diff --git a/compiler/rustc_mir_transform/src/patch.rs b/compiler/rustc_mir_transform/src/patch.rs
index c781d1a5324b7..d831ab50b1ac0 100644
--- a/compiler/rustc_mir_transform/src/patch.rs
+++ b/compiler/rustc_mir_transform/src/patch.rs
@@ -1,4 +1,5 @@
-use rustc_index::{Idx, IndexVec};
+use rustc_data_structures::fx::FxHashMap;
+use rustc_index::Idx;
 use rustc_middle::mir::*;
 use rustc_middle::ty::Ty;
 use rustc_span::Span;
@@ -9,7 +10,7 @@ use tracing::debug;
 /// and replacement of terminators, and then apply the queued changes all at
 /// once with `apply`. This is useful for MIR transformation passes.
 pub(crate) struct MirPatch<'tcx> {
-    term_patch_map: IndexVec<BasicBlock, Option<TerminatorKind<'tcx>>>,
+    term_patch_map: FxHashMap<BasicBlock, TerminatorKind<'tcx>>,
     new_blocks: Vec<BasicBlockData<'tcx>>,
     new_statements: Vec<(Location, StatementKind<'tcx>)>,
     new_locals: Vec<LocalDecl<'tcx>>,
@@ -22,17 +23,21 @@ pub(crate) struct MirPatch<'tcx> {
     terminate_block: Option<(BasicBlock, UnwindTerminateReason)>,
     body_span: Span,
     next_local: usize,
+    /// The number of blocks at the start of the transformation. New blocks
+    /// get appended at the end.
+    next_block: usize,
 }
 
 impl<'tcx> MirPatch<'tcx> {
     /// Creates a new, empty patch.
     pub(crate) fn new(body: &Body<'tcx>) -> Self {
         let mut result = MirPatch {
-            term_patch_map: IndexVec::from_elem(None, &body.basic_blocks),
+            term_patch_map: Default::default(),
             new_blocks: vec![],
             new_statements: vec![],
             new_locals: vec![],
             next_local: body.local_decls.len(),
+            next_block: body.basic_blocks.len(),
             resume_block: None,
             unreachable_cleanup_block: None,
             unreachable_no_cleanup_block: None,
@@ -141,7 +146,7 @@ impl<'tcx> MirPatch<'tcx> {
 
     /// Has a replacement of this block's terminator been queued in this patch?
     pub(crate) fn is_term_patched(&self, bb: BasicBlock) -> bool {
-        self.term_patch_map[bb].is_some()
+        self.term_patch_map.contains_key(&bb)
     }
 
     /// Universal getter for block data, either it is in 'old' blocks or in patched ones
@@ -194,18 +199,17 @@ impl<'tcx> MirPatch<'tcx> {
 
     /// Queues the addition of a new basic block.
     pub(crate) fn new_block(&mut self, data: BasicBlockData<'tcx>) -> BasicBlock {
-        let block = self.term_patch_map.next_index();
+        let block = BasicBlock::from_usize(self.next_block + self.new_blocks.len());
         debug!("MirPatch: new_block: {:?}: {:?}", block, data);
         self.new_blocks.push(data);
-        self.term_patch_map.push(None);
         block
     }
 
     /// Queues the replacement of a block's terminator.
     pub(crate) fn patch_terminator(&mut self, block: BasicBlock, new: TerminatorKind<'tcx>) {
-        assert!(self.term_patch_map[block].is_none());
+        assert!(!self.term_patch_map.contains_key(&block));
         debug!("MirPatch: patch_terminator({:?}, {:?})", block, new);
-        self.term_patch_map[block] = Some(new);
+        self.term_patch_map.insert(block, new);
     }
 
     /// Queues the insertion of a statement at a given location. The statement
@@ -244,6 +248,7 @@ impl<'tcx> MirPatch<'tcx> {
             self.new_blocks.len(),
             body.basic_blocks.len()
         );
+        debug_assert_eq!(self.next_block, body.basic_blocks.len());
         let bbs = if self.term_patch_map.is_empty() && self.new_blocks.is_empty() {
             body.basic_blocks.as_mut_preserves_cfg()
         } else {
@@ -251,11 +256,12 @@ impl<'tcx> MirPatch<'tcx> {
         };
         bbs.extend(self.new_blocks);
         body.local_decls.extend(self.new_locals);
-        for (src, patch) in self.term_patch_map.into_iter_enumerated() {
-            if let Some(patch) = patch {
-                debug!("MirPatch: patching block {:?}", src);
-                bbs[src].terminator_mut().kind = patch;
-            }
+
+        // The order in which we patch terminators does not change the result.
+        #[allow(rustc::potential_query_instability)]
+        for (src, patch) in self.term_patch_map {
+            debug!("MirPatch: patching block {:?}", src);
+            bbs[src].terminator_mut().kind = patch;
         }
 
         let mut new_statements = self.new_statements;
@@ -273,8 +279,8 @@ impl<'tcx> MirPatch<'tcx> {
             }
             debug!("MirPatch: adding statement {:?} at loc {:?}+{}", stmt, loc, delta);
             loc.statement_index += delta;
-            let source_info = Self::source_info_for_index(&body[loc.block], loc);
-            body[loc.block]
+            let source_info = Self::source_info_for_index(&bbs[loc.block], loc);
+            bbs[loc.block]
                 .statements
                 .insert(loc.statement_index, Statement::new(source_info, stmt));
             delta += 1;