cxx: implement slice support for nostd

OVERVIEW:
This patch adds experimental support for us to use `slices` in a nostd/freestanding environment, aim is to run
c++ <=> cxx-rs <=> rust on a baremental target environment and use
slices, with the intention of supporting more features later.

In the application `Cargo.toml` file, the following feature cfgs are required:

```
[dependencies]
cxx = { ... , default-features = false }

[build-dependencies]
cxx-build = { ... , default-features = false }
```

Then build with:
```
RUSTFLAGS='--cfg cxx_experimental_no_alloc' cargo build
```

and in the particular `.cpp` file you may need to declare the following

```
void __assert_func (const char *__a, int, const char *__b, const char *__c) {
	while (true) {}
}

void *__gxx_personality_v0;
```

FUNCTIONALITY:
Currently, this only supports slices (outside of trivial features).

TESTING:
Tested by compiling:
	- cargo test (run existing tests)
	- cxx/demo/ running it.
	- compiling with a arm toolchain setup with cmake/corrosion
	  and running the binary on QEMU arm with basic logic assertions [1].

Current testing has been done in the direction of C++ -> Rust with a
simple callback test to C++. A
simple test setup can be seen here [2].

TODO:
- Get features such as `Results<Ok, Err>` working.

- When we build for the none `std` case (no `default-features`), instead of creating a symlink
     to the original `cxx.h` file, we copy it over and define the macro to
     disable stdlib dependencies. Perhaps there's a better way to do this?

NOTES:
By default, all the standard features are enabled gaurded by the
`#ifndef CXXBRIDGE1_RUST_STD`, so this **shoudn't** break anything.

[1] https://github.com/twilfredo/qemu_misc/tree/master/bm_arm
[2] https://github.com/twilfredo/qemu_misc/blob/master/bm_arm/main.cpp

Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>

build.rs

@@ -3,13 +3,26 @@ use std::path::Path;
 use std::process::Command;
 
 fn main() {
-    cc::Build::new()
-        .file("src/cxx.cc")
-        .cpp(true)
-        .cpp_link_stdlib(None) // linked via link-cplusplus crate
-        .flag_if_supported(cxxbridge_flags::STD)
-        .warnings_into_errors(cfg!(deny_warnings))
-        .compile("cxxbridge1");
+    // Building without alloc, for a nostd target.
+    if !cfg!(feature = "alloc") {
+        cc::Build::new()
+            .file("src/cxx.cc")
+            .cpp(true)
+            .cpp_link_stdlib(None) // linked via link-cplusplus crate
+            .flag_if_supported(cxxbridge_flags::STD)
+            .warnings_into_errors(cfg!(deny_warnings))
+            .define("CXXBRIDGE1_RUST_STD", None) // Exclude all dependencies on std
+            .compile("cxxbridge1");
+    } else {
+        // Building for STD with all features
+        cc::Build::new()
+            .file("src/cxx.cc")
+            .cpp(true)
+            .cpp_link_stdlib(None) // linked via link-cplusplus crate
+            .flag_if_supported(cxxbridge_flags::STD)
+            .warnings_into_errors(cfg!(deny_warnings))
+            .compile("cxxbridge1");
+    }
 
     println!("cargo:rerun-if-changed=src/cxx.cc");
     println!("cargo:rerun-if-changed=include/cxx.h");

gen/build/Cargo.toml

@@ -13,9 +13,11 @@ repository = "https://github.com/dtolnay/cxx"
 rust-version = "1.48"
 
 [features]
+default = ["std"]
 parallel = ["cc/parallel"]
 # incomplete features that are not covered by a compatibility guarantee:
 experimental-async-fn = []
+std = []
 
 [dependencies]
 cc = "1.0.49"

