Fix WASI compilation for C++ (#1083)

* Add support for WASI cpp build

* Look for WASI_SDK & link libc++

* Fixes & change wasi sdk to sysroot

* Fix fmt issues

* wasi sysroot now return a pathbuf

* Replaced PathBuf to Arc

* Ensure no-exceptions is set only for wasm32-wasi

* Update src/lib.rs

---------

Co-authored-by: Jiahao XU <[email protected]>

Author: antaalt

src/lib.rs

@@ -1318,6 +1318,14 @@ impl Build {
                 self.cargo_output
                     .print_metadata(&format_args!("cargo:rustc-link-lib={}", stdlib));
             }
+            // Link c++ lib from WASI sysroot
+            if self.get_target()?.contains("wasi") {
+                let wasi_sysroot = self.wasi_sysroot()?;
+                self.cargo_output.print_metadata(&format_args!(
+                    "cargo:rustc-flags=-L {}/lib/wasm32-wasi -lstatic=c++ -lstatic=c++abi",
+                    wasi_sysroot
+                ));
+            }
         }
 
         let cudart = match &self.cudart {
@@ -1917,6 +1925,14 @@ impl Build {
                         cmd.push_cc_arg("-fno-plt".into());
                     }
                 }
+                if target == "wasm32-wasip1" {
+                    // WASI does not support exceptions yet.
+                    // https://github.com/WebAssembly/exception-handling
+                    cmd.push_cc_arg("-fno-exceptions".into());
+                    // Link clang sysroot
+                    let wasi_sysroot = self.wasi_sysroot()?;
+                    cmd.push_cc_arg(format!("--sysroot={}", wasi_sysroot).into());
+                }
             }
         }
 
@@ -2859,7 +2875,11 @@ impl Build {
                     || target == "wasm32-unknown-wasi"
                     || target == "wasm32-unknown-unknown"
                 {
-                    "clang".to_string()
+                    if self.cpp {
+                        "clang++".to_string()
+                    } else {
+                        "clang".to_string()
+                    }
                 } else if target.contains("vxworks") {
                     if self.cpp {
                         "wr-c++".to_string()
@@ -3099,6 +3119,7 @@ impl Build {
                         | target.contains("openbsd")
                         | target.contains("aix")
                         | target.contains("linux-ohos")
+                        | target.contains("-wasi")
                     {
                         Ok(Some("c++".to_string()))
                     } else if target.contains("android") {
@@ -3853,6 +3874,17 @@ impl Build {
         }
     }
 
+    fn wasi_sysroot(&self) -> Result<Arc<str>, Error> {
+        if let Some(wasi_sysroot_path) = self.getenv("WASI_SYSROOT") {
+            Ok(wasi_sysroot_path)
+        } else {
+            Err(Error::new(
+                ErrorKind::EnvVarNotFound,
+                "Environment variable WASI_SYSROOT not defined. Download sysroot from github & setup environment variable WASI_SYSROOT targetting the folder.",
+            ))
+        }
+    }
+
     fn cuda_file_count(&self) -> usize {
         self.files
             .iter()