std: Use native `#[thread_local]` TLS on wasm

This commit moves `thread_local!` on WebAssembly targets to using the
`#[thread_local]` attribute in LLVM. This was recently implemented
upstream and is [in the process of being documented][dox]. This change
only takes affect if modules are compiled with `+atomics` which is
currently unstable and a pretty esoteric method of compiling wasm
artifacts.

This "new power" of the wasm toolchain means that the old
`wasm-bindgen-threads` feature of the standard library can be removed
since it should now be possible to create a fully functioning threaded
wasm module without intrusively dealing with libstd symbols or
intrinsics. Yay!

[dox]: https://github.com/WebAssembly/tool-conventions/pull/116

1 files changed, 9 insertions, 0 deletions

diff --git a/src/libstd/sys/wasm/fast_thread_local.rs b/src/libstd/sys/wasm/fast_thread_local.rs
new file mode 100644
index 00000000000..ff2198175f0
--- /dev/null
+++ b/src/libstd/sys/wasm/fast_thread_local.rs
@@ -0,0 +1,9 @@
+#![unstable(feature = "thread_local_internals", issue = "0")]
+
+pub unsafe fn register_dtor(_t: *mut u8, _dtor: unsafe extern fn(*mut u8)) {
+    // FIXME: right now there is no concept of "thread exit", but this is likely
+    // going to show up at some point in the form of an exported symbol that the
+    // wasm runtime is oging to be expected to call. For now we basically just
+    // ignore the arguments, but if such a function starts to exist it will
+    // likely look like the OSX implementation in `unix/fast_thread_local.rs`
+}