Upgrade rust toolchain to 2023-09-07

`VarDebugInfoContents::Composite` no longer exists, and therefore had to
be removed. We hadn't fully implemented this case, so this is an
improvement.

Fixes: model-checking#2742

kani-compiler/src/codegen_cprover_gotoc/codegen/span.rs

@@ -42,18 +42,6 @@ impl<'tcx> GotocCtx<'tcx> {
  self.current_fn().mir().var_debug_info.iter().find(|info| match info.value {
  VarDebugInfoContents::Place(p) => p.local == *l && p.projection.len() == 0,
  VarDebugInfoContents::Const(_) => false,
- // This variant was added in
- // https://github.com/rust-lang/rust/pull/102570 and is concerned
- // with a scalar replacement of aggregates (SROA) MIR optimization
- // that is only enabled with `--mir-opt-level=3` or higher.
- // TODO: create a test and figure out if we should return debug info
- // for this case:
- // https://github.com/model-checking/kani/issues/1933
- VarDebugInfoContents::Composite { .. } => {
- // Fail in debug mode to determine if we ever hit this case
- debug_assert!(false, "Unhandled VarDebugInfoContents::Composite");
- false
- }
  })
  }
 }

rust-toolchain.toml

@@ -2,5 +2,5 @@
 # SPDX-License-Identifier: Apache-2.0 OR MIT
 
 [toolchain]
-channel = "nightly-2023-09-06"
+channel = "nightly-2023-09-07"
 components = ["llvm-tools-preview", "rustc-dev", "rust-src", "rustfmt"]