Rollup merge of rust-lang#121716 - Nadrieril:simple-binding-order, r=matthewjasper

match lowering: Lower bindings in a predictable order

After the recent refactorings, we can now lower bindings in a truly predictable order. The order in rust-lang#120214 was an improvement but not very clear. With this PR, we lower bindings from left to right, with the special case that `x @ pat` is traversed as `pat @ x` (i.e. `x` is lowered after any bindings in `pat`).

This description only applies in the absence of or-patterns. Or-patterns make everything complicated, because the binding place depends on the subpattern. Until I have a better idea I leave them to be handled in whatever weird order arises from today's code.

r? `@matthewjasper`

Author: Nadrieril

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -701,7 +701,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         self.bind_pattern(
             self.source_info(irrefutable_pat.span),
             candidate,
-            &fake_borrow_temps,
+            fake_borrow_temps.as_slice(),
             irrefutable_pat.span,
             None,
             false,
@@ -1938,7 +1938,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let post_guard_block = self.bind_pattern(
             self.source_info(pat.span),
             guard_candidate,
-            &fake_borrow_temps,
+            fake_borrow_temps.as_slice(),
             expr_span,
             None,
             false,
@@ -2425,7 +2425,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             let matching = this.bind_pattern(
                 this.source_info(pattern.span),
                 candidate,
-                &fake_borrow_temps,
+                fake_borrow_temps.as_slice(),
                 initializer_span,
                 None,
                 true,
@@ -2434,7 +2434,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             let failure = this.bind_pattern(
                 this.source_info(else_block_span),
                 wildcard,
-                &fake_borrow_temps,
+                fake_borrow_temps.as_slice(),
                 initializer_span,
                 None,
                 true,

compiler/rustc_mir_build/src/build/matches/simplify.rs

@@ -41,60 +41,30 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         //     let y = x;
         // }
         //
-        // We can't just reverse the binding order, because we must preserve pattern-order
-        // otherwise, e.g. in `let (Some(a), Some(b)) = (x, y)`. Our rule then is: deepest-first,
-        // and bindings at the same depth stay in source order.
-        //
-        // To do this, every time around the loop we prepend the newly found bindings to the
-        // bindings we already had.
-        //
-        // example:
-        // candidate.bindings = [1, 2, 3]
-        // bindings in iter 1: [4, 5]
-        // bindings in iter 2: [6, 7]
-        //
-        // final bindings: [6, 7, 4, 5, 1, 2, 3]
-        let mut accumulated_bindings = mem::take(candidate_bindings);
-        let mut simplified_match_pairs = Vec::new();
-        // Repeatedly simplify match pairs until we're left with only unsimplifiable ones.
-        loop {
-            for mut match_pair in mem::take(match_pairs) {
-                if let TestCase::Irrefutable { binding, ascription } = match_pair.test_case {
-                    if let Some(binding) = binding {
-                        candidate_bindings.push(binding);
-                    }
-                    if let Some(ascription) = ascription {
-                        candidate_ascriptions.push(ascription);
-                    }
-                    // Simplifiable pattern; we replace it with its subpairs and simplify further.
-                    match_pairs.append(&mut match_pair.subpairs);
-                } else {
-                    // Unsimplifiable pattern; we recursively simplify its subpairs and don't
-                    // process it further.
-                    self.simplify_match_pairs(
-                        &mut match_pair.subpairs,
-                        candidate_bindings,
-                        candidate_ascriptions,
-                    );
-                    simplified_match_pairs.push(match_pair);
+        // We therefore lower bindings from left-to-right, except we lower the `x` in `x @ pat`
+        // after any bindings in `pat`. This doesn't work for or-patterns: the current structure of
+        // match lowering forces us to lower bindings inside or-patterns last.
+        for mut match_pair in mem::take(match_pairs) {
+            self.simplify_match_pairs(
+                &mut match_pair.subpairs,
+                candidate_bindings,
+                candidate_ascriptions,
+            );
+            if let TestCase::Irrefutable { binding, ascription } = match_pair.test_case {
+                if let Some(binding) = binding {
+                    candidate_bindings.push(binding);
                 }
-            }
-
-            // This does: accumulated_bindings = candidate.bindings.take() ++ accumulated_bindings
-            candidate_bindings.extend_from_slice(&accumulated_bindings);
-            mem::swap(candidate_bindings, &mut accumulated_bindings);
-            candidate_bindings.clear();
-
-            if match_pairs.is_empty() {
-                break;
+                if let Some(ascription) = ascription {
+                    candidate_ascriptions.push(ascription);
+                }
+                // Simplifiable pattern; we replace it with its already simplified subpairs.
+                match_pairs.append(&mut match_pair.subpairs);
+            } else {
+                // Unsimplifiable pattern; we keep it.
+                match_pairs.push(match_pair);
             }
         }
 
