Bug 1177599 - always pass --target to rustc; r=mshal

rustc, unlike our typical C++ compilers, can target multiple platforms
with ease through its use of the --target flag.  To support
cross-compiling, we just need to pass the appropriate --target option.

rustc uses specific names for its accepted --target option, however, and
they are slightly different from the values we get out of autoconf.  So
in addition to checking whether rustc can accept --target for our chosen
platform, we also need to munge autoconf's idea of the target into
something rustc understands.

build/autoconf/rust.m4

@@ -32,5 +32,87 @@ AC_DEFUN([MOZ_RUST_SUPPORT], [
       first in your path.
       You can verify this by typing 'rustc --version'.])
   fi
+
+  if test -n "$RUSTC" -a -n "$MOZ_RUST"; then
+    # Rust's --target options are similar to, but not exactly the same
+    # as, the autoconf-derived targets we use.  An example would be that
+    # Rust uses distinct target triples for targetting the GNU C++ ABI
+    # and the MSVC C++ ABI on Win32, whereas autoconf has a single
+    # triple and relies on the user to ensure that everything is
+    # compiled for the appropriate ABI.  We need to perform appropriate
+    # munging to get the correct option to rustc.
+    #
+    # The canonical list of targets supported can be derived from:
+    #
+    # https://github.com/rust-lang/rust/tree/master/mk/cfg
+    rust_target=
+    case "$target" in
+      # Linux
+      i*86*linux-gnu)
+          rust_target=i686-unknown-linux-gnu
+          ;;
+      x86_64*linux-gnu)
+          rust_target=x86_64-unknown-linux-gnu
+          ;;
+
+      # OS X and iOS
+      i*86-apple-darwin*)
+          rust_target=i686-apple-darwin
+          ;;
+      i*86-apple-ios*)
+          rust_target=i386-apple-ios
+          ;;
+      x86_64-apple-darwin*)
+          rust_target=x86_64-apple-darwin
+          ;;
+
+      # Android
+      i*86*linux-android)
+          rust_target=i686-linux-android
+          ;;
+      arm*linux-android*)
+          rust_target=arm-linux-androideabi
+          ;;
+
+      # Windows
+      i*86-pc-mingw32)
+          # XXX better detection of CXX needed here, to figure out whether
+          # we need i686-pc-windows-gnu instead, since mingw32 builds work.
+          rust_target=i686-pc-windows-msvc
+          ;;
+      x86_64-pc-mingw32)
+          # XXX and here as well
+          rust_target=x86_64-pc-windows-msvc
+          ;;
+      *)
+          AC_ERROR([don't know how to translate $target for rustc])
+    esac
+
+    # Check to see whether we need to pass --target to RUSTC.  This can
+    # happen when building Firefox on Windows: js's configure will receive
+    # a RUSTC from the toplevel configure that already has --target added to
+    # it.
+    rustc_target_arg=
+    case "$RUSTC" in
+      *--target=${rust_target}*)
+        ;;
+      *)
+        rustc_target_arg=--target=${rust_target}
+        ;;
+    esac
+
+    # Check to see whether our rustc has a reasonably functional stdlib
+    # for our chosen target.
+    echo 'pub extern fn hello() { println!("Hello world"); }' > conftest.rs
+    if AC_TRY_COMMAND(${RUSTC} --crate-type staticlib ${rustc_target_arg} -o conftest.rlib conftest.rs > /dev/null) && test -s conftest.rlib; then
+      RUSTC="${RUSTC} ${rustc_target_arg}"
+    else
+      AC_ERROR([cannot compile for ${rust_target} with ${RUSTC}])
+    fi
+    rm -f conftest.rs conftest.rlib
+  fi
+
+  # TODO: separate HOST_RUSTC and RUSTC variables
+
   AC_SUBST(MOZ_RUST)
 ])

configure.in

@@ -1637,7 +1637,6 @@ dnl ========================================================
 if test -n "$MACOSX_DEPLOYMENT_TARGET" -a -n "$MOZ_RUST"; then
   AC_MSG_CHECKING([if we're targeting 32-bit])
   if test -z "$HAVE_64BIT_BUILD"; then
-    RUSTC="$RUSTC --target=i686-apple-darwin"
     AC_MSG_RESULT([using $RUSTC])
   else
     AC_MSG_RESULT([no])