Auto merge of #117611 - Nadrieril:linear-pass-take-4, r=cjgillot

Rewrite exhaustiveness in one pass

This is at least my 4th attempt at this in as many years x) Previous attempts were all too complicated or too slow. But we're finally here!

The previous version of the exhaustiveness algorithm computed reachability for each arm then exhaustiveness of the whole match. Since each of these steps does roughly the same things, this rewrites the algorithm to do them all in one go. I also think this makes things much simpler.

I also rewrote the documentation of the algorithm in depth. Hopefully it's up-to-date and easier to follow now. Plz comment if anything's unclear.

r? `@oli-obk` I think you're one of the rare other people to understand the exhaustiveness algorithm?

cc `@varkor` I know you're not active anymore, but if you feel like having a look you might enjoy this :D

Fixes #79307

Author: bors

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -279,8 +279,10 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
         } else {
             // Check the pattern for some things unrelated to exhaustiveness.
             let refutable = if cx.refutable { Refutable } else { Irrefutable };
-            pat.walk_always(|pat| check_borrow_conflicts_in_at_patterns(self, pat));
-            pat.walk_always(|pat| check_for_bindings_named_same_as_variants(self, pat, refutable));
+            pat.walk_always(|pat| {
+                check_borrow_conflicts_in_at_patterns(self, pat);
+                check_for_bindings_named_same_as_variants(self, pat, refutable);
+            });
             Ok(cx.pattern_arena.alloc(DeconstructedPat::from_pat(cx, pat)))
         }
     }
@@ -289,6 +291,7 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
         &self,
         refutability: RefutableFlag,
         match_span: Option<Span>,
+        scrut_span: Span,
     ) -> MatchCheckCtxt<'p, 'tcx> {
         let refutable = match refutability {
             Irrefutable => false,
@@ -299,7 +302,9 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
             param_env: self.param_env,
             module: self.tcx.parent_module(self.lint_level).to_def_id(),
             pattern_arena: self.pattern_arena,
+            match_lint_level: self.lint_level,
             match_span,
+            scrut_span,
             refutable,
         }
     }
@@ -330,7 +335,8 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
         source: hir::MatchSource,
         expr_span: Span,
     ) {
-        let cx = self.new_cx(Refutable, Some(expr_span));
+        let scrut = &self.thir[scrut];
+        let cx = self.new_cx(Refutable, Some(expr_span), scrut.span);
 
         let mut tarms = Vec::with_capacity(arms.len());
         for &arm in arms {
@@ -346,9 +352,8 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
             }
         }
 
-        let scrut = &self.thir[scrut];
         let scrut_ty = scrut.ty;
-        let report = compute_match_usefulness(&cx, &tarms, self.lint_level, scrut_ty, scrut.span);
+        let report = compute_match_usefulness(&cx, &tarms, scrut_ty);
 
         match source {
             // Don't report arm reachability of desugared `match $iter.into_iter() { iter => .. }`
@@ -453,10 +458,10 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
         pat: &Pat<'tcx>,
         refutability: RefutableFlag,
     ) -> Result<(MatchCheckCtxt<'p, 'tcx>, UsefulnessReport<'p, 'tcx>), ErrorGuaranteed> {
-        let cx = self.new_cx(refutability, None);
+        let cx = self.new_cx(refutability, None, pat.span);
         let pat = self.lower_pattern(&cx, pat)?;
         let arms = [MatchArm { pat, hir_id: self.lint_level, has_guard: false }];
-        let report = compute_match_usefulness(&cx, &arms, self.lint_level, pat.ty(), pat.span());
+        let report = compute_match_usefulness(&cx, &arms, pat.ty());
         Ok((cx, report))
     }
 

compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs

@@ -35,6 +35,17 @@ fn main() {
         ((0, 0) | (1, 0),) => {}
         _ => {}
     }
+    match ((0, 0),) {
+        // Note how the second one would be redundant without the guard.
+        ((x, y) | (y, x),) if x == 0 => {}
+        _ => {}
+    }
+    match 0 {
+        // We don't warn the second one as redundant in general because of cases like the one above.
+        // We could technically do it if there are no bindings.
+        0 | 0 if 0 == 0 => {}
+        _ => {}
+    }
 
     // This one caused ICE https://github.com/rust-lang/rust/issues/117378
     match (0u8, 0) {

compiler/rustc_mir_build/src/thir/pattern/usefulness.rs

@@ -1,6 +1,7 @@
 #![deny(unreachable_patterns)]
 
