Do not consider match/let/raw-ref of deref that evalautes to ! to diverge

Author: compiler-errors

Diff for: compiler/rustc_hir_typeck/src/expr.rs

@@ -237,9 +237,31 @@ 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, or a scrutinee of a match.
         if ty.is_never() {
-            self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
+            if matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Deref, _)) {
+                match self.tcx.parent_hir_node(expr.hir_id) {
+                    hir::Node::Expr(hir::Expr {
+                        kind: hir::ExprKind::AddrOf(hir::BorrowKind::Raw, ..),
+                        ..
+                    }) => {}
+                    hir::Node::Expr(hir::Expr {
+                        kind: hir::ExprKind::Let(hir::LetExpr { init: target, .. }),
+                        ..
+                    })
+                    | hir::Node::Expr(hir::Expr {
+                        kind: hir::ExprKind::Match(target, _, _), ..
+                    })
+                    | hir::Node::LetStmt(hir::LetStmt { init: Some(target), .. })
+                        if expr.hir_id == target.hir_id => {}
+                    _ => {
+                        self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
+                    }
+                }
+            } else {
+                self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
+            }
         }
 
         // Record the type, which applies it effects.

Diff for: compiler/rustc_hir_typeck/src/pat.rs

@@ -27,6 +27,7 @@ use rustc_trait_selection::traits::{ObligationCause, ObligationCauseCode};
 use ty::VariantDef;
 
 use super::report_unexpected_variant_res;
+use crate::diverges::Diverges;
 use crate::gather_locals::DeclOrigin;
 use crate::{errors, FnCtxt, LoweredTy};
 
@@ -276,6 +277,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         };
 
+        // All other patterns constitute a read, which causes us to diverge
+        // if the type is never.
+        if !matches!(pat.kind, PatKind::Wild | PatKind::Never) {
+            if ty.is_never() {
+                self.diverges.set(self.diverges.get() | Diverges::always(pat.span));
+            }
+        }
+
         self.write_ty(pat.hir_id, ty);
 
         // (note_1): In most of the cases where (note_1) is referenced

Diff for: tests/ui/never_type/diverging-place-match.rs

@@ -0,0 +1,13 @@
+#![feature(never_type)]
+
+fn make_up_a_value<T>() -> T {
+    unsafe {
+    //~^ ERROR mismatched types
+        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() {}

Diff for: tests/ui/never_type/diverging-place-match.stderr

@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.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 | |         let _: ! = *x;
+LL | |         // Since `*x` "diverges" in HIR, but doesn't count as a read in MIR, this
+LL | |         // is unsound since we act as if it diverges but it doesn't.
+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`.

Diff for: 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() {}

Diff for: 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`.