diff --git a/compiler/rustc_mir_build/src/builder/custom/mod.rs b/compiler/rustc_mir_build/src/builder/custom/mod.rs
index 6d0c415b7358a..91da85000a796 100644
--- a/compiler/rustc_mir_build/src/builder/custom/mod.rs
+++ b/compiler/rustc_mir_build/src/builder/custom/mod.rs
@@ -20,7 +20,7 @@ use rustc_attr_ir as attrs;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::HirId;
 use rustc_hir::def_id::DefId;
-use rustc_index::{IndexSlice, IndexVec};
+use rustc_index::{IndexSlice, IndexVec, indexvec};
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -64,7 +64,6 @@ pub(super) fn build_custom_mir<'tcx>(
     };
 
     body.local_decls.push(LocalDecl::new(return_ty, return_ty_span));
-    body.basic_blocks_mut().push(BasicBlockData::maybe_unterminated(None, false));
     body.source_scopes.push(SourceScopeData {
         span,
         parent_scope: None,
@@ -80,6 +79,8 @@ pub(super) fn build_custom_mir<'tcx>(
         thir,
         source_scope: OUTERMOST_SOURCE_SCOPE,
         body: &mut body,
+        // Single `false` for entry block which is declared implicitly
+        block_decls: indexvec![false],
         local_map: FxHashMap::default(),
         block_map: FxHashMap::default(),
     };
@@ -138,6 +139,8 @@ struct ParseCtxt<'a, 'tcx> {
     thir: &'a Thir<'tcx>,
     source_scope: SourceScope,
     body: &'a mut Body<'tcx>,
+    /// list of declared blocks, with a flag indicating whether it is a cleanup block
+    block_decls: IndexVec<BasicBlock, bool>,
     local_map: FxHashMap<LocalVarId, Local>,
     block_map: FxHashMap<LocalVarId, BasicBlock>,
 }
diff --git a/compiler/rustc_mir_build/src/builder/custom/parse.rs b/compiler/rustc_mir_build/src/builder/custom/parse.rs
index 0cc73e381ea2b..55252ea5c95d9 100644
--- a/compiler/rustc_mir_build/src/builder/custom/parse.rs
+++ b/compiler/rustc_mir_build/src/builder/custom/parse.rs
@@ -180,10 +180,18 @@ impl<'a, 'tcx> ParseCtxt<'a, 'tcx> {
             ExprKind::Block { block } => &self.thir[*block].stmts,
         );
         for (i, block_def) in block_defs.iter().enumerate() {
-            let is_cleanup = self.body.basic_blocks_mut()[BasicBlock::from_usize(i)].is_cleanup;
+            let bb = BasicBlock::from_usize(i);
+            let is_cleanup = self.block_decls[bb];
             let block = self.parse_block_def(self.statement_as_expr(*block_def)?, is_cleanup)?;
-            self.body.basic_blocks_mut()[BasicBlock::from_usize(i)] = block;
+            let inserted_bb = self.body.basic_blocks_mut().push(block);
+            // just a sanity check to make sure that we inserted the block
+            // to the correct position (basic block idx)
+            assert_eq!(inserted_bb, bb);
         }
+        // check that declarations, definitions and block_map counts match
+        assert_eq!(self.block_decls.len(), self.body.basic_blocks.len());
+        // +1 because block_map doesn't contain the entry block
+        assert_eq!(self.block_decls.len(), self.block_map.len() + 1);
 
         Ok(())
     }
@@ -199,14 +207,11 @@ impl<'a, 'tcx> ParseCtxt<'a, 'tcx> {
         match &self.thir[stmt].kind {
             StmtKind::Let { pattern, initializer: Some(initializer), .. } => {
                 let (var, ..) = self.parse_var(pattern)?;
-                let data = BasicBlockData::maybe_unterminated(
-                    None,
-                    parse_by_kind!(self, *initializer, _, "basic block declaration",
-                        @variant(mir_basic_block, Normal) => false,
-                        @variant(mir_basic_block, Cleanup) => true,
-                    ),
+                let is_cleanup = parse_by_kind!(self, *initializer, _, "basic block declaration",
+                    @variant(mir_basic_block, Normal) => false,
+                    @variant(mir_basic_block, Cleanup) => true,
                 );
-                let block = self.body.basic_blocks_mut().push(data);
+                let block = self.block_decls.push(is_cleanup);
                 self.block_map.insert(var, block);
                 Ok(())
             }
@@ -303,23 +308,24 @@ impl<'a, 'tcx> ParseCtxt<'a, 'tcx> {
             ExprKind::Block { block } => &self.thir[*block],
         );
 
-        let mut data = BasicBlockData::maybe_unterminated(None, is_cleanup);
+        let mut statements = vec![];
         for stmt_id in &*block.stmts {
             let stmt = self.statement_as_expr(*stmt_id)?;
             let span = self.thir[stmt].span;
             let statement = self.parse_statement(stmt)?;
-            data.statements
+            statements
                 .push(Statement::new(SourceInfo { span, scope: self.source_scope }, statement));
         }
 
         let Some(trailing) = block.expr else { return Err(self.expr_error(expr_id, "terminator")) };
         let span = self.thir[trailing].span;
         let terminator = self.parse_terminator(trailing)?;
-        data.terminator = Some(Terminator {
+
+        let terminator = Terminator {
             source_info: SourceInfo { span, scope: self.source_scope },
             kind: terminator,
-        });
+        };
 
-        Ok(data)
+        Ok(BasicBlockData::new_stmts(statements, terminator, is_cleanup))
     }
 }