Add comments, and hopefully fix iOS cross-build

.cirrus.yml

@@ -8,9 +8,6 @@ cargo_cache:
 # the system's binaries, so the environment shouldn't matter.
 task:
   name: FreeBSD amd64 & i686
-  env:
-    # Temporary workaround for https://github.com/rust-lang/rustup/issues/2774
-    RUSTUP_IO_THREADS: 1
   freebsd_instance:
     image: freebsd-11-4-release-amd64
   setup_script:
@@ -60,7 +57,6 @@ task:
         TARGET: arm-unknown-linux-gnueabi
     - name: Linux armv7 gnueabihf
       env:
-        RUST_BACKTRACE: 1 # XXX temporary for debugging
         TARGET: armv7-unknown-linux-gnueabihf
     - name: Linux arm-musleabi
       env:
@@ -101,7 +97,8 @@ task:
   script:
     - . $HOME/.cargo/env || true
     - cross build --target $TARGET
-    - cross test --target $TARGET
+    - env RUST_BACKTRACE=1 cross test --target $TARGET
+    #- cross test --target $TARGET
   before_cache_script: rm -rf $CARGO_HOME/registry/index
 
 # Tasks for Linux amd64 builds
@@ -168,16 +165,22 @@ task:
     - name: iOS aarch64
       env:
         TARGET: aarch64-apple-ios
+        # Rustup only supports cross-building from arbitrary hosts for iOS at
+        # 1.49.0 and above.  Below that it's possible to cross-build from an OSX
+        # host, but OSX VMs
+        # are more expensive than Linux VMs.
+        TOOLCHAIN: 1.49.0
     - name: iOS x86_64
       env:
         TARGET: x86_64-apple-ios
+        TOOLCHAIN: 1.49.0
     # Cross testing on powerpc fails with "undefined reference to renameat2".
     # Perhaps cross is using too-old a version?
     - name: Linux powerpc
       env:
         TARGET: powerpc-unknown-linux-gnu
-    # Cross claims to support Linux powerpc64, but the tests fail with "file in
-    # wrong format"
+    # Cross claims to support Linux powerpc64, but it really doesn't.
+    # https://github.com/rust-embedded/cross/issues/441
     - name: Linux powerpc64
       env:
         TARGET: powerpc64-unknown-linux-gnu