Add a couple of comments

Co-Authored-By: Ralf Jung <post@ralfj.de>

Author: bjorn3

src/tools/miri/src/shims/foreign_items.rs

@@ -51,6 +51,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         // Some shims forward to other MIR bodies.
         match link_name.as_str() {
+            // This allocator function has forwarding shims synthesized during normal codegen
+            // (see `allocator_shim_contents`); this is where we emulate that behavior.
             // FIXME should use global_fn_name, but mangle_internal_symbol requires a static str.
             name if name == this.mangle_internal_symbol("__rust_alloc_error_handler") => {
                 // Forward to the right symbol that implements this function.
@@ -60,14 +62,16 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                         "`__rust_alloc_error_handler` cannot be called when no alloc error handler is set"
                     );
                 };
-                if handler_kind == AllocatorKind::Global {
+                if handler_kind == AllocatorKind::Default {
                     let name =
                         Symbol::intern(this.mangle_internal_symbol("__rdl_alloc_error_handler"));
                     let handler = this
                         .lookup_exported_symbol(name)?
                         .expect("missing alloc error handler symbol");
                     return interp_ok(Some(handler));
                 }
+                // Fall through to the `lookup_exported_symbol` below which should find
+                // a `__rust_alloc_error_handler`.
             }
             _ => {}
         }