Compute predecessors in mir_build query and use existing cache for generating ReadOnlyBodyCache, remove unneeded fns

src/librustc/mir/cache.rs

@@ -181,8 +181,6 @@ impl BodyCache<'tcx> {
         ReadOnlyBodyCache::new(&self.cache, &self.body)
     }
 
-    pub fn cache(&self) -> &Cache { &self.cache }
-
     pub fn basic_blocks_mut(&mut self) -> &mut IndexVec<BasicBlock, BasicBlockData<'tcx>> {
         self.cache.basic_blocks_mut(&mut self.body)
     }
@@ -240,14 +238,6 @@ impl ReadOnlyBodyCache<'a, 'tcx> {
         }
     }
 
-    pub fn from_external_cache(cache: &'a mut Cache, body: &'a Body<'tcx>) -> Self {
-        cache.ensure_predecessors(body);
-        Self {
-            cache,
-            body,
-        }
-    }
-
     #[inline]
     pub fn predecessors(&self) -> &IndexVec<BasicBlock, Vec<BasicBlock>> {
         self.cache.predecessors.as_ref().unwrap()

src/librustc_mir/build/mod.rs

@@ -196,7 +196,9 @@ pub fn mir_build(tcx: TyCtxt<'_>, def_id: DefId) -> BodyCache<'_> {
 
         lints::check(tcx, &body, def_id);
 
-        BodyCache::new(body)
+        let mut body = BodyCache::new(body);
+        body.ensure_predecessors();
+        body
     })
 }
 

src/librustc_mir/transform/check_unsafety.rs

@@ -528,9 +528,10 @@ fn unsafety_check_result(tcx: TyCtxt<'_>, def_id: DefId) -> UnsafetyCheckResult
         hir::BodyOwnerKind::Static(_) => (true, false),
     };
     let mut checker = UnsafetyChecker::new(const_context, min_const_fn, body, tcx, param_env);
-    let mut cache = body.cache().clone();
-    let read_only_cache = ReadOnlyBodyCache::from_external_cache(&mut cache, body);
-    checker.visit_body(read_only_cache);
+    // mir_built ensures that body has a computed cache, so we don't (and can't) attempt to
+    // recompute it here.
+    let body = body.unwrap_read_only();
+    checker.visit_body(body);
 
     check_unused_unsafe(tcx, def_id, &checker.used_unsafe, &mut checker.inherited_blocks);
     UnsafetyCheckResult {