
+    /// A test which needs to cross the FFI.
+    #[derive(Debug)]
+    pub struct FFITest {
+        pub name: &'static str,
+        pub func: fn(),
+    }
+
+    /// Add a new test.
+    /// Example usage:
+    /// ```
+    ///   add_test!("test_name", || {
+    ///       assert!(1 + 2 == 3);
+    ///   });
+    /// ```
+    macro_rules! add_test {
+        ($name:literal, $func:expr) => {
+            inventory::submit!(crate::ffi_tests::FFITest {
+                name: $name,
+                func: $func,
+            });
+        };
+    }
+    pub(crate) use add_test;
+
+    inventory::collect!(crate::ffi_tests::FFITest);
+
+    /// Runs all ffi tests.
+    pub fn run_ffi_tests() {
+        for test in inventory::iter::<crate::ffi_tests::FFITest> {
+            println!("Running ffi test {}", test.name);
+            (test.func)();
+        }
+    }
+}
+
+#[cfg(not(all(feature = "fish-ffi-tests", not(test))))]
+mod ffi_tests_impl {
+    macro_rules! add_test {
+        ($name:literal, $func:expr) => {};
+    }
+    pub(crate) use add_test;
+    pub fn run_ffi_tests() {}
+}
+
+pub(crate) use ffi_tests_impl::*;
+
+#[cxx::bridge(namespace = rust)]
+mod ffi_tests {
+    extern "Rust" {
+        fn run_ffi_tests();
+    }
+}

fish-rust/src/lib.rs

@@ -10,6 +10,7 @@ mod fd_readable_set;
 mod fds;
 mod ffi;
 mod ffi_init;
+mod ffi_tests;
 mod flog;
 mod signal;
 mod smoke;

fish-rust/src/smoke.rs

@@ -19,3 +19,8 @@ mod tests {
         assert_eq!(result, 4);
     }
 }
+
+use crate::ffi_tests::add_test;
+add_test!("test_add", || {
+    assert_eq!(add(2, 3), 5);
+});

src/fish_tests.cpp

@@ -63,6 +63,7 @@
 #include "fd_readable_set.rs.h"
 #include "fds.h"
 #include "ffi_init.rs.h"
+#include "ffi_tests.rs.h"
 #include "function.h"
 #include "future_feature_flags.h"
 #include "global_safety.h"
@@ -7148,6 +7149,8 @@ void test_rust_smoke() {
     do_test(x == 42);
 }
 
+void test_rust_ffi() { rust::run_ffi_tests(); }
+
 // typedef void (test_entry_point_t)();
 using test_entry_point_t = void (*)();
 struct test_t {
@@ -7269,6 +7272,7 @@ static const test_t s_tests[]{
     {TEST_GROUP("re"), test_re_substitute},
     {TEST_GROUP("wgetopt"), test_wgetopt},
     {TEST_GROUP("rust_smoke"), test_rust_smoke},
+    {TEST_GROUP("rust_ffi"), test_rust_ffi},
 };
 
 void list_tests() {