Special-case raw ref op for diverging check in HIR typeck

compiler/rustc_hir_typeck/src/expr.rs

@@ -237,9 +237,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => self.warn_if_unreachable(expr.hir_id, expr.span, "expression"),
         }
 
-        // Any expression that produces a value of type `!` must have diverged
+        // Any expression that produces a value of type `!` must have diverged,
+        // unless it's the place of a raw ref expr.
         if ty.is_never() {
-            self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
+            if let hir::Node::Expr(hir::Expr {
+                kind: hir::ExprKind::AddrOf(hir::BorrowKind::Raw, ..),
+                ..
+            }) = self.tcx.parent_hir_node(expr.hir_id)
+            {
+                // Taking a raw ref does not constitute a read, so this expression
+                // evaluating to `!` does not guarantee that the program diverges.
+                // So do nothing here.
+            } else {
+                self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
+            }
         }
 
         // Record the type, which applies it effects.

tests/ui/never_type/diverging-place-match.rs

@@ -0,0 +1,15 @@
+//@ known-bug: #117288
+//@ check-pass
+
+#![feature(never_type)]
+
+fn make_up_a_value<T>() -> T {
+    unsafe {
+        let x: *const ! = 0 as _;
+        let _: ! = *x;
+        // Since `*x` "diverges" in HIR, but doesn't count as a read in MIR, this
+        // is unsound since we act as if it diverges but it doesn't.
+    }
+}
+
+fn main() {}

tests/ui/raw-ref-op/never-place-isnt-diverging.rs

@@ -0,0 +1,13 @@
+#![feature(never_type)]
+
+fn make_up_a_value<T>() -> T {
+    unsafe {
+    //~^ ERROR mismatched types
+        let x: *const ! = 0 as _;
+        &raw const *x;
+        // Since `*x` is `!`, HIR typeck used to think that it diverges
+        // and allowed the block to coerce to any value, leading to UB.
+    }
+}
+
+fn main() {}

tests/ui/raw-ref-op/never-place-isnt-diverging.stderr

@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+  --> $DIR/never-place-isnt-diverging.rs:4:5
+   |
+LL |   fn make_up_a_value<T>() -> T {
+   |                      - expected this type parameter
+LL | /     unsafe {
+LL | |
+LL | |         let x: *const ! = 0 as _;
+LL | |         &raw const *x;
+LL | |         // Since `*x` is `!`, HIR typeck used to think that it diverges
+LL | |         // and allowed the block to coerce to any value, leading to UB.
+LL | |     }
+   | |_____^ expected type parameter `T`, found `()`
+   |
+   = note: expected type parameter `T`
+                   found unit type `()`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.