Auto merge of #142540 - cjgillot:renumber-cfg, r=<try>

Pre-compute MIR CFG caches for borrowck and other analyses

I was puzzled that #142390 introduces additional computations of CFG traversals: borrowck computes them, right?

It turns out that borrowck clones the MIR body, so doesn't share its cache with other analyses.

This PR:
- forces the computation of all caches in `mir_promoted` query;
- modifies region renumbering to avoid dropping that cache.

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Author: rust-bors[bot]

compiler/rustc_borrowck/src/renumber.rs

@@ -21,10 +21,10 @@ pub(crate) fn renumber_mir<'tcx>(
     let mut renumberer = RegionRenumberer { infcx };
 
     for body in promoted.iter_mut() {
-        renumberer.visit_body(body);
+        renumberer.visit_body_preserves_cfg(body);
     }
 
-    renumberer.visit_body(body);
+    renumberer.visit_body_preserves_cfg(body);
 }
 
 // The fields are used only for debugging output in `sccs_info`.

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();