Update the rust toolchain to 2023-12-20 (#2961)

Fixes needed due to renaming of a few items:
 - rust-lang/rust#119063

kani-compiler/src/session.rs

@@ -9,7 +9,7 @@ use rustc_errors::{
     ColorConfig, Diagnostic, TerminalUrl,
 };
 use rustc_session::config::ErrorOutputType;
-use rustc_session::EarlyErrorHandler;
+use rustc_session::EarlyDiagCtxt;
 use std::io::IsTerminal;
 use std::panic;
 use std::sync::LazyLock;
@@ -71,7 +71,7 @@ pub fn init_session(args: &Arguments, json_hook: bool) {
     // Initialize the rustc logger using value from RUSTC_LOG. We keep the log control separate
     // because we cannot control the RUSTC log format unless if we match the exact tracing
     // version used by RUSTC.
-    let handler = EarlyErrorHandler::new(ErrorOutputType::default());
+    let handler = EarlyDiagCtxt::new(ErrorOutputType::default());
     rustc_driver::init_rustc_env_logger(&handler);
 
     // Install Kani panic hook.

rust-toolchain.toml

@@ -2,5 +2,5 @@
 # SPDX-License-Identifier: Apache-2.0 OR MIT
 
 [toolchain]
-channel = "nightly-2023-12-18"
+channel = "nightly-2023-12-20"
 components = ["llvm-tools-preview", "rustc-dev", "rust-src", "rustfmt"]

tools/bookrunner/librustdoc/doctest.rs

@@ -66,7 +66,7 @@ pub fn make_test(
     let result = rustc_driver::catch_fatal_errors(|| {
         rustc_span::create_session_if_not_set_then(edition, |_| {
             use rustc_errors::emitter::{Emitter, EmitterWriter};
-            use rustc_errors::Handler;
+            use rustc_errors::DiagCtxt;
             use rustc_parse::maybe_new_parser_from_source_str;
             use rustc_parse::parser::ForceCollect;
             use rustc_session::parse::ParseSess;
@@ -88,8 +88,8 @@ pub fn make_test(
             let emitter = EmitterWriter::new(Box::new(io::sink()), fallback_bundle);
 
             // FIXME(misdreavus): pass `-Z treat-err-as-bug` to the doctest parser
-            let handler = Handler::with_emitter(Box::new(emitter));
-            let sess = ParseSess::with_span_handler(handler, sm);
+            let handler = DiagCtxt::with_emitter(Box::new(emitter));
+            let sess = ParseSess::with_dcx(handler, sm);
 
             let mut found_main = false;
             let mut found_extern_crate = crate_name.is_none();
@@ -152,7 +152,7 @@ pub fn make_test(
             // handler. Any errors in the tests will be reported when the test file is compiled,
             // Note that we still need to cancel the errors above otherwise `DiagnosticBuilder`
             // will panic on drop.
-            sess.span_diagnostic.reset_err_count();
+            sess.dcx.reset_err_count();
 
             (found_main, found_extern_crate, found_macro)
         })