Prepare for merging from rust-lang/rust

This updates the rust-version file to 2f3f27bf79ec147fec9d2e7980605307a74067f4.
diff --git a/rust-version b/rust-version
index f412399..df2bac8 100644
--- a/rust-version
+++ b/rust-version
@@ -1 +1 @@
-a1dbb443527bd126452875eb5d5860c1d001d761
+2f3f27bf79ec147fec9d2e7980605307a74067f4