Fix dirent to match WASI libc's definition.

dirent contains a flexible array member, so don't test its sizeof, don't
allow it to be copied, and don't represent it with an artificial size.

Author: sunfishcode

libc-test/build.rs

@@ -1931,5 +1931,9 @@ fn test_wasi(target: &str) {
     // import the same thing but have different function pointers
     cfg.skip_fn_ptrcheck(|f| f.starts_with("__wasi"));
 
+    // d_name is declared as a flexible array in WASI libc, so it
+    // doesn't support sizeof.
+    cfg.skip_field(|s, field| s == "dirent" && field == "d_name");
+
     cfg.generate("../src/lib.rs", "main.rs");
 }

src/wasi.rs

@@ -141,12 +141,6 @@ s! {
         fds_bits: [c_ulong; FD_SETSIZE / ULONG_SIZE],
     }
 
-    pub struct dirent {
-        pub d_ino: ino_t,
-        pub d_type: c_uchar,
-        pub d_name: [c_char; 1024],
-    }
-
     pub struct lconv {
         pub decimal_point: *mut c_char,
         pub thousands_sep: *mut c_char,
@@ -303,6 +297,20 @@ s_no_extra_traits! {
 
 }
 
+// Declare dirent outside of s! so that it doesn't implement Copy, Eq, Hash,
+// etc., since it contains a flexible array member with a dynamic size.
+#[repr(C)]
+#[allow(missing_copy_implementations)]
+#[cfg_attr(feature = "extra_traits", derive(Debug))]
+pub struct dirent {
+    pub d_ino: ino_t,
+    pub d_type: c_uchar,
+    /// d_name is declared in WASI libc as a flexible array member, which
+    /// can't be directly expressed in Rust. As an imperfect workaround,
+    /// declare it as a zero-length array instead.
+    pub d_name: [c_char; 0],
+}
+
 // intentionally not public, only used for fd_set
 cfg_if! {
     if #[cfg(target_pointer_width = "32")] {