diff --git a/compiler/rustc_mir/src/transform/match_identity.rs b/compiler/rustc_mir/src/transform/match_identity.rs
new file mode 100644
index 0000000000000..2b255fe6e5f7d
--- /dev/null
+++ b/compiler/rustc_mir/src/transform/match_identity.rs
@@ -0,0 +1,76 @@
+use crate::transform::MirPass;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+use rustc_target::abi::VariantIdx;
+
+pub struct MatchIdentitySimplification;
+
+impl<'tcx> MirPass<'tcx> for MatchIdentitySimplification {
+ fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+ //let param_env = tcx.param_env(body.source.def_id());
+ let (bbs, local_decls) = body.basic_blocks_and_local_decls_mut();
+ for bb_idx in bbs.indices() {
+ let (read_discr, og_match) = match &bbs[bb_idx].statements[..] {
+ &[Statement {
+ kind: StatementKind::Assign(box (dst, Rvalue::Discriminant(src))),
+ ..
+ }] => (dst, src),
+ _ => continue,
+ };
+ let (var_idx, fst, snd) = match bbs[bb_idx].terminator().kind {
+ TerminatorKind::SwitchInt {
+ discr: Operand::Copy(ref place) | Operand::Move(ref place),
+ ref targets,
+ ref values,
+ ..
+ } if targets.len() == 2
+ && values.len() == 1
+ && targets[0] != targets[1]
+ // check that we're switching on the read discr
+ && place == &read_discr
+ // check that this is actually
+ && place.ty(local_decls, tcx).ty.is_enum() =>
+ {
+ (VariantIdx::from(values[0] as usize), targets[0], targets[1])
+ }
+ // Only optimize switch int statements
+ _ => continue,
+ };
+ let stmts_ok = |stmts: &[Statement<'_>], expected_variant| match stmts {
+ [Statement {
+ kind:
+ StatementKind::Assign(box (
+ dst0,
+ Rvalue::Use(Operand::Copy(from) | Operand::Move(from)),
+ )),
+ ..
+ }, Statement {
+ kind: StatementKind::SetDiscriminant { place: box dst1, variant_index },
+ ..
+ }] => *variant_index == expected_variant && dst0 == dst1 && og_match == *from,
+ _ => false,
+ };
+ let bb1 = &bbs[fst];
+ let bb2 = &bbs[snd];
+ if bb1.terminator().kind != bb2.terminator().kind
+ || stmts_ok(&bb1.statements[..], var_idx)
+ || stmts_ok(&bb2.statements[..], var_idx + 1)
+ {
+ continue;
+ }
+ let dst = match (&bb1.statements[0], &bb2.statements[0]) {
+ (
+ Statement { kind: StatementKind::Assign(box (dst0, _)), .. },
+ Statement { kind: StatementKind::Assign(box (dst1, _)), .. },
+ ) if dst0 == dst1 => dst0.clone(),
+ _ => continue,
+ };
+ let term_kind = bb1.terminator().kind.clone();
+ // Reassign the output to just be the original
+ // Replace the terminator with the terminator of the output
+ bbs[bb_idx].statements[0].kind =
+ StatementKind::Assign(box (dst, Rvalue::Use(Operand::Copy(og_match))));
+ bbs[bb_idx].terminator_mut().kind = term_kind;
+ }
+ }
+}
diff --git a/compiler/rustc_mir/src/transform/mod.rs b/compiler/rustc_mir/src/transform/mod.rs
index b4f5947f5a339..54f0fb4f8148b 100644
--- a/compiler/rustc_mir/src/transform/mod.rs
+++ b/compiler/rustc_mir/src/transform/mod.rs
@@ -33,6 +33,7 @@ pub mod inline;
pub mod instcombine;
pub mod instrument_coverage;
pub mod match_branches;
+pub mod match_identity;
pub mod multiple_return_terminators;
pub mod no_landing_pads;
pub mod nrvo;
@@ -412,6 +413,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
&nrvo::RenameReturnPlace,
&simplify::SimplifyLocals,
&multiple_return_terminators::MultipleReturnTerminators,
+ &match_identity::MatchIdentitySimplification,
];
// Optimizations to run even if mir optimizations have been disabled.
