output tree representation for thir-tree

compiler/rustc_mir_build/src/build/mod.rs

@@ -439,6 +439,10 @@ fn construct_fn<'tcx>(
     let fn_id = tcx.hir().local_def_id_to_hir_id(fn_def.did);
     let generator_kind = tcx.generator_kind(fn_def.did);
 
+    // The representation of thir for `-Zunpretty=thir-tree` relies on
+    // the entry expression being the last element of `thir.exprs`.
+    assert_eq!(expr.as_usize(), thir.exprs.len() - 1);
+
     // Figure out what primary body this item has.
     let body_id = tcx.hir().body_owned_by(fn_def.did);
     let span_with_body = tcx.hir().span_with_body(fn_id);

compiler/rustc_mir_build/src/thir/cx/mod.rs

@@ -54,7 +54,10 @@ pub(crate) fn thir_body(
 
 pub(crate) fn thir_tree(tcx: TyCtxt<'_>, owner_def: ty::WithOptConstParam<LocalDefId>) -> String {
     match thir_body(tcx, owner_def) {
-        Ok((thir, _)) => format!("{:#?}", thir.steal()),
+        Ok((thir, _)) => {
+            let thir = thir.steal();
+            tcx.thir_tree_representation(&thir)
+        }
         Err(_) => "error".into(),
     }
 }