-        // Store computed bindings back in `candidate_bindings`.
-        mem::swap(candidate_bindings, &mut accumulated_bindings);
-        // Store simplified match pairs back in `match_pairs`.
-        mem::swap(match_pairs, &mut simplified_match_pairs);
-
         // Move or-patterns to the end, because they can result in us
         // creating additional candidates, so we want to test them as
         // late as possible.

tests/mir-opt/early_otherwise_branch.opt1.EarlyOtherwiseBranch.diff

@@ -51,13 +51,13 @@
 -     }
 - 
 -     bb3: {
-          StorageLive(_9);
-          _9 = (((_3.1: std::option::Option<u32>) as Some).0: u32);
           StorageLive(_8);
           _8 = (((_3.0: std::option::Option<u32>) as Some).0: u32);
+          StorageLive(_9);
+          _9 = (((_3.1: std::option::Option<u32>) as Some).0: u32);
           _0 = const 0_u32;
-          StorageDead(_8);
           StorageDead(_9);
+          StorageDead(_8);
 -         goto -> bb4;
 +         goto -> bb3;
       }

tests/mir-opt/early_otherwise_branch.opt2.EarlyOtherwiseBranch.diff

@@ -57,13 +57,13 @@
 -     }
 - 
 -     bb4: {
-          StorageLive(_10);
-          _10 = (((_3.1: std::option::Option<u32>) as Some).0: u32);
           StorageLive(_9);
           _9 = (((_3.0: std::option::Option<u32>) as Some).0: u32);
+          StorageLive(_10);
+          _10 = (((_3.1: std::option::Option<u32>) as Some).0: u32);
           _0 = const 0_u32;
-          StorageDead(_9);
           StorageDead(_10);
+          StorageDead(_9);
 -         goto -> bb6;
 +         goto -> bb4;
       }

tests/mir-opt/early_otherwise_branch.opt3.EarlyOtherwiseBranch.diff

@@ -51,13 +51,13 @@
 -     }
 - 
 -     bb3: {
-          StorageLive(_9);
-          _9 = (((_3.1: std::option::Option<bool>) as Some).0: bool);
           StorageLive(_8);
           _8 = (((_3.0: std::option::Option<u32>) as Some).0: u32);
+          StorageLive(_9);
+          _9 = (((_3.1: std::option::Option<bool>) as Some).0: bool);
           _0 = const 0_u32;
-          StorageDead(_8);
           StorageDead(_9);
+          StorageDead(_8);
 -         goto -> bb4;
 +         goto -> bb3;
       }

tests/mir-opt/early_otherwise_branch_3_element_tuple.opt1.EarlyOtherwiseBranch.diff

@@ -69,16 +69,16 @@
 
 -     bb4: {
 +     bb3: {
-          StorageLive(_13);
-          _13 = (((_4.2: std::option::Option<u32>) as Some).0: u32);
-          StorageLive(_12);
-          _12 = (((_4.1: std::option::Option<u32>) as Some).0: u32);
           StorageLive(_11);
           _11 = (((_4.0: std::option::Option<u32>) as Some).0: u32);
+          StorageLive(_12);
+          _12 = (((_4.1: std::option::Option<u32>) as Some).0: u32);
+          StorageLive(_13);
+          _13 = (((_4.2: std::option::Option<u32>) as Some).0: u32);
           _0 = const 0_u32;
-          StorageDead(_11);
-          StorageDead(_12);
           StorageDead(_13);
+          StorageDead(_12);
+          StorageDead(_11);
 -         goto -> bb5;
 +         goto -> bb4;
       }