diff --git a/src/test/mir-opt/match_identity.flip_flop.MatchBranchSimplification.64bit.diff b/src/test/mir-opt/match_identity.flip_flop.MatchBranchSimplification.64bit.diff
new file mode 100644
index 0000000000000..3eb66e4e03c79
--- /dev/null
+++ b/src/test/mir-opt/match_identity.flip_flop.MatchBranchSimplification.64bit.diff
@@ -0,0 +1,52 @@
+- // MIR for `flip_flop` before MatchBranchSimplification
++ // MIR for `flip_flop` after MatchBranchSimplification
+
+ fn flip_flop(_1: std::result::Result<i32, u32>) -> std::result::Result<u32, i32> {
+ debug a => _1; // in scope 0 at $DIR/match_identity.rs:16:18: 16:19
+ let mut _0: std::result::Result<u32, i32>; // return place in scope 0 at $DIR/match_identity.rs:16:38: 16:50
+ let mut _2: isize; // in scope 0 at $DIR/match_identity.rs:18:9: 18:14
+ let _3: i32; // in scope 0 at $DIR/match_identity.rs:18:12: 18:13
+ let mut _4: i32; // in scope 0 at $DIR/match_identity.rs:18:22: 18:23
+ let _5: u32; // in scope 0 at $DIR/match_identity.rs:19:13: 19:14
+ let mut _6: u32; // in scope 0 at $DIR/match_identity.rs:19:22: 19:23
+ scope 1 {
+ debug x => _3; // in scope 1 at $DIR/match_identity.rs:18:12: 18:13
+ }
+ scope 2 {
+ debug y => _5; // in scope 2 at $DIR/match_identity.rs:19:13: 19:14
+ }
+
+ bb0: {
+ _2 = discriminant(_1); // scope 0 at $DIR/match_identity.rs:18:9: 18:14
+ switchInt(move _2) -> [0_isize: bb2, otherwise: bb1]; // scope 0 at $DIR/match_identity.rs:18:9: 18:14
+ }
+
+ bb1: {
+ StorageLive(_5); // scope 0 at $DIR/match_identity.rs:19:13: 19:14
+ _5 = ((_1 as Err).0: u32); // scope 0 at $DIR/match_identity.rs:19:13: 19:14
+ StorageLive(_6); // scope 2 at $DIR/match_identity.rs:19:22: 19:23
+ _6 = _5; // scope 2 at $DIR/match_identity.rs:19:22: 19:23
+ ((_0 as Ok).0: u32) = move _6; // scope 2 at $DIR/match_identity.rs:19:19: 19:24
+ discriminant(_0) = 0; // scope 2 at $DIR/match_identity.rs:19:19: 19:24
+ StorageDead(_6); // scope 2 at $DIR/match_identity.rs:19:23: 19:24
+ StorageDead(_5); // scope 0 at $DIR/match_identity.rs:19:23: 19:24
+ goto -> bb3; // scope 0 at $DIR/match_identity.rs:17:5: 20:6
+ }
+
+ bb2: {
+ StorageLive(_3); // scope 0 at $DIR/match_identity.rs:18:12: 18:13
+ _3 = ((_1 as Ok).0: i32); // scope 0 at $DIR/match_identity.rs:18:12: 18:13
+ StorageLive(_4); // scope 1 at $DIR/match_identity.rs:18:22: 18:23
+ _4 = _3; // scope 1 at $DIR/match_identity.rs:18:22: 18:23
+ ((_0 as Err).0: i32) = move _4; // scope 1 at $DIR/match_identity.rs:18:18: 18:24
+ discriminant(_0) = 1; // scope 1 at $DIR/match_identity.rs:18:18: 18:24
+ StorageDead(_4); // scope 1 at $DIR/match_identity.rs:18:23: 18:24
