Description of changes:

Update rust toolchain to nightly-2023-09-19.

Source changes required by the following upstream commits:

Propagate changes from rust-lang/rust#115748

Resolved issues:

n/a

Related RFC:

n/a

Call-outs:

n/a

Testing:

• How is this change tested? CI

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• n/a Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.