Rollup merge of rust-lang#128443 - compiler-errors:async-unreachable, r=fmease

Properly mark loop as diverging if it has no breaks

Due to specifics about the desugaring of the `.await` operator, HIR typeck doesn't recognize that `.await`ing an `impl Future<Output = !>` will diverge in the same way as calling a `fn() -> !`.

This is because the await operator desugars to approximately:

```rust
loop {
    match future.poll(...) {
        Poll::Ready(x) => break x,
        Poll::Pending => {}
    }
}
```

We know that the value of `x` is `!`, however since `break` is a coercion site, we coerce `!` to some `?0` (the type of the loop expression). Then since the type of the `loop {...}` expression is `?0`, we will not detect the loop as diverging like we do with other expressions that evaluate to `!`:

https://github.com/rust-lang/rust/blob/0b5eb7ba7bd796fb39c8bb6acd9ef6c140f28b65/compiler/rustc_hir_typeck/src/expr.rs#L240-L243

We can technically fix this in two ways:
1. Make coercion of loop exprs more eagerly result in a type of `!` when the only break expressions have type `!`.
2. Make loops understand that all of that if they have only diverging break values, then the loop diverges as well.

(1.) likely has negative effects on inference, and seems like a weird special case to drill into coercion. However, it turns out that (2.) is very easy to implement, we already record whether a loop has any break expressions, and when we do so, we actually skip over any break expressions with diverging values!:

https://github.com/rust-lang/rust/blob/0b5eb7ba7bd796fb39c8bb6acd9ef6c140f28b65/compiler/rustc_hir_typeck/src/expr.rs#L713-L716

Thus, we can consider the loop as diverging if we see that it has no breaks, which is the change implemented in this PR.

This is not usually a problem in regular code for two reasons:
1. In regular code, we already mark `break diverging()` as unreachable if `diverging()` is unreachable. We don't do this for `.await`, since we suppress unreachable errors within `.await` (rust-lang#64930). Un-suppressing this code will result in spurious unreachable expression errors pointing to internal await machinery.
3. In loops that truly have no breaks (e.g. `loop {}`), we already evaluate the type of the loop to `!`, so this special case is kinda moot. This only affects loops that have `break`s with values of type `!`.

Thus, this seems like a change that may affect more code than just `.await`, but it likely does not in meaningful ways; if it does, it's certainly correct to apply.

Fixes rust-lang#128434

Author: matthiaskrgr

compiler/rustc_hir_typeck/src/expr.rs

@@ -1306,6 +1306,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // No way to know whether it's diverging because
             // of a `break` or an outer `break` or `return`.
             self.diverges.set(Diverges::Maybe);
+        } else {
+            self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
         }
 
         // If we permit break with a value, then result type is

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -48,30 +48,42 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Produces warning on the given node, if the current point in the
     /// function is unreachable, and there hasn't been another warning.
     pub(crate) fn warn_if_unreachable(&self, id: HirId, span: Span, kind: &str) {
-        // FIXME: Combine these two 'if' expressions into one once
-        // let chains are implemented
-        if let Diverges::Always { span: orig_span, custom_note } = self.diverges.get() {
-            // If span arose from a desugaring of `if` or `while`, then it is the condition itself,
-            // which diverges, that we are about to lint on. This gives suboptimal diagnostics.
-            // Instead, stop here so that the `if`- or `while`-expression's block is linted instead.
-            if !span.is_desugaring(DesugaringKind::CondTemporary)
-                && !span.is_desugaring(DesugaringKind::Async)
-                && !orig_span.is_desugaring(DesugaringKind::Await)
-            {
-                self.diverges.set(Diverges::WarnedAlways);
+        // If span arose from a desugaring of `if` or `while`, then it is the condition itself,
+        // which diverges, that we are about to lint on. This gives suboptimal diagnostics.
+        // Instead, stop here so that the `if`- or `while`-expression's block is linted instead.
+        if span.is_desugaring(DesugaringKind::CondