Use new thread-local const-init

Let's see if this gives us any speedup - some of the TLS state modified in this
commit *is* pretty heavily accessed, so we can hope!

compiler/rustc_middle/src/lib.rs

@@ -50,6 +50,7 @@
 #![feature(control_flow_enum)]
 #![feature(associated_type_defaults)]
 #![feature(iter_zip)]
+#![feature(thread_local_const_init)]
 #![recursion_limit = "512"]
 
 #[macro_use]

compiler/rustc_middle/src/ty/context.rs

@@ -1705,7 +1705,7 @@ pub mod tls {
     #[cfg(not(parallel_compiler))]
     thread_local! {
         /// A thread local variable that stores a pointer to the current `ImplicitCtxt`.
-        static TLV: Cell<usize> = Cell::new(0);
+        static TLV: Cell<usize> = const { Cell::new(0) };
     }
 
     /// Sets TLV to `value` during the call to `f`.

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -55,10 +55,10 @@ macro_rules! define_scoped_cx {
 }
 
 thread_local! {
-    static FORCE_IMPL_FILENAME_LINE: Cell<bool> = Cell::new(false);
-    static SHOULD_PREFIX_WITH_CRATE: Cell<bool> = Cell::new(false);
-    static NO_TRIMMED_PATH: Cell<bool> = Cell::new(false);
-    static NO_QUERIES: Cell<bool> = Cell::new(false);
+    static FORCE_IMPL_FILENAME_LINE: Cell<bool> = const { Cell::new(false) };
+    static SHOULD_PREFIX_WITH_CRATE: Cell<bool> = const { Cell::new(false) };
+    static NO_TRIMMED_PATH: Cell<bool> = const { Cell::new(false) };
+    static NO_QUERIES: Cell<bool> = const { Cell::new(false) };
 }
 
 /// Avoids running any queries during any prints that occur