 // We wrap patterns in a tuple because top-level or-patterns were special-cased.
+#[rustfmt::skip]
 fn main() {
     match (0u8,) {
         (1 | 2,) => {}
@@ -73,6 +74,11 @@ fn main() {
             | 0] => {} //~ ERROR unreachable
         _ => {}
     }
+    match (true, 0) {
+        (true, 0 | 0) => {} //~ ERROR unreachable
+        (_, 0 | 0) => {} //~ ERROR unreachable
+        _ => {}
+    }
     match &[][..] {
         [0] => {}
         [0, _] => {}
@@ -149,4 +155,8 @@ fn main() {
             | true, //~ ERROR unreachable
             false | true) => {}
     }
+    match (true, true) {
+        (x, y)
+            | (y, x) => {} //~ ERROR unreachable
+    }
 }

tests/ui/or-patterns/exhaustiveness-pass.rs

@@ -1,5 +1,5 @@
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:7:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:8:9
    |
 LL |         (1,) => {}
    |         ^^^^
@@ -11,128 +11,140 @@ LL | #![deny(unreachable_patterns)]
    |         ^^^^^^^^^^^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:12:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:13:9
    |
 LL |         (2,) => {}
    |         ^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:18:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:19:9
    |
 LL |         (1 | 2,) => {}
    |         ^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:23:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:24:9
    |
 LL |         (1, 3) => {}
    |         ^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:24:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:25:9
    |
 LL |         (1, 4) => {}
    |         ^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:25:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:26:9
    |
 LL |         (2, 4) => {}
    |         ^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:26:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:27:9
    |
 LL |         (2 | 1, 4) => {}
    |         ^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:28:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:29:9
    |
 LL |         (1, 4 | 5) => {}
    |         ^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:36:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:37:9
    |
 LL |         (Some(1),) => {}
    |         ^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:37:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:38:9
    |
 LL |         (None,) => {}
    |         ^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:42:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:43:9
    |
 LL |         ((1..=4,),) => {}
    |         ^^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:47:14
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:48:14
    |
 LL |         (1 | 1,) => {}
    |              ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:51:19
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:52:19
    |
 LL |         (0 | 1) | 1 => {}
    |                   ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:57:14
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:58:14
    |
 LL |         0 | (0 | 0) => {}
    |              ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:57:18
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:58:18
    |
 LL |         0 | (0 | 0) => {}
    |                  ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:65:13
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:66:13
    |
 LL | /             Some(
 LL | |                 0 | 0) => {}
    | |______________________^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:71:15
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:72:15
    |
 LL |             | 0
    |               ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:73:15
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:74:15
    |
 LL |             | 0] => {}
    |               ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:81:10
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:78:20
+   |
+LL |         (true, 0 | 0) => {}
+   |                    ^
+
+error: unreachable pattern
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:79:17
+   |
+LL |         (_, 0 | 0) => {}
+   |                 ^
+
+error: unreachable pattern
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:87:10
    |
 LL |         [1
    |          ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:93:10
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:99:10
    |
 LL |         [true
    |          ^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:100:36
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:106:36
    |
 LL |         (true | false, None | Some(true
    |                                    ^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:105:14
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:111:14
    |
 LL |             (true
    |              ^^^^
@@ -143,28 +155,34 @@ LL |         (true | false, None | Some(t_or_f!())) => {}
    = note: this error originates in the macro `t_or_f` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:116:14
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:122:14
    |
 LL |         Some(0
    |              ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:135:19
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:141:19
    |
 LL |                 | false) => {}
    |                   ^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:143:15
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:149:15
    |
 LL |             | true) => {}
    |               ^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:149:15
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:155:15
    |
 LL |             | true,
    |               ^^^^
 
-error: aborting due to 26 previous errors
+error: unreachable pattern
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:160:15
+   |
+LL |             | (y, x) => {}
+   |               ^^^^^^
+
+error: aborting due to 29 previous errors
 

tests/ui/or-patterns/exhaustiveness-unreachable-pattern.rs

@@ -1,5 +1,5 @@
 error[E0004]: non-exhaustive patterns: type `usize` is non-empty
-  --> $DIR/pointer-sized-int.rs:54:11
+  --> $DIR/pointer-sized-int.rs:59:11
    |
 LL |     match 7usize {}
    |           ^^^^^^