Update rust-toolchain

Also update the rustfmt option fn_args_density to fn_args_layout. See
rust-lang/rustfmt#3581.

type_alias_enum_variants is apparently now in stable, so there's no
need to require it as a feature.

Author: nickdrozd

rust-toolchain

@@ -1 +1 @@
-nightly-2019-05-01
+nightly-2019-07-18

rust_src/rustfmt.toml

@@ -12,7 +12,7 @@ disable_all_formatting = false
 empty_item_single_line = true
 error_on_line_overflow = true
 error_on_unformatted = false
-fn_args_density = "Tall"
+fn_args_layout = "Tall"
 fn_single_line = false
 force_explicit_abi = true
 force_multiline_blocks = false

rust_src/src/lib.rs

@@ -17,7 +17,6 @@
 #![feature(slice_patterns)]
 #![feature(specialization)]
 #![feature(stmt_expr_attributes)]
-#![feature(type_alias_enum_variants)]
 #![feature(untagged_unions)]
 
 extern crate errno;