tests/mir-opt/early_otherwise_branch_68867.try_sum.EarlyOtherwiseBranch.diff

@@ -116,12 +116,12 @@
       }
 
       bb6: {
-          StorageLive(_13);
-          _39 = deref_copy (_4.1: &ViewportPercentageLength);
-          _13 = (((*_39) as Vw).0: f32);
           StorageLive(_12);
-          _40 = deref_copy (_4.0: &ViewportPercentageLength);
-          _12 = (((*_40) as Vw).0: f32);
+          _39 = deref_copy (_4.0: &ViewportPercentageLength);
+          _12 = (((*_39) as Vw).0: f32);
+          StorageLive(_13);
+          _40 = deref_copy (_4.1: &ViewportPercentageLength);
+          _13 = (((*_40) as Vw).0: f32);
           StorageLive(_14);
           StorageLive(_15);
           _15 = _12;
@@ -132,18 +132,18 @@
           StorageDead(_15);
           _3 = ViewportPercentageLength::Vw(move _14);
           StorageDead(_14);
-          StorageDead(_12);
           StorageDead(_13);
+          StorageDead(_12);
           goto -> bb10;
       }
 
       bb7: {
-          StorageLive(_18);
-          _41 = deref_copy (_4.1: &ViewportPercentageLength);
-          _18 = (((*_41) as Vh).0: f32);
           StorageLive(_17);
-          _42 = deref_copy (_4.0: &ViewportPercentageLength);
-          _17 = (((*_42) as Vh).0: f32);
+          _41 = deref_copy (_4.0: &ViewportPercentageLength);
+          _17 = (((*_41) as Vh).0: f32);
+          StorageLive(_18);
+          _42 = deref_copy (_4.1: &ViewportPercentageLength);
+          _18 = (((*_42) as Vh).0: f32);
           StorageLive(_19);
           StorageLive(_20);
           _20 = _17;
@@ -154,18 +154,18 @@
           StorageDead(_20);
           _3 = ViewportPercentageLength::Vh(move _19);
           StorageDead(_19);
-          StorageDead(_17);
           StorageDead(_18);
+          StorageDead(_17);
           goto -> bb10;
       }
 
       bb8: {
-          StorageLive(_23);
-          _43 = deref_copy (_4.1: &ViewportPercentageLength);
-          _23 = (((*_43) as Vmin).0: f32);
           StorageLive(_22);
-          _44 = deref_copy (_4.0: &ViewportPercentageLength);
-          _22 = (((*_44) as Vmin).0: f32);
+          _43 = deref_copy (_4.0: &ViewportPercentageLength);
+          _22 = (((*_43) as Vmin).0: f32);
+          StorageLive(_23);
+          _44 = deref_copy (_4.1: &ViewportPercentageLength);
+          _23 = (((*_44) as Vmin).0: f32);
           StorageLive(_24);
           StorageLive(_25);
           _25 = _22;
@@ -176,18 +176,18 @@
           StorageDead(_25);
           _3 = ViewportPercentageLength::Vmin(move _24);
           StorageDead(_24);
-          StorageDead(_22);
           StorageDead(_23);
+          StorageDead(_22);
           goto -> bb10;
       }
 
       bb9: {
-          StorageLive(_28);
-          _45 = deref_copy (_4.1: &ViewportPercentageLength);
-          _28 = (((*_45) as Vmax).0: f32);
           StorageLive(_27);
-          _46 = deref_copy (_4.0: &ViewportPercentageLength);
-          _27 = (((*_46) as Vmax).0: f32);
+          _45 = deref_copy (_4.0: &ViewportPercentageLength);
+          _27 = (((*_45) as Vmax).0: f32);
+          StorageLive(_28);
+          _46 = deref_copy (_4.1: &ViewportPercentageLength);
+          _28 = (((*_46) as Vmax).0: f32);
           StorageLive(_29);
           StorageLive(_30);
           _30 = _27;
@@ -198,8 +198,8 @@
           StorageDead(_30);
           _3 = ViewportPercentageLength::Vmax(move _29);
           StorageDead(_29);
-          StorageDead(_27);
           StorageDead(_28);
+          StorageDead(_27);
           goto -> bb10;
       }
 

