Auto merge of rust-lang#116042 - Nadrieril:linear-pass-take-2, r=<try>

[Experiment] Rewrite exhaustiveness in one pass

Arm reachability checking does a quadratic amount of work: for each arm we check if it is reachable given the arms above it. This feels wasteful since we often end up re-exploring the same cases when we check for exhaustiveness.

This PR is an attempt to check reachability at the same time as exhaustiveness. This opens the door to a bunch of code simplifications I'm very excited about. The main question is whether I can get actual performance gains out of this.

I had started the experiment in rust-lang#111720 but I can't reopen it.

r? `@ghost`

compiler/rustc_lint_defs/src/builtin.rs

@@ -3915,8 +3915,10 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The `non_exhaustive_omitted_patterns` lint detects when a wildcard (`_` or `..`) in a
-    /// pattern for a `#[non_exhaustive]` struct or enum is reachable.
+    /// The `non_exhaustive_omitted_patterns` lint detects when some variants or fields of a
+    /// `#[non_exhaustive]` struct or enum are not mentioned explicitly in a pattern. This allows
+    /// downstream crates to be warned when new variants or fields are added to the upstream struct
+    /// or enum.
     ///
     /// ### Example
     ///
@@ -3930,9 +3932,9 @@ declare_lint! {
     ///
     /// // in crate B
     /// #![feature(non_exhaustive_omitted_patterns_lint)]
+    /// #[warn(non_exhaustive_omitted_patterns)]
     /// match Bar::A {
     ///     Bar::A => {},
-    ///     #[warn(non_exhaustive_omitted_patterns)]
     ///     _ => {},
     /// }
     /// ```
@@ -3943,8 +3945,8 @@ declare_lint! {
     /// warning: reachable patterns not covered of non exhaustive enum
     ///    --> $DIR/reachable-patterns.rs:70:9
     ///    |
-    /// LL |         _ => {}
-    ///    |         ^ pattern `B` not covered
+    /// LL |         match Bar::A {
+    ///    |               ^ pattern `Bar::B` not covered
     ///    |
     ///  note: the lint level is defined here
     ///   --> $DIR/reachable-patterns.rs:69:16
@@ -3957,12 +3959,11 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// Structs and enums tagged with `#[non_exhaustive]` force the user to add a
-    /// (potentially redundant) wildcard when pattern-matching, to allow for future
-    /// addition of fields or variants. The `non_exhaustive_omitted_patterns` lint
-    /// detects when such a wildcard happens to actually catch some fields/variants.
-    /// In other words, when the match without the wildcard would not be exhaustive.
-    /// This lets the user be informed if new fields/variants were added.
+    /// Structs and enums tagged with `#[non_exhaustive]` force the user to add a (potentially
+    /// redundant) wildcard when pattern-matching, to allow for future addition of fields or
+    /// variants. The `non_exhaustive_omitted_patterns` lint detects when such a wildcard happens to
+    /// actually catch some fields/variants. This lets the user be informed if new fields/variants
+    /// were added.
     pub NON_EXHAUSTIVE_OMITTED_PATTERNS,
     Allow,
     "detect when patterns of types marked `non_exhaustive` are missed",

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

@@ -269,7 +269,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
 
         let scrut = &self.thir[scrut];
         let scrut_ty = scrut.ty;
-        let report = compute_match_usefulness(&cx, &tarms, self.lint_level, scrut_ty);
+        let report = compute_match_usefulness(&cx, &tarms, self.lint_level, scrut_ty, scrut.span);
 
         match source {
             // Don't report arm reachability of desugared `match $iter.into_iter() { iter => .. }`
@@ -431,7 +431,8 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
         let pattern = self.lower_pattern(&mut cx, pat);
         let pattern_ty = pattern.ty();
         let arm = MatchArm { pat: pattern, hir_id: self.lint_level, has_guard: false };
-        let report = compute_match_usefulness(&cx, &[arm], self.lint_level, pattern_ty);
+        let report =
+            compute_match_usefulness(&cx, &[arm], self.lint_level, pattern_ty, pattern.span());
 
         // Note: we ignore whether the pattern is unreachable (i.e. whether the type is empty). We
         // only care about exhaustiveness here.
@@ -622,7 +623,7 @@ fn is_let_irrefutable<'p, 'tcx>(
     pat: &'p DeconstructedPat<'p, 'tcx>,
 ) -> bool {
     let arms = [MatchArm { pat, hir_id: pat_id, has_guard: false }];
-    let report = compute_match_usefulness(&cx, &arms, pat_id, pat.ty());
+    let report = compute_match_usefulness(&cx, &arms, pat_id, pat.ty(), pat.span());
 
     // Report if the pattern is unreachable, which can only occur when the type is uninhabited.
     // This also reports unreachable sub-patterns though, so we can't just replace it with an