Reinstate code that makes sure the CompiledTrace isn't dropped.

Also add comments to make clear what's going on here.

ykrt/src/mt.rs

@@ -10,7 +10,7 @@ use std::{
     },
     thread::{self, JoinHandle},
 };
-use ykcompile::TraceCompiler;
+use ykcompile::{CompiledTrace, TraceCompiler};
 use yktrace::{sir::SIR, start_tracing, tir::TirTrace, ThreadTracer, TracingKind};
 
 pub type HotThreshold = usize;
@@ -305,7 +305,13 @@ impl MTThread {
                 }
                 PHASE_COMPILED => {
                     let p = (lp & !PHASE_TAG) as *const ();
-                    let f = unsafe { mem::transmute::<_, fn(&mut I) -> bool>(p) };
+                    // Retrieve the CompiledTrace to gain access to the memory containing the
+                    // trace.
+                    let bct = unsafe { Box::from_raw(p as *mut CompiledTrace<I>) };
+                    let f = unsafe { mem::transmute::<_, fn(&mut I) -> bool>(bct.ptr()) };
+                    // Forget the CompiledTrace again to make sure it isn't dropped before we had a
+                    // chance to execute it.
+                    mem::forget(bct);
                     return Some(f);
                 }
                 _ => unreachable!(),