tests/mir-opt/early_otherwise_branch_noopt.noopt1.EarlyOtherwiseBranch.diff

@@ -59,13 +59,13 @@
       }
 
       bb5: {
-          StorageLive(_10);
-          _10 = (((_3.1: std::option::Option<u32>) as Some).0: u32);
           StorageLive(_9);
           _9 = (((_3.0: std::option::Option<u32>) as Some).0: u32);
+          StorageLive(_10);
+          _10 = (((_3.1: std::option::Option<u32>) as Some).0: u32);
           _0 = const 0_u32;
-          StorageDead(_9);
           StorageDead(_10);
+          StorageDead(_9);
           goto -> bb8;
       }
 

tests/mir-opt/exponential_or.match_tuple.SimplifyCfg-initial.after.mir

@@ -19,8 +19,7 @@ fn match_tuple(_1: (u32, bool, Option<i32>, u32)) -> u32 {
 
     bb0: {
         PlaceMention(_1);
-        _2 = discriminant((_1.2: std::option::Option<i32>));
-        switchInt(move _2) -> [0: bb3, 1: bb2, otherwise: bb1];
+        switchInt((_1.0: u32)) -> [1: bb2, 4: bb2, otherwise: bb1];
     }
 
     bb1: {
@@ -29,11 +28,12 @@ fn match_tuple(_1: (u32, bool, Option<i32>, u32)) -> u32 {
     }
 
     bb2: {
-        switchInt((((_1.2: std::option::Option<i32>) as Some).0: i32)) -> [1: bb3, 8: bb3, otherwise: bb1];
+        _2 = discriminant((_1.2: std::option::Option<i32>));
+        switchInt(move _2) -> [0: bb4, 1: bb3, otherwise: bb1];
     }
 
     bb3: {
-        switchInt((_1.0: u32)) -> [1: bb4, 4: bb4, otherwise: bb1];
+        switchInt((((_1.2: std::option::Option<i32>) as Some).0: i32)) -> [1: bb4, 8: bb4, otherwise: bb1];
     }
 
     bb4: {

tests/ui/pattern/bindings-after-at/bind-by-copy-or-pat.rs

@@ -0,0 +1,17 @@
+//@ known-bug: unknown
+#![allow(unused)]
+
+struct A(u32);
+
+pub fn main() {
+    // The or-pattern bindings are lowered after `x`, which triggers the error.
+    let x @ (A(a) | A(a)) = A(10);
+    // ERROR: use of moved value
+    assert!(x.0 == 10);
+    assert!(a == 10);
+
+    // This works.
+    let (x @ A(a) | x @ A(a)) = A(10);
+    assert!(x.0 == 10);
+    assert!(a == 10);
+}

tests/ui/pattern/bindings-after-at/bind-by-copy-or-pat.stderr

@@ -0,0 +1,31 @@
+error[E0382]: use of moved value
+  --> $DIR/bind-by-copy-or-pat.rs:8:16
+   |
+LL |     let x @ (A(a) | A(a)) = A(10);
+   |         -      ^            ----- move occurs because value has type `A`, which does not implement the `Copy` trait
+   |         |      |
+   |         |      value used here after move
+   |         value moved here
+   |
+help: borrow this binding in the pattern to avoid moving the value
+   |
+LL |     let ref x @ (A(a) | A(a)) = A(10);
+   |         +++
+
+error[E0382]: use of moved value
+  --> $DIR/bind-by-copy-or-pat.rs:8:23
+   |
+LL |     let x @ (A(a) | A(a)) = A(10);
+   |         -             ^     ----- move occurs because value has type `A`, which does not implement the `Copy` trait
+   |         |             |
+   |         |             value used here after move
+   |         value moved here
+   |
+help: borrow this binding in the pattern to avoid moving the value
+   |
+LL |     let ref x @ (A(a) | A(a)) = A(10);
+   |         +++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0382`.