Auto merge of #28420 - dominiccooney:rlsnightly, r=jdm

Update toolchain to nightly-2021-05-18

The nightly-2021-03-12 toolchain doesn't have RLS on Windows. This
breaks code completion in Visual Studio Code because the rust plugin
picks up the project toolchain.

RLS is available in all tier one platforms in nightly-2021-05-18 per:
https://rust-lang.github.io/rustup-components-history/

Signed-off-by: Dominic Cooney <dominic.cooney@gmail.com>

<!-- Please describe your changes on the following line: -->
Update toolchain to nightly-2021-05-18

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X ] `./mach build -d` does not report any errors
- [ X] `./mach test-tidy` does not report any errors (note, except tabs/spaces issue in an unrelated file... happy to fix in this PR)
- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because changing the toolchain, covered by existing tests

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components/script/lib.rs

@@ -2,9 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-#![feature(assoc_char_funcs)]
-#![feature(const_fn)]
 #![feature(const_fn_fn_ptr_basics)]
+#![feature(const_fn_trait_bound)]
 #![feature(core_intrinsics)]
 #![feature(drain_filter)]
 #![feature(plugin)]