[std=c++17] Builder flag_if_supported/flag ignored

[jwinarske]

Describe the bug
Using b.flag_if_supported("-std=c++17") has no impact, and the log output will still use -std=c++14. Same for b.flag("-std=c++17")

Snippet from RUST_LOG=autocxx_engine=info cargo build when using with b.flag_if_supported("-std=c++17"). Notice -std=c++14 in log:

  cargo:warning=MESSAGE:Bindgen flags would be: "--use-specific-virtual-function-receiver" "--cpp-semantic-attributes" "--represent-cxx-operators" "--use-distinct-char16-t" "--blocklist-item" "std::unique_ptr" "--blocklist-item" "std::vector" "--blocklist-item" "std::shared_ptr" "--blocklist-item" "std::weak_ptr" "--blocklist-item" "std::string" "--blocklist-item" "rust::Str" "--blocklist-item" "rust::String" "--blocklist-item" "rust::Box" "--allowlist-type" "mbgl::ffi::Context::Context" "--allowlist-type" "autocxx_make_string_0x87976c9e4c91ff15" "--allowlist-function" "mbgl::ffi::Context::Context" "--allowlist-function" "autocxx_make_string_0x87976c9e4c91ff15" "--allowlist-var" "mbgl::ffi::Context::Context" "--allowlist-var" "autocxx_make_string_0x87976c9e4c91ff15" "--default-enum-style" "rust" "--enable-cxx-namespaces" "--no-layout-tests" "--no-derive-copy" "--no-derive-debug" "--no-derive-default" "--generate" "functions,types,vars,methods,constructors,destructors" "--generate-inline-functions" "--rust-target" "1.68" "--respect-cxx-access-specs" "--" "-x" "c++" "-std=c++14" "-DBINDGEN"
... 

To Reproduce
Add .flag_if_supported("-std=c++17") to build.rs. Alternatively use .flag("-std=c++17")

Expected behavior
"-std=c++17" is applied to compiler invocation instead of "-std=c++14"

[jwinarske]

If I use

    let mut b = autocxx_build::Builder::new("src/lib.rs", &[...])
        .extra_clang_args(&["-std=c++17", "-Wc++17-extensions", "-Wunused-parameter"])
        .build()?;

    b.file("cxx/context.cpp")
        .compile("example");

it appends "-std=c++17" but "-std=c++14" is still present.

[jwinarske]

works fine.

[jwinarske]

Actually it doesn't work.

Setting c++ version using extra_clang_args ends up as the last command line option. Which won't affect any of the include files.

It looks like autocxx itself is dependent on 14, as changing engine/src/lib.rs from "-std=c++14" to "-std=c++17" in local folder (using relative path to point to autocxx and autocxx-build) ends up with a build error.

This is the error I get when I update both autocxx and autocxx-build to -std=c++17:

warning: In file included from /home/joel/work/app/maplibre-native/ffi/target/debug/build/maplibre_ffi-78ae8e6a3d2fea4a/out/autocxx-build-dir/cxx/gen0.cxx:2:
warning: /home/joel/work/app/maplibre-native/ffi/target/debug/build/maplibre_ffi-78ae8e6a3d2fea4a/out/autocxx-build-dir/include/autocxxgen_ffi.h:35:78: error: ‘::rust’ has not been declared
warning:    35 | inline std::unique_ptr<std::string> autocxx_make_string_0xef637c73314d0a8e(::rust::Str str) { return std::make_unique<std::string>(std::string(str)); }
warning:       |                                                                              ^~~~
warning: /home/joel/work/app/maplibre-native/ffi/target/debug/build/maplibre_ffi-78ae8e6a3d2fea4a/out/autocxx-build-dir/include/autocxxgen_ffi.h:35:93: error: expected ‘,’ or ‘;’ before ‘{’ token
warning:    35 | inline std::unique_ptr<std::string> autocxx_make_string_0xef637c73314d0a8e(::rust::Str str) { return std::make_unique<std::string>(std::string(str)); }
warning:       |                                                                                             ^
warning: /home/joel/work/app/maplibre-native/ffi/target/debug/build/maplibre_ffi-78ae8e6a3d2fea4a/out/autocxx-build-dir/cxx/gen0.cxx: In function ‘std::string* cxxbridge1$autocxx_make_string_0xef637c73314d0a8e(rust::cxxbridge1::Str)’:
warning: /home/joel/work/app/maplibre-native/ffi/target/debug/build/maplibre_ffi-78ae8e6a3d2fea4a/out/autocxx-build-dir/cxx/gen0.cxx:149:96: error: cannot convert ‘std::unique_ptr<std::__cxx11::basic_string<char> >’ to ‘std::unique_ptr<std::__cxx11::basic_string<char> > (*)(rust::cxxbridge1::Str)’ in initialization
warning:   149 |   ::std::unique_ptr<::std::string> (*autocxx_make_string_0xef637c73314d0a8e$)(::rust::Str) = ::autocxx_make_string_0xef637c73314d0a8e;
warning:       |                                                                                              ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
warning:       |                                                                                                |
warning:       |                                                                                                std::unique_ptr<std::__cxx11::basic_string<char> >

This is just a simple scenario of trying to use #include <optional> which is only available with >= -std=c++17

[adetaylor]

You're right to be using extra_clang_args.

Applying this diff to the code in demo seems to build OK for me:

diff --git a/demo/build.rs b/demo/build.rs
index 69c632c4..f91eae0d 100644
--- a/demo/build.rs
+++ b/demo/build.rs
@@ -8,8 +8,10 @@
 
 fn main() -> miette::Result<()> {
     let path = std::path::PathBuf::from("src");
-    let mut b = autocxx_build::Builder::new("src/main.rs", [&path]).build()?;
-    b.flag_if_supported("-std=c++14").compile("autocxx-demo");
+    let mut b = autocxx_build::Builder::new("src/main.rs", [&path])
+        .extra_clang_args(&["-std=c++17"])
+        .build()?;
+    b.flag_if_supported("-std=c++17").compile("autocxx-demo");
 
     println!("cargo:rerun-if-changed=src/main.rs");
     println!("cargo:rerun-if-changed=src/input.h");
diff --git a/demo/src/input.h b/demo/src/input.h
index b6cdd874..5cc93955 100644
--- a/demo/src/input.h
+++ b/demo/src/input.h
@@ -12,6 +12,7 @@
 #include <sstream>
 #include <stdint.h>
 #include <string>
+#include <optional>
 
 class Goat {
 public:

[adetaylor]

(At the very least the documentation needs to be improved here, so I'll keep this issue open. But I want to make sure you can resolve your problem first - can you try the above diff against the demo, and report back on whether it works for you?)

[jwinarske]

@adetaylor Using your suggest pattern works! The only combo I didn't try :)

This gets me past the issue:

    let mut b = autocxx_build::Builder::new("src/lib.rs", &[
        &path27, &path28, &path29, &path30,
        &path1, &path2,                          &path5, &path6, &path7, &path8, &path9, &path10,
        &path11, &path12, &path13, &path14, &path15, &path16, &path17, &path18, &path19, &path20,
        &path21, &path22, &path23, &path24, &path25
        ])
        .extra_clang_args(&["-std=c++17"])
        .build()?;

    b.flag_if_supported("-std=c++17").file("cxx/context.cpp")
        .compile("autocxx-maplibre-example");

Yes I agree the docs need to improve around this

[adetaylor]

OK great, thank you.

[adetaylor]

There was already a test for C++17 compatibility, so I just improved the documentation here. Thanks again for the report.