Specify consistent c++ standard between cxx and cxx-test-suite

Cargo.toml

@@ -14,9 +14,10 @@ keywords = ["ffi"]
 categories = ["development-tools::ffi", "api-bindings"]
 
 [features]
-default = [] # c++11
-"c++14" = []
-"c++17" = []
+default = ["cxxbridge-flags/default"] # c++11
+"c++14" = ["cxxbridge-flags/c++14"]
+"c++17" = ["cxxbridge-flags/c++17"]
+"c++20" = ["cxxbridge-flags/c++20"]
 
 [dependencies]
 cxxbridge-macro = { version = "=0.3.6", path = "macro" }
@@ -24,6 +25,7 @@ link-cplusplus = "1.0"
 
 [build-dependencies]
 cc = "1.0.49"
+cxxbridge-flags = { version = "=0.3.6", path = "flags", default-features = false }
 
 [dev-dependencies]
 cxx-build = { version = "=0.3.6", path = "gen/build" }
@@ -32,7 +34,7 @@ rustversion = "1.0"
 trybuild = { version = "1.0.33", features = ["diff"] }
 
 [workspace]
-members = ["demo-rs", "gen/build", "gen/cmd", "macro", "tests/ffi"]
+members = ["demo-rs", "flags", "gen/build", "gen/cmd", "macro", "tests/ffi"]
 
 [package.metadata.docs.rs]
 targets = ["x86_64-unknown-linux-gnu"]

build.rs

@@ -3,13 +3,7 @@ fn main() {
         .file("src/cxx.cc")
         .cpp(true)
         .cpp_link_stdlib(None) // linked via link-cplusplus crate
-        .flag_if_supported(if cfg!(feature = "c++17") {
-            "-std=c++17"
-        } else if cfg!(feature = "c++14") {
-            "-std=c++14"
-        } else {
-            "-std=c++11"
-        })
+        .flag_if_supported(cxxbridge_flags::STD)
         .compile("cxxbridge03");
     println!("cargo:rerun-if-changed=src/cxx.cc");
     println!("cargo:rerun-if-changed=include/cxx.h");

flags/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "cxxbridge-flags"
+version = "0.3.6"
+authors = ["David Tolnay <dtolnay@gmail.com>"]
+edition = "2018"
+license = "MIT OR Apache-2.0"
+description = "Compiler configuration of the `cxx` crate (implementation detail)"
+repository = "https://github.com/dtolnay/cxx"
+
+[features]
+default = [] # c++11
+"c++14" = []
+"c++17" = []
+"c++20" = []
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]

flags/src/impl.rs

@@ -0,0 +1,20 @@
+#[allow(unused_assignments, unused_mut, unused_variables)]
+pub const STD: &str = {
+    let mut flags = ["-std=c++11", "/std:c++11"];
+
+    #[cfg(feature = "c++14")]
+    (flags = ["-std=c++14", "/std:c++14"]);
+
+    #[cfg(feature = "c++17")]
+    (flags = ["-std=c++17", "/std:c++17"]);
+
+    #[cfg(feature = "c++20")]
+    (flags = ["-std=c++20", "/std:c++20"]);
+
+    let [mut flag, msvc_flag] = flags;
+
+    #[cfg(target_env = "msvc")]
+    (flag = msvc_flag);
+
+    flag
+};

flags/src/lib.rs

@@ -0,0 +1,7 @@
+//! This crate is an implementation detail of the `cxx` and `cxx-build` crates,
+//! and does not expose any public API.
+
+mod r#impl;
+
+#[doc(hidden)]
+pub use r#impl::*;

tests/ffi/Cargo.toml

@@ -12,3 +12,4 @@ cxx = { path = "../.." }
 
 [build-dependencies]
 cxx-build = { path = "../../gen/build" }
+cxxbridge-flags = { path = "../../flags" }

tests/ffi/build.rs

@@ -6,6 +6,6 @@ fn main() {
     let sources = vec!["lib.rs", "module.rs"];
     cxx_build::bridges(sources)
         .file("tests.cc")
-        .flag_if_supported("-std=c++11")
+        .flag_if_supported(cxxbridge_flags::STD)
         .compile("cxx-test-suite");
 }

third-party/Cargo.lock

@@ -70,6 +70,7 @@ dependencies = [
  "cc",
  "cxx-build",
  "cxx-test-suite",
+ "cxxbridge-flags",
  "cxxbridge-macro",
  "link-cplusplus",
  "rustversion",
@@ -94,6 +95,7 @@ version = "0.0.0"
 dependencies = [
  "cxx",
  "cxx-build",
+ "cxxbridge-flags",
 ]
 
 [[package]]
@@ -116,6 +118,10 @@ dependencies = [
  "cxx-build",
 ]
 
+[[package]]
+name = "cxxbridge-flags"
+version = "0.3.6"
+
 [[package]]
 name = "cxxbridge-macro"
 version = "0.3.6"