+ StorageDead(_3); // scope 0 at $DIR/match_identity.rs:18:23: 18:24
+ goto -> bb3; // scope 0 at $DIR/match_identity.rs:17:5: 20:6
+ }
+
+ bb3: {
+ return; // scope 0 at $DIR/match_identity.rs:21:2: 21:2
+ }
+ }
+
diff --git a/src/test/mir-opt/match_identity.id_result.MatchIdentitySimplification.64bit.diff b/src/test/mir-opt/match_identity.id_result.MatchIdentitySimplification.64bit.diff
new file mode 100644
index 0000000000000..f7bebbe6607de
--- /dev/null
+++ b/src/test/mir-opt/match_identity.id_result.MatchIdentitySimplification.64bit.diff
@@ -0,0 +1,32 @@
+- // MIR for `id_result` before MatchIdentitySimplification
++ // MIR for `id_result` after MatchIdentitySimplification
+
+ fn id_result(_1: std::result::Result<i32, u32>) -> std::result::Result<i32, u32> {
+ debug a => _1; // in scope 0 at $DIR/match_identity.rs:8:18: 8:19
+ let mut _0: std::result::Result<i32, u32>; // return place in scope 0 at $DIR/match_identity.rs:8:38: 8:50
+ let mut _2: isize; // in scope 0 at $DIR/match_identity.rs:10:9: 10:14
+ scope 1 {
+ debug x => ((_0 as Ok).0: i32); // in scope 1 at $DIR/match_identity.rs:10:12: 10:13
+ }
+ scope 2 {
+ debug y => ((_0 as Err).0: u32); // in scope 2 at $DIR/match_identity.rs:11:13: 11:14
+ }
+
+ bb0: {
+ _2 = discriminant(_1); // scope 0 at $DIR/match_identity.rs:10:9: 10:14
+ switchInt(move _2) -> [0_isize: bb2, otherwise: bb1]; // scope 0 at $DIR/match_identity.rs:10:9: 10:14
+ }
+
+ bb1: {
+ ((_0 as Err).0: u32) = ((_1 as Err).0: u32); // scope 0 at $DIR/match_identity.rs:11:13: 11:14
+ discriminant(_0) = 1; // scope 2 at $DIR/match_identity.rs:11:19: 11:25
+ return; // scope 0 at $DIR/match_identity.rs:9:5: 12:6
+ }
+
+ bb2: {
+ ((_0 as Ok).0: i32) = ((_1 as Ok).0: i32); // scope 0 at $DIR/match_identity.rs:10:12: 10:13
+ discriminant(_0) = 0; // scope 1 at $DIR/match_identity.rs:10:18: 10:23
+ return; // scope 0 at $DIR/match_identity.rs:9:5: 12:6
+ }
+ }
+
diff --git a/src/test/mir-opt/match_identity.rs b/src/test/mir-opt/match_identity.rs
new file mode 100644
index 0000000000000..5e54cbcda31c2
--- /dev/null
+++ b/src/test/mir-opt/match_identity.rs
@@ -0,0 +1,21 @@
+#![crate_type = "lib"]
+
+type T = i32;
+type E = u32;
+
+// EMIT_MIR_FOR_EACH_BIT_WIDTH
+// EMIT_MIR match_identity.id_result.MatchIdentitySimplification.diff
+pub fn id_result(a: Result<T, E>) -> Result<T, E> {
+ match a {
+ Ok(x) => Ok(x),
+ Err(y) => Err(y),
+ }
+}
+
+// EMIT_MIR match_identity.flip_flop.MatchBranchSimplification.diff
+pub fn flip_flop(a: Result<T, E>) -> Result<E, T> {
+ match a {
+ Ok(x) => Err(x),
+ Err(y) => Ok(y),
+ }
+}