gen/build/src/lib.rs

@@ -385,12 +385,29 @@ fn make_include_dir(prj: &Project) -> Result<PathBuf> {
     let include_dir = prj.out_dir.join("cxxbridge").join("include");
     let cxx_h = include_dir.join("rust").join("cxx.h");
     let ref shared_cxx_h = prj.shared_dir.join("rust").join("cxx.h");
-    if let Some(ref original) = env::var_os("DEP_CXXBRIDGE1_HEADER") {
-        out::symlink_file(original, cxx_h)?;
-        out::symlink_file(original, shared_cxx_h)?;
-    } else {
-        out::write(shared_cxx_h, gen::include::HEADER.as_bytes())?;
+
+    if !cfg!(feature = "std") {
+        // For a nonstd target, specify `#define CXXBRIDGE1_RUST_STD` to a copy of "include/cxx.h" and
+        // don't create a symlink to the original header (which has all std features enabled).
+        // This copy can now be included by a C++ file
+        // compiled/linked with `-nostdlib`
+        let mut new = "// Allow building for a nostd target by excluding std dependencies\n#define CXXBRIDGE1_RUST_STD\n"
+            .to_string()
+            .as_bytes()
+            .to_vec();
+        let mut byte_vec = gen::include::HEADER.as_bytes().to_vec();
+        new.append(&mut byte_vec);
+        let slice: &[u8] = &new;
+        out::write(shared_cxx_h, slice)?;
         out::symlink_file(shared_cxx_h, cxx_h)?;
+    } else {
+        if let Some(ref original) = env::var_os("DEP_CXXBRIDGE1_HEADER") {
+            out::symlink_file(original, cxx_h)?;
+            out::symlink_file(original, shared_cxx_h)?;
+        } else {
+            out::write(shared_cxx_h, gen::include::HEADER.as_bytes())?;
+            out::symlink_file(shared_cxx_h, cxx_h)?;
+        }
     }
     Ok(include_dir)
 }

include/cxx.h

@@ -1,16 +1,18 @@
 #pragma once
-#include <algorithm>
 #include <array>
-#include <cassert>
 #include <cstddef>
 #include <cstdint>
-#include <exception>
 #include <initializer_list>
+#include <stdexcept>
+#include <string>
+
+#ifndef CXXBRIDGE1_RUST_STD
+#include <algorithm>
+#include <cassert>
+#include <exception>
 #include <iosfwd>
 #include <iterator>
 #include <new>
-#include <stdexcept>
-#include <string>
 #include <type_traits>
 #include <utility>
 #include <vector>
@@ -19,6 +21,7 @@
 #else
 #include <sys/types.h>
 #endif
+#endif // CXXBRIDGE1_RUST_STD
 
 namespace rust {
 inline namespace cxxbridge1 {
@@ -30,6 +33,7 @@ template <typename T>
 class impl;
 }
 
+#ifndef CXXBRIDGE1_RUST_STD
 #ifndef CXXBRIDGE1_RUST_STRING
 #define CXXBRIDGE1_RUST_STRING
 // https://cxx.rs/binding/string.html
@@ -151,6 +155,7 @@ class Str final {
   std::array<std::uintptr_t, 2> repr;
 };
 #endif // CXXBRIDGE1_RUST_STR
+#endif
 
 #ifndef CXXBRIDGE1_RUST_SLICE
 namespace detail {
@@ -412,8 +417,10 @@ using isize = ssize_t;
 #endif
 #endif // CXXBRIDGE1_RUST_ISIZE
 
+#ifndef CXXBRIDGE1_RUST_STD
 std::ostream &operator<<(std::ostream &, const String &);
 std::ostream &operator<<(std::ostream &, const Str &);
+#endif
 
 #ifndef CXXBRIDGE1_RUST_OPAQUE
 #define CXXBRIDGE1_RUST_OPAQUE
@@ -465,14 +472,17 @@ using f32 = float;
 using f64 = double;
 
 // Snake case aliases for use in code that uses this style for type names.
-using string = String;
-using str = Str;
 template <typename T>
 using slice = Slice<T>;
+#ifndef CXXBRIDGE1_RUST_STD
+using string = String;
+using str = Str;
 template <typename T>
 using box = Box<T>;
 template <typename T>
 using vec = Vec<T>;
+#endif
+
 using error = Error;
 template <typename Signature>
 using fn = Fn<Signature>;

src/cxx.cc

@@ -1,9 +1,12 @@
 #include "../include/cxx.h"
 #include <cstring>
+#ifndef CXXBRIDGE1_RUST_STD
 #include <iostream>
 #include <memory>
+#endif
 
 extern "C" {
+#ifndef CXXBRIDGE1_RUST_STD
 void cxxbridge1$cxx_string$init(std::string *s, const std::uint8_t *ptr,
                                 std::size_t len) noexcept {
   new (s) std::string(reinterpret_cast<const char *>(ptr), len);
@@ -62,6 +65,7 @@ bool cxxbridge1$str$from(rust::Str *self, const char *ptr,
                          std::size_t len) noexcept;
 const char *cxxbridge1$str$ptr(const rust::Str *self) noexcept;
 std::size_t cxxbridge1$str$len(const rust::Str *self) noexcept;
+#endif
 
