Rollup merge of rust-lang#126553 - Nadrieril:expand-or-pat-into-above, r=matthewjasper

match lowering: expand or-candidates mixed with candidates above

This PR tweaks match lowering of or-patterns. Consider this:
```rust
match (x, y) {
    (1, true) => 1,
    (2, false) => 2,
    (1 | 2, true | false) => 3,
    (3 | 4, true | false) => 4,
    _ => 5,
}
```
One might hope that this can be compiled to a single `SwitchInt` on `x` followed by some boolean checks. Before this PR, we compile this to 3 `SwitchInt`s on `x`, because an arm that contains more than one or-pattern was compiled on its own. This PR groups branch `3` with the two branches above, getting us down to 2 `SwitchInt`s on `x`.

We can't in general expand or-patterns freely, because this interacts poorly with another optimization we do: or-pattern simplification. When an or-pattern doesn't involve bindings, we branch the success paths of all its alternatives to the same block. The drawback is that in a case like:
```rust
match (1, true) {
    (1 | 2, false) => unreachable!(),
    (2, _) => unreachable!(),
    _ => {}
}
```
if we used a single `SwitchInt`, by the time we test `false` we don't know whether we came from the `1` case or the `2` case, so we don't know where to go if `false` doesn't match.

Hence the limitation: we can process or-pattern alternatives alongside candidates that precede it, but not candidates that follow it. (Unless the or-pattern is the only remaining match pair of its candidate, in which case we can process it alongside whatever).

This PR allows the processing of or-pattern alternatives alongside candidates that precede it. One benefit is that we now process or-patterns in a single place in `mod.rs`.

r? ``@matthewjasper``

Author: fmease

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

@@ -0,0 +1,24 @@
+// skip-filecheck
+
+// EMIT_MIR or_pattern.shortcut_second_or.SimplifyCfg-initial.after.mir
+fn shortcut_second_or() {
+    // Check that after matching `0`, failing to match `2 | 3` skips trying to match `(1, 2 | 3)`.
+    match ((0, 0), 0) {
+        (x @ (0, _) | x @ (_, 1), y @ 2 | y @ 3) => {}
+        _ => {}
+    }
+}
+
+// EMIT_MIR or_pattern.single_switchint.SimplifyCfg-initial.after.mir
+fn single_switchint() {
+    // Check how many `SwitchInt`s we do. In theory a single one is necessary.
+    match (1, true) {
+        (1, true) => 1,
+        (2, false) => 2,
+        (1 | 2, true | false) => 3,
+        (3 | 4, true | false) => 4,
+        _ => 5,
+    };
+}
+
+fn main() {}

tests/mir-opt/or_pattern.rs

@@ -0,0 +1,100 @@
+// MIR for `shortcut_second_or` after SimplifyCfg-initial
+
+fn shortcut_second_or() -> () {
+    let mut _0: ();
+    let mut _1: ((i32, i32), i32);
+    let mut _2: (i32, i32);
+    let _3: (i32, i32);
+    let _4: i32;
+    scope 1 {
+        debug x => _3;
+        debug y => _4;
+    }
+
+    bb0: {
+        StorageLive(_1);
+        StorageLive(_2);
+        _2 = (const 0_i32, const 0_i32);
+        _1 = (move _2, const 0_i32);
+        StorageDead(_2);
+        PlaceMention(_1);
+        switchInt(((_1.0: (i32, i32)).0: i32)) -> [0: bb4, otherwise: bb2];
+    }
+
+    bb1: {
+        _0 = const ();
+        goto -> bb14;
+    }
+
+    bb2: {
+        switchInt(((_1.0: (i32, i32)).1: i32)) -> [1: bb3, otherwise: bb1];
+    }
+
+    bb3: {
+        switchInt((_1.1: i32)) -> [2: bb7, 3: bb8, otherwise: bb1];
+    }
+
+    bb4: {
+        switchInt((_1.1: i32)) -> [2: bb5, 3: bb6, otherwise: bb1];
+    }
+
+    bb5: {
+        falseEdge -> [real: bb10, imaginary: bb6];
+    }
+
+    bb6: {
+        falseEdge -> [real: bb11, imaginary: bb2];
+    }
+
+    bb7: {
+        falseEdge -> [real: bb12, imaginary: bb8];
+    }
+
+    bb8: {
+        falseEdge -> [real: bb13, imaginary: bb1];
+    }
+
+    bb9: {
+        _0 = const ();
+        StorageDead(_4);
+        StorageDead(_3);
+        goto -> bb14;
+    }
+
+    bb10: {
+        StorageLive(_3);
+        _3 = (_1.0: (i32, i32));
+        StorageLive(_4);
+        _4 = (_1.1: i32);
+        goto -> bb9;
+    }
+
+    bb11: {
+        StorageLive(_3);
+        _3 = (_1.0: (i32, i32));
+        StorageLive(_4);
+        _4 = (_1.1: i32);
+        goto -> bb9;
+    }
+
+    bb12: {
+        StorageLive(_3);
+        _3 = (_1.0: (i32, i32));
+        StorageLive(_4);
+        _4 = (_1.1: i32);
+        goto -> bb9;
+    }
+
+    bb13: {
+        StorageLive(_3);
+        _3 = (_1.0: (i32, i32));
+        StorageLive(_4);
+        _4 = (_1.1: i32);
+        goto -> bb9;
+    }
+
+    bb14: {
+        StorageDead(_1);
+        return;
+    }
+}

