Auto merge of #80632 - Nadrieril:fix-80501, r=varkor

Identify unreachable subpatterns more reliably

In https://github.com/rust-lang/rust/pull/80104 I used `Span`s to identify unreachable sub-patterns in the presence of or-patterns during exhaustiveness checking. In https://github.com/rust-lang/rust/issues/80501 it was revealed that `Span`s are complicated and that this was not a good idea.
Instead, this PR identifies subpatterns logically: as a path in the tree of subpatterns of a given pattern. I made a struct that captures a set of such subpatterns. This is a bit complex, but thankfully self-contained; the rest of the code does not need to know anything about it.
Fixes https://github.com/rust-lang/rust/issues/80501. I think I managed to keep the perf neutral.

r? `@varkor`

src/test/ui/or-patterns/exhaustiveness-unreachable-pattern.rs

@@ -48,6 +48,25 @@ fn main() {
         (1 | 1,) => {} //~ ERROR unreachable
         _ => {}
     }
+    match 0 {
+        (0 | 1) | 1 => {} //~ ERROR unreachable
+        _ => {}
+    }
+    match 0 {
+        // We get two errors because recursive or-pattern expansion means we don't notice the two
+        // errors span a whole pattern. This could be better but doesn't matter much
+        0 | (0 | 0) => {}
+        //~^ ERROR unreachable
+        //~| ERROR unreachable
+        _ => {}
+    }
+    match None {
+        // There is only one error that correctly points to the whole subpattern
+        Some(0) |
+            Some( //~ ERROR unreachable
+                0 | 0) => {}
+        _ => {}
+    }
     match [0; 2] {
         [0
             | 0 //~ ERROR unreachable
@@ -84,8 +103,8 @@ fn main() {
     }
     macro_rules! t_or_f {
         () => {
-            (true // FIXME: should be unreachable
-                        | false)
+            (true //~ ERROR unreachable
+            | false)
         };
     }
     match (true, None) {

src/test/ui/or-patterns/exhaustiveness-unreachable-pattern.stderr

@@ -77,58 +77,94 @@ LL |         (1 | 1,) => {}
    |              ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:53:15
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:52:19
+   |
+LL |         (0 | 1) | 1 => {}
+   |                   ^
+
+error: unreachable pattern
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:58:14
+   |
+LL |         0 | (0 | 0) => {}
+   |              ^
+
+error: unreachable pattern
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:58:18
+   |
+LL |         0 | (0 | 0) => {}
+   |                  ^
+
+error: unreachable pattern
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:66:13
+   |
+LL | /             Some(
+LL | |                 0 | 0) => {}
+   | |______________________^
+
+error: unreachable pattern
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:72:15
    |
 LL |             | 0
    |               ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:55:15
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:74:15
    |
 LL |             | 0] => {}
    |               ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:63:10
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:82:10
    |
 LL |         [1
    |          ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:75:10
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:94:10
    |
 LL |         [true
    |          ^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:82:36
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:101:36
    |
 LL |         (true | false, None | Some(true
    |                                    ^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:98:14
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:106:14
+   |
+LL |             (true
+   |              ^^^^
+...
+LL |         (true | false, None | Some(t_or_f!())) => {}
+   |                                    --------- in this macro invocation
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: unreachable pattern
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:117:14
    |
 LL |         Some(0
    |              ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:117:19
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:136:19
    |
 LL |                 | false) => {}
    |                   ^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:125:15
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:144:15
    |
 LL |             | true) => {}
    |               ^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:131:15
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:150:15
    |
 LL |             | true,
    |               ^^^^
 
-error: aborting due to 21 previous errors
+error: aborting due to 26 previous errors
 

src/test/ui/pattern/usefulness/issue-80501-or-pat-and-macro.rs

@@ -0,0 +1,27 @@
+// check-pass
+#![deny(unreachable_patterns)]
+pub enum TypeCtor {
+    Slice,
+    Array,
+}
+
+pub struct ApplicationTy(TypeCtor);
+
+macro_rules! ty_app {
+    ($ctor:pat) => {
+        ApplicationTy($ctor)
+    };
+}
+
+fn _foo(ty: ApplicationTy) {
+    match ty {
+        ty_app!(TypeCtor::Array) | ty_app!(TypeCtor::Slice) => {}
+    }
+
+    // same as above, with the macro expanded
+    match ty {
+        ApplicationTy(TypeCtor::Array) | ApplicationTy(TypeCtor::Slice) => {}
+    }
+}
+
+fn main() {}