refactor: fix canonicalize_refs in libxml transforms (#1450)

`canonicalize_refs` used `cx.hir_map().node_to_hir_id` and
unconditionally called `typeck(parent)`. When `MutVisitNodes` descends
into attribute tokens or items without bodies (e.g., `extern` decls) the
HIR map lacks a node or typeck tables, so the walk panicked with
`span_bug!("can't type-check body …")`.

This PR swaps to `opt_node_to_hir_id` so attribute-only nodes
short-circuit before we ever try to fetch HIR expressions, keeping the
transform aligned with what rustc actually lowers, and checks
`maybe_body_owned_by(parent)` before querying typeck tables; this skips
extern items and other definitions without bodies, avoiding the
`primary_body_of` unwrap panic while still running the transform over
real bodies.

Author: oinoom

c2rust-refactor/src/transform/canonicalize_refs.rs

@@ -15,9 +15,21 @@ struct CanonicalizeRefs;
 impl Transform for CanonicalizeRefs {
     fn transform(&self, krate: &mut Crate, _st: &CommandState, cx: &RefactorCtxt) {
         MutVisitNodes::visit(krate, |expr: &mut P<Expr>| {
-            let hir_id = cx.hir_map().node_to_hir_id(expr.id);
+            // `MutVisitNodes` walks into attributes even though rustc never lowers their bodies to HIR.
+            // We skip such nodes by probing with `opt_node_to_hir_id`; if we ever need an attribute-only
+            // walk we’d have to reimplement the traversal with an attribute-depth-aware visitor instead.
+            let Some(hir_id) = cx.hir_map().opt_node_to_hir_id(expr.id) else {
+                return;
+            };
             let hir_expr = cx.hir_map().expect_expr(hir_id);
             let parent = cx.hir_map().get_parent_item(hir_id);
+            if cx.hir_map().maybe_body_owned_by(parent).is_none() {
+                // `tcx.typeck(parent)` unwraps `primary_body_of` (rustc_typeck::check::mod.rs),
+                // so querying items without bodies (e.g. extern statics/functions) triggers a
+                // `span_bug!("can't type-check body ...")`. Skip any expressions owned by such
+                // items instead of poking the typeck tables.
+                return;
+            }
             let tables = cx.ty_ctxt().typeck(parent);
             for adjustment in tables.expr_adjustments(hir_expr) {
                 match adjustment.kind {