Add alloc feature behind feature gate

BUCK

@@ -1,6 +1,10 @@
 rust_library(
     name = "cxx",
     srcs = glob(["src/**"]),
+    features = [
+        "alloc",
+        "std",
+    ],
     visibility = ["PUBLIC"],
     deps = [
         ":core",

BUILD

@@ -4,6 +4,10 @@ load("@rules_rust//rust:defs.bzl", "rust_binary", "rust_library", "rust_proc_mac
 rust_library(
     name = "cxx",
     srcs = glob(["src/**/*.rs"]),
+    crate_features = [
+        "alloc",
+        "std",
+    ],
     proc_macro_deps = [
         ":cxxbridge-macro",
     ],

Cargo.toml

@@ -20,7 +20,8 @@ default = ["std", "cxxbridge-flags/default"] # c++11
 "c++14" = ["cxxbridge-flags/c++14"]
 "c++17" = ["cxxbridge-flags/c++17"]
 "c++20" = ["cxxbridge-flags/c++20"]
-std = []
+alloc = []
+std = ["alloc"]
 
 [dependencies]
 cxxbridge-macro = { version = "=1.0.57", path = "macro" }

src/c_char.rs

@@ -1,3 +1,5 @@
+#![cfg(feature = "alloc")]
+
 pub type c_char = c_char_definition::c_char;
 
 // Validate that our definition is consistent with libstd's definition, without

src/cxx_string.rs

@@ -1,7 +1,10 @@
 use crate::actually_private::Private;
+#[cfg(feature = "alloc")]
 use alloc::borrow::Cow;
+#[cfg(feature = "alloc")]
 use alloc::string::String;
 use core::cmp::Ordering;
+#[cfg(feature = "alloc")]
 use core::fmt::{self, Debug, Display};
 use core::hash::{Hash, Hasher};
 use core::marker::{PhantomData, PhantomPinned};
@@ -144,6 +147,7 @@ impl CxxString {
     /// Cow::Owned String.
     ///
     /// [replacement character]: https://doc.rust-lang.org/std/char/constant.REPLACEMENT_CHARACTER.html
+    #[cfg(feature = "alloc")]
     pub fn to_string_lossy(&self) -> Cow<str> {
         String::from_utf8_lossy(self.as_bytes())
     }
@@ -202,12 +206,14 @@ impl CxxString {
     }
 }
 
+#[cfg(feature = "alloc")]
 impl Display for CxxString {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         Display::fmt(self.to_string_lossy().as_ref(), f)
     }
 }
 
+#[cfg(feature = "alloc")]
 impl Debug for CxxString {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         Debug::fmt(self.to_string_lossy().as_ref(), f)

src/exception.rs

@@ -1,3 +1,5 @@
+#![cfg(feature = "alloc")]
+
 use alloc::boxed::Box;
 use core::fmt::{self, Display};
 

src/extern_type.rs

@@ -1,5 +1,6 @@
 use self::kind::{Kind, Opaque, Trivial};
 use crate::CxxString;
+#[cfg(feature = "alloc")]
 use alloc::string::String;
 
 /// A type for which the layout is determined by its C++ definition.
@@ -212,8 +213,13 @@ impl_extern_type! {
     isize = "rust::isize"
     f32 = "float"
     f64 = "double"
-    String = "rust::String"
 
     [Opaque]
     CxxString = "std::string"
 }
+
+#[cfg(feature = "alloc")]
+impl_extern_type! {
+    [Trivial]
+    String = "rust::String"
+}

src/lib.rs

@@ -395,9 +395,14 @@
 #[cfg(built_with_cargo)]
 extern crate link_cplusplus;
 
-extern crate alloc;
 extern crate self as cxx;
 
+#[cfg(feature = "alloc")]
+extern crate alloc;
+
+#[cfg(not(feature = "alloc"))]
+extern crate core as alloc;
+
 #[cfg(feature = "std")]
 extern crate std;
 
@@ -406,6 +411,11 @@ extern crate std;
 #[cfg(not(feature = "std"))]
 extern crate core as std;
 
+#[cfg(not(any(feature = "alloc", cxx_experimental_no_alloc)))]
+compile_error! {
+    r#"cxx support for no_alloc is incomplete and semver exempt; you must build with at least one of feature="std", feature="alloc", or RUSTFLAGS='--cfg cxx_experimental_no_alloc'"#
+}
+
 #[macro_use]
 mod macros;
 
@@ -434,6 +444,7 @@ pub mod vector;
 mod weak_ptr;
 
 pub use crate::cxx_vector::CxxVector;
+#[cfg(feature = "alloc")]
 pub use crate::exception::Exception;
 pub use crate::extern_type::{kind, ExternType};
 pub use crate::shared_ptr::SharedPtr;
@@ -463,11 +474,14 @@ pub mod private {
     pub use crate::extern_type::{verify_extern_kind, verify_extern_type};
     pub use crate::function::FatFunction;
     pub use crate::opaque::Opaque;
+    #[cfg(feature = "alloc")]
     pub use crate::result::{r#try, Result};
     pub use crate::rust_slice::RustSlice;
     pub use crate::rust_str::RustStr;
+    #[cfg(feature = "alloc")]
     pub use crate::rust_string::RustString;
     pub use crate::rust_type::{ImplBox, ImplVec, RustType};
+    #[cfg(feature = "alloc")]
     pub use crate::rust_vec::RustVec;
     pub use crate::shared_ptr::SharedPtrTarget;
     pub use crate::string::StackString;

src/result.rs

@@ -1,3 +1,4 @@
+#![cfg(feature = "alloc")]
 #![allow(missing_docs)]
 
 use crate::exception::Exception;

src/rust_string.rs

@@ -1,3 +1,4 @@
+#![cfg(feature = "alloc")]
 #![allow(missing_docs)]
 
 use alloc::string::String;

src/rust_vec.rs

@@ -1,3 +1,4 @@
+#![cfg(feature = "alloc")]
 #![allow(missing_docs)]
 
 use crate::rust_string::RustString;

src/symbols/exception.rs

@@ -1,3 +1,5 @@
+#![cfg(feature = "alloc")]
+
 use alloc::boxed::Box;
 use alloc::string::String;
 use core::slice;

src/symbols/rust_str.rs

@@ -1,3 +1,4 @@
+#[cfg(feature = "alloc")]
 use alloc::string::String;
 use core::mem::MaybeUninit;
 use core::ptr;
@@ -10,6 +11,7 @@ unsafe extern "C" fn str_new(this: &mut MaybeUninit<&str>) {
     unsafe { ptr::write(this, "") }
 }
 
+#[cfg(feature = "alloc")]
 #[export_name = "cxxbridge1$str$ref"]
 unsafe extern "C" fn str_ref<'a>(this: &mut MaybeUninit<&'a str>, string: &'a String) {
     let this = this.as_mut_ptr();

src/symbols/rust_string.rs

@@ -1,3 +1,5 @@
+#![cfg(feature = "alloc")]
+
 use alloc::borrow::ToOwned;
 use alloc::string::String;
 use core::mem::{ManuallyDrop, MaybeUninit};

src/symbols/rust_vec.rs

@@ -1,3 +1,5 @@
+#![cfg(feature = "alloc")]
+
 use crate::c_char::c_char;
 use crate::rust_string::RustString;
 use crate::rust_vec::RustVec;