Pre-cache traversals in mir_promoted.

Author: cjgillot

compiler/rustc_middle/src/mir/basic_blocks.rs

@@ -52,6 +52,15 @@ impl<'tcx> BasicBlocks<'tcx> {
         BasicBlocks { basic_blocks, cache: Cache::default() }
     }
 
+    /// Force caching of traversals.
+    pub fn cache_traversals(&self) {
+        let _ = self.predecessors();
+        let _ = self.switch_sources();
+        let _ = self.reverse_postorder();
+        let _ = self.dominators();
+    }
+
+    #[inline]
     pub fn dominators(&self) -> &Dominators<BasicBlock> {
         self.cache.dominators.get_or_init(|| dominators(self))
     }

compiler/rustc_mir_transform/src/lib.rs

@@ -446,6 +446,10 @@ fn mir_promoted(
         pm::Optimizations::Allowed,
     );
 
+    // The returned MIR will be used for analyses. Make sure that the traversals are computed
+    // early, so we don't have to re-run them.
+    body.basic_blocks.cache_traversals();
+
     lint_tail_expr_drop_order::run_lint(tcx, def, &body);
 
     let promoted = promote_pass.promoted_fragments.into_inner();