don't warn on explicit casts of never to any

compiler/rustc_hir_typeck/src/expr.rs

@@ -290,6 +290,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             | ExprKind::Let(..)
             | ExprKind::Loop(..)
             | ExprKind::Match(..) => {}
+            // Do not warn on `as` casts from never to any,
+            // they are sometimes required to appeal typeck.
+            ExprKind::Cast(_, _) => {}
             // If `expr` is a result of desugaring the try block and is an ok-wrapped
             // diverging expression (e.g. it arose from desugaring of `try { return }`),
             // we skip issuing a warning because it is autogenerated code.

tests/ui/reachable/expr_cast.rs

@@ -1,12 +1,21 @@
+//@ check-pass
 //@ edition: 2024
+//
+// Check that we don't warn on `as` casts of never to any as unreachable.
+// While they *are* unreachable, sometimes they are required to appeal typeck.
 #![deny(unreachable_code)]
 
 fn a() {
-    _ = {return} as u32; //~ error: unreachable
+    _ = {return} as u32;
 }
 
 fn b() {
-    (return) as u32; //~ error: unreachable
+    (return) as u32;
+}
+
+// example that needs an explicit never-to-any `as` cast
+fn example() -> impl Iterator<Item = u8> {
+    todo!() as std::iter::Empty<_>
 }
 
 fn main() {}

tests/ui/reachable/expr_cast.stderr

@@ -1,26 +0,0 @@
-error: unreachable expression
-  --> $DIR/expr_cast.rs:5:9
-   |
-LL |     _ = {return} as u32;
-   |         ^------^^^^^^^^
-   |         ||
-   |         |any code following this expression is unreachable
-   |         unreachable expression
-   |
-note: the lint level is defined here
-  --> $DIR/expr_cast.rs:2:9
-   |
-LL | #![deny(unreachable_code)]
-   |         ^^^^^^^^^^^^^^^^
-
-error: unreachable expression
-  --> $DIR/expr_cast.rs:9:5
-   |
-LL |     (return) as u32;
-   |     --------^^^^^^^
-   |     |
-   |     unreachable expression
-   |     any code following this expression is unreachable
-
-error: aborting due to 2 previous errors
-

tests/ui/reachable/unreachable-try-pattern.rs

@@ -18,7 +18,7 @@ fn bar(x: Result<!, i32>) -> Result<u32, i32> {
 fn foo(x: Result<!, i32>) -> Result<u32, i32> {
     let y = (match x { Ok(n) => Ok(n as u32), Err(e) => Err(e) })?;
     //~^ WARN unreachable pattern
-    //~| WARN unreachable expression
+    //~| WARN unreachable call
     Ok(y)
 }
 

tests/ui/reachable/unreachable-try-pattern.stderr

@@ -1,11 +1,10 @@
-warning: unreachable expression
-  --> $DIR/unreachable-try-pattern.rs:19:36
+warning: unreachable call
+  --> $DIR/unreachable-try-pattern.rs:19:33
    |
 LL |     let y = (match x { Ok(n) => Ok(n as u32), Err(e) => Err(e) })?;
-   |                                    -^^^^^^^
-   |                                    |
-   |                                    unreachable expression
-   |                                    any code following this expression is unreachable
+   |                                 ^^ - any code following this expression is unreachable
+   |                                 |
+   |                                 unreachable call
    |
 note: the lint level is defined here
   --> $DIR/unreachable-try-pattern.rs:3:9