tests/mir-opt/or_pattern.shortcut_second_or.SimplifyCfg-initial.after.mir

@@ -0,0 +1,75 @@
+// MIR for `single_switchint` after SimplifyCfg-initial
+
+fn single_switchint() -> () {
+    let mut _0: ();
+    let _1: i32;
+    let mut _2: (i32, bool);
+
+    bb0: {
+        StorageLive(_1);
+        StorageLive(_2);
+        _2 = (const 1_i32, const true);
+        PlaceMention(_2);
+        switchInt((_2.0: i32)) -> [1: bb2, 2: bb4, otherwise: bb1];
+    }
+
+    bb1: {
+        switchInt((_2.0: i32)) -> [3: bb8, 4: bb8, otherwise: bb7];
+    }
+
+    bb2: {
+        switchInt((_2.1: bool)) -> [0: bb6, otherwise: bb3];
+    }
+
+    bb3: {
+        falseEdge -> [real: bb9, imaginary: bb4];
+    }
+
+    bb4: {
+        switchInt((_2.1: bool)) -> [0: bb5, otherwise: bb6];
+    }
+
+    bb5: {
+        falseEdge -> [real: bb10, imaginary: bb6];
+    }
+
+    bb6: {
+        falseEdge -> [real: bb11, imaginary: bb1];
+    }
+
+    bb7: {
+        _1 = const 5_i32;
+        goto -> bb13;
+    }
+
+    bb8: {
+        falseEdge -> [real: bb12, imaginary: bb7];
+    }
+
+    bb9: {
+        _1 = const 1_i32;
+        goto -> bb13;
+    }
+
+    bb10: {
+        _1 = const 2_i32;
+        goto -> bb13;
+    }
+
+    bb11: {
+        _1 = const 3_i32;
+        goto -> bb13;
+    }
+
+    bb12: {
+        _1 = const 4_i32;
+        goto -> bb13;
+    }
+
+    bb13: {
+        StorageDead(_2);
+        StorageDead(_1);
+        _0 = const ();
+        return;
+    }
+}

tests/mir-opt/or_pattern.single_switchint.SimplifyCfg-initial.after.mir

@@ -26,5 +26,6 @@ fn main() {
     assert_eq!(or_at(Err(7)), 207);
     assert_eq!(or_at(Err(8)), 8);
     assert_eq!(or_at(Err(20)), 220);
+    assert_eq!(or_at(Err(34)), 134);
     assert_eq!(or_at(Err(50)), 500);
 }

tests/ui/or-patterns/bindings-runpass-2.rs

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/inner-or-pat.rs:38:54
+  --> $DIR/inner-or-pat.rs:36:54
    |
 LL |     match x {
    |           - this expression has type `&str`

tests/ui/or-patterns/inner-or-pat.or3.stderr

@@ -1,5 +1,5 @@
 error[E0408]: variable `x` is not bound in all patterns
-  --> $DIR/inner-or-pat.rs:53:37
+  --> $DIR/inner-or-pat.rs:51:37
    |
 LL |         (x @ "red" | (x @ "blue" |  "red")) => {
    |                       -             ^^^^^ pattern doesn't bind `x`

tests/ui/or-patterns/inner-or-pat.or4.stderr

@@ -1,7 +1,5 @@
-//@ revisions: or1 or2 or3 or4 or5
+//@ revisions: or1 or3 or4
 //@ [or1] run-pass
-//@ [or2] run-pass
-//@ [or5] run-pass
 
 #![allow(unreachable_patterns)]
 #![allow(unused_variables)]

tests/ui/or-patterns/inner-or-pat.rs

@@ -1,21 +1,20 @@
-//@ check-pass
+//@ run-pass
 
 #![deny(unreachable_patterns)]
 
 fn main() {
-    match (3,42) {
-        (a,_) | (_,a) if a > 10 => {println!("{}", a)}
-        _ => ()
+    match (3, 42) {
+        (a, _) | (_, a) if a > 10 => {}
+        _ => unreachable!(),
     }
 
-    match Some((3,42)) {
-        Some((a, _)) | Some((_, a)) if a > 10 => {println!("{}", a)}
-        _ => ()
-
+    match Some((3, 42)) {
+        Some((a, _)) | Some((_, a)) if a > 10 => {}
+        _ => unreachable!(),
     }
 
-    match Some((3,42)) {
-        Some((a, _) | (_, a)) if a > 10 => {println!("{}", a)}
-        _ => ()
+    match Some((3, 42)) {
+        Some((a, _) | (_, a)) if a > 10 => {}
+        _ => unreachable!(),
     }
 }

tests/ui/or-patterns/issue-70413-no-unreachable-pat-and-guard.rs

@@ -42,6 +42,23 @@ fn search_old_style(target: (bool, bool, bool)) -> u32 {
     }
 }
 
+// Check that a dummy or-pattern also leads to running the guard multiple times.
+fn search_with_dummy(target: (bool, bool)) -> u32 {
+    let x = ((false, true), (false, true), ());
+    let mut guard_count = 0;
+    match x {
+        ((a, _) | (_, a), (b, _) | (_, b), _ | _)
+            if {
+                guard_count += 1;
+                (a, b) == target
+            } =>
+        {
+            guard_count
+        }
+        _ => unreachable!(),
+    }
+}
+
 fn main() {
     assert_eq!(search((false, false, false)), 1);
     assert_eq!(search((false, false, true)), 2);
@@ -60,4 +77,9 @@ fn main() {
     assert_eq!(search_old_style((true, false, true)), 6);
     assert_eq!(search_old_style((true, true, false)), 7);
     assert_eq!(search_old_style((true, true, true)), 8);
+
+    assert_eq!(search_with_dummy((false, false)), 1);
+    assert_eq!(search_with_dummy((false, true)), 3);
+    assert_eq!(search_with_dummy((true, false)), 5);
+    assert_eq!(search_with_dummy((true, true)), 7);
 }