Switch to Cargo's -Zbuild-std for building libstd (#1796)

This commit switches away from `xargo` to using `-Zbuild-std` to
building the standard library for the raytrace-parallel example (which
needs to rebuild std with new target features).

azure-pipelines.yml

@@ -211,13 +211,11 @@ jobs:
     steps:
       - template: ci/azure-install-rust.yml
         parameters:
-          toolchain: nightly-2019-08-27
+          toolchain: nightly-2019-09-26
       # Temporarily disable sccache because it is failing on CI.
       # - template: ci/azure-install-sccache.yml
       - script: rustup component add rust-src
         displayName: "install rust-src"
-      - script: cargo install xargo
-        displayName: "install xargo"
       - script: |
           set -e
           sed -i 's/python/#python/' examples/raytrace-parallel/build.sh

examples/raytrace-parallel/Xargo.toml

@@ -1,2 +0,0 @@
-[dependencies.std]
-stage = 0

examples/raytrace-parallel/build.sh

@@ -2,16 +2,24 @@
 
 set -ex
 
-# Two critical steps are required here to get this working:
+# A few steps are necessary to get this build working which makes it slightly
+# nonstandard compared to most other builds.
 #
-# * First, the Rust standard library needs to be compiled. The default version
+# * First, the Rust standard library needs to be recompiled with atomics
-#   is not compatible with atomics so we need to compile a version, with xargo,
+#   enabled. to do that we use Cargo's unstable `-Zbuild-std` feature.
-#   that is compatible.
 #
-# * Next we need to compile everything with the `atomics` feature enabled,
+# * Next we need to compile everything with the `atomics` and `bulk-memory`
-#   ensuring that LLVM will generate atomic instructions and such.
+#   features enabled, ensuring that LLVM will generate atomic instructions,
+#   shared memory, passive segments, etc.
+#
+# * Finally, `-Zbuild-std` is still in development, and one of its downsides
+#   right now is rust-lang/wg-cargo-std-aware#47 where using `rust-lld` doesn't
+#   work by default, which the wasm target uses. To work around that we find it
+#   and put it in PATH
+
+PATH=$PATH:$(dirname $(find $(rustc --print sysroot) -name 'rust-lld')) \
   RUSTFLAGS='-C target-feature=+atomics,+bulk-memory' \
-  xargo build --target wasm32-unknown-unknown --release
+  cargo build --target wasm32-unknown-unknown --release -Z build-std -Z timings=html
 
 # Note the usage of `--no-modules` here which is used to create an output which
 # is usable from Web Workers. We notably can't use `--target bundler` since