chore: update nightly to 2021-09-22 (#117)

* chore: update to nightly-2021-09-22, fix clippy (#115)

This just updates Rust to the latest nightly, and fixes a new Clippy
lint on that nightly.

* chore(x86_64): add `-z nostart-stop-gc` to linker args

when linking with `rust-lld` on recent nightlies, `__start` and
`__stop` labels are now gc'd by default. this breaks the linker hax
we use for tests. this argument makes `lld` stop doing that.

rust-toolchain.toml

@@ -1,5 +1,5 @@
 [toolchain]
-channel = "nightly-2021-06-05"
+channel = "nightly-2021-09-22"
 components = [
     "clippy",
     "rustfmt",

util/src/io/impls.rs

@@ -258,7 +258,7 @@ impl Write for &mut [u8] {
     #[inline]
     fn write(&mut self, data: &[u8]) -> io::Result<usize> {
         let amt = cmp::min(data.len(), self.len());
-        let (a, b) = mem::replace(self, &mut []).split_at_mut(amt);
+        let (a, b) = mem::take(self).split_at_mut(amt);
         a.copy_from_slice(&data[..amt]);
         *self = b;
         Ok(amt)

x86_64-mycelium.json

@@ -10,7 +10,13 @@
     "executables": true,
     "linker-flavor": "ld.lld",
     "linker": "rust-lld",
+    "post-link-args": {
+        "ld.lld": [
+            "-z",
+            "nostart-stop-gc"
+        ]
+    },
     "panic-strategy": "abort",
     "disable-redzone": true,
     "features": "-mmx,-sse,+soft-float"
-}
+}