OpenBSD under x86 has particular ABI for returning a struct.

It is like OSX or Windows: small structs are returned as integers.

src/librustc_back/target/mod.rs

@@ -302,6 +302,9 @@ pub struct TargetOptions {
     pub staticlib_suffix: String,
     /// OS family to use for conditional compilation. Valid options: "unix", "windows".
     pub target_family: Option<String>,
+    /// Whether the target toolchain is like OpenBSD's.
+    /// Only useful for compiling against OpenBSD, for configuring abi when returning a struct.
+    pub is_like_openbsd: bool,
     /// Whether the target toolchain is like OSX's. Only useful for compiling against iOS/OS X, in
     /// particular running dsymutil and some other stuff like `-dead_strip`. Defaults to false.
     pub is_like_osx: bool,
@@ -406,6 +409,7 @@ impl Default for TargetOptions {
             staticlib_prefix: "lib".to_string(),
             staticlib_suffix: ".a".to_string(),
             target_family: None,
+            is_like_openbsd: false,
             is_like_osx: false,
             is_like_solaris: false,
             is_like_windows: false,
@@ -572,6 +576,7 @@ impl Target {
         key!(staticlib_prefix);
         key!(staticlib_suffix);
         key!(target_family, optional);
+        key!(is_like_openbsd, bool);
         key!(is_like_osx, bool);
         key!(is_like_solaris, bool);
         key!(is_like_windows, bool);
@@ -733,6 +738,7 @@ impl ToJson for Target {
         target_option_val!(staticlib_prefix);
         target_option_val!(staticlib_suffix);
         target_option_val!(target_family);
+        target_option_val!(is_like_openbsd);
         target_option_val!(is_like_osx);
         target_option_val!(is_like_solaris);
         target_option_val!(is_like_windows);

src/librustc_back/target/openbsd_base.rs

@@ -17,6 +17,7 @@ pub fn opts() -> TargetOptions {
         executables: true,
         linker_is_gnu: true,
         has_rpath: true,
+        is_like_openbsd: true,
         pre_link_args: vec![
             // GNU-style linkers will use this to omit linking to libraries
             // which don't actually fulfill any relocations, but only for

src/librustc_trans/cabi_x86.rs

@@ -25,7 +25,8 @@ pub fn compute_abi_info(ccx: &CrateContext, fty: &mut FnType) {
             // http://www.angelcode.com/dev/callconv/callconv.html
             // Clang's ABI handling is in lib/CodeGen/TargetInfo.cpp
             let t = &ccx.sess().target.target;
-            if t.options.is_like_osx || t.options.is_like_windows {
+            if t.options.is_like_osx || t.options.is_like_windows
+                || t.options.is_like_openbsd {
                 match llsize_of_alloc(ccx, fty.ret.ty) {
                     1 => fty.ret.cast = Some(Type::i8(ccx)),
                     2 => fty.ret.cast = Some(Type::i16(ccx)),