Auto merge of #118308 - Nadrieril:sound-exhaustive-patterns-take-3, r=compiler-errors

Don't warn an empty pattern unreachable if we're not sure the data is valid

Exhaustiveness checking used to be naive about the possibility of a place containing invalid data. This could cause it to emit an "unreachable pattern" lint on an arm that was in fact reachable, as in https://github.com/rust-lang/rust/issues/117119.

This PR fixes that. We now track whether a place that is matched on may hold invalid data. This also forced me to be extra precise about how exhaustiveness manages empty types.

Note that this now errs in the opposite direction: the following arm is truly unreachable (because the binding causes a read of the value) but not linted as such. I'd rather not recommend writing a `match ... {}` that has the implicit side-effect of loading the value. [Never patterns](https://github.com/rust-lang/rust/issues/118155) will solve this cleanly.
```rust
match union.value {
    _x => unreachable!(),
}
```

I recommend reviewing commit by commit. I went all-in on the test suite because this went through a lot of iterations and I kept everything. The bit I'm least confident in is `is_known_valid_scrutinee` in `check_match.rs`.

Fixes https://github.com/rust-lang/rust/issues/117119.

compiler/rustc_middle/src/ty/inhabitedness/inhabited_predicate.rs

@@ -1,3 +1,5 @@
+use smallvec::SmallVec;
+
 use crate::ty::context::TyCtxt;
 use crate::ty::{self, DefId, ParamEnv, Ty};
 
@@ -31,27 +33,31 @@ impl<'tcx> InhabitedPredicate<'tcx> {
     /// Returns true if the corresponding type is inhabited in the given
     /// `ParamEnv` and module
     pub fn apply(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>, module_def_id: DefId) -> bool {
-        let Ok(result) = self
-            .apply_inner::<!>(tcx, param_env, &|id| Ok(tcx.is_descendant_of(module_def_id, id)));
+        let Ok(result) = self.apply_inner::<!>(tcx, param_env, &mut Default::default(), &|id| {
+            Ok(tcx.is_descendant_of(module_def_id, id))
+        });
         result
     }
 
     /// Same as `apply`, but returns `None` if self contains a module predicate
     pub fn apply_any_module(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> Option<bool> {
-        self.apply_inner(tcx, param_env, &|_| Err(())).ok()
+        self.apply_inner(tcx, param_env, &mut Default::default(), &|_| Err(())).ok()
     }
 
     /// Same as `apply`, but `NotInModule(_)` predicates yield `false`. That is,
     /// privately uninhabited types are considered always uninhabited.
     pub fn apply_ignore_module(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> bool {
-        let Ok(result) = self.apply_inner::<!>(tcx, param_env, &|_| Ok(true));
+        let Ok(result) =
+            self.apply_inner::<!>(tcx, param_env, &mut Default::default(), &|_| Ok(true));
         result
     }
 
-    fn apply_inner<E>(
+    #[instrument(level = "debug", skip(tcx, param_env, in_module), ret)]
+    fn apply_inner<E: std::fmt::Debug>(
         self,
         tcx: TyCtxt<'tcx>,
         param_env: ParamEnv<'tcx>,
+        eval_stack: &mut SmallVec<[Ty<'tcx>; 1]>, // for cycle detection
         in_module: &impl Fn(DefId) -> Result<bool, E>,
     ) -> Result<bool, E> {
         match self {
@@ -71,11 +77,25 @@ impl<'tcx> InhabitedPredicate<'tcx> {
                 match normalized_pred {
                     // We don't have more information than we started with, so consider inhabited.
                     Self::GenericType(_) => Ok(true),
-                    pred => pred.apply_inner(tcx, param_env, in_module),
+                    pred => {
+                        // A type which is cyclic when monomorphized can happen here since the
+                        // layout error would only trigger later. See e.g. `tests/ui/sized/recursive-type-2.rs`.
+                        if eval_stack.contains(&t) {
+                            return Ok(true); // Recover; this will error later.
+                        }
+                        eval_stack.push(t);
+                        let ret = pred.apply_inner(tcx, param_env, eval_stack, in_module);
+                        eval_stack.pop();
+                        ret
+                    }
                 }
             }
-            Self::And([a, b]) => try_and(a, b, |x| x.apply_inner(tcx, param_env, in_module)),
-            Self::Or([a, b]) => try_or(a, b, |x| x.apply_inner(tcx, param_env, in_module)),
+            Self::And([a, b]) => {
+                try_and(a, b, |x| x.apply_inner(tcx, param_env, eval_stack, in_module))
+            }
+            Self::Or([a, b]) => {
+                try_or(a, b, |x| x.apply_inner(tcx, param_env, eval_stack, in_module))
+            }
         }
     }
 
@@ -197,7 +217,7 @@ impl<'tcx> InhabitedPredicate<'tcx> {
 
 // this is basically like `f(a)? && f(b)?` but different in the case of
 // `Ok(false) && Err(_) -> Ok(false)`
-fn try_and<T, E>(a: T, b: T, f: impl Fn(T) -> Result<bool, E>) -> Result<bool, E> {
+fn try_and<T, E>(a: T, b: T, mut f: impl FnMut(T) -> Result<bool, E>) -> Result<bool, E> {
     let a = f(a);
     if matches!(a, Ok(false)) {
         return Ok(false);
@@ -209,7 +229,7 @@ fn try_and<T, E>(a: T, b: T, f: impl Fn(T) -> Result<bool, E>) -> Result<bool, E
     }
 }
 
-fn try_or<T, E>(a: T, b: T, f: impl Fn(T) -> Result<bool, E>) -> Result<bool, E> {
+fn try_or<T, E>(a: T, b: T, mut f: impl FnMut(T) -> Result<bool, E>) -> Result<bool, E> {
     let a = f(a);
     if matches!(a, Ok(true)) {
         return Ok(true);

compiler/rustc_middle/src/ty/inhabitedness/mod.rs

@@ -103,6 +103,7 @@ impl<'tcx> VariantDef {
 }
 
 impl<'tcx> Ty<'tcx> {
+    #[instrument(level = "debug", skip(tcx), ret)]
     pub fn inhabited_predicate(self, tcx: TyCtxt<'tcx>) -> InhabitedPredicate<'tcx> {
         match self.kind() {
             // For now, unions are always considered inhabited