 // rust::Slice
 void cxxbridge1$slice$new(void *self, const void *ptr,
@@ -73,6 +77,13 @@ std::size_t cxxbridge1$slice$len(const void *self) noexcept;
 namespace rust {
 inline namespace cxxbridge1 {
 
+template <typename T>
+static bool is_aligned(const void *ptr) noexcept {
+  auto iptr = reinterpret_cast<std::uintptr_t>(ptr);
+  return !(iptr % alignof(T));
+}
+
+#ifndef CXXBRIDGE1_RUST_STD
 template <typename Exception>
 void panic [[noreturn]] (const char *msg) {
 #if defined(RUST_CXX_NO_EXCEPTIONS)
@@ -85,12 +96,6 @@ void panic [[noreturn]] (const char *msg) {
 
 template void panic<std::out_of_range> [[noreturn]] (const char *msg);
 
-template <typename T>
-static bool is_aligned(const void *ptr) noexcept {
-  auto iptr = reinterpret_cast<std::uintptr_t>(ptr);
-  return !(iptr % alignof(T));
-}
-
 String::String() noexcept { cxxbridge1$string$new(this); }
 
 String::String(const String &other) noexcept {
@@ -377,6 +382,7 @@ std::ostream &operator<<(std::ostream &os, const Str &s) {
   os.write(s.data(), s.size());
   return os;
 }
+#endif
 
 void sliceInit(void *self, const void *ptr, std::size_t len) noexcept {
   cxxbridge1$slice$new(self, ptr, len);
@@ -406,11 +412,13 @@ static_assert(sizeof(rust::isize) == sizeof(std::intptr_t),
 static_assert(alignof(rust::isize) == alignof(std::intptr_t),
               "unsupported ssize_t alignment");
 
+#ifndef CXXBRIDGE1_RUST_STD
 static_assert(std::is_trivially_copy_constructible<Str>::value,
               "trivial Str(const Str &)");
 static_assert(std::is_trivially_copy_assignable<Str>::value,
               "trivial operator=(const Str &)");
 static_assert(std::is_trivially_destructible<Str>::value, "trivial ~Str()");
+#endif
 
 static_assert(
     std::is_trivially_copy_constructible<Slice<const std::uint8_t>>::value,
@@ -448,6 +456,7 @@ static_assert(!std::is_same<Vec<std::uint8_t>::const_iterator,
                             Vec<std::uint8_t>::iterator>::value,
               "Vec<T>::const_iterator != Vec<T>::iterator");
 
+#ifndef CXXBRIDGE1_RUST_STD
 static const char *errorCopy(const char *ptr, std::size_t len) {
   char *copy = new char[len];
   std::memcpy(copy, ptr, len);
@@ -494,6 +503,7 @@ Error &Error::operator=(Error &&other) &noexcept {
   other.len = 0;
   return *this;
 }
+#endif
 
 const char *Error::what() const noexcept { return this->msg; }
 
@@ -513,9 +523,11 @@ struct PtrLen final {
 };
 } // namespace repr
 
+#ifndef CXXBRIDGE1_RUST_STD
 extern "C" {
 repr::PtrLen cxxbridge1$exception(const char *, std::size_t len) noexcept;
 }
+#endif
 
 namespace detail {
 // On some platforms size_t is the same C++ type as one of the sized integer
@@ -530,7 +542,7 @@ using isize_if_unique =
     typename std::conditional<std::is_same<rust::isize, int64_t>::value ||
                                   std::is_same<rust::isize, int32_t>::value,
                               struct isize_ignore, rust::isize>::type;
-
+#ifndef CXXBRIDGE1_RUST_STD
 class Fail final {
   repr::PtrLen &throw$;
 
@@ -547,6 +559,7 @@ void Fail::operator()(const char *catch$) noexcept {
 void Fail::operator()(const std::string &catch$) noexcept {
   throw$ = cxxbridge1$exception(catch$.data(), catch$.length());
 }
+#endif
 } // namespace detail
 
 } // namespace cxxbridge1
@@ -559,6 +572,7 @@ void destroy(T *ptr) {
 }
 } // namespace
 
+#ifndef CXXBRIDGE1_RUST_STD
 extern "C" {
 void cxxbridge1$unique_ptr$std$string$null(
     std::unique_ptr<std::string> *ptr) noexcept {
@@ -790,3 +804,4 @@ inline namespace cxxbridge1 {
 FOR_EACH_RUST_VEC(RUST_VEC_OPS)
 } // namespace cxxbridge1
 } // namespace rust
+#endif // CXXBRIDGE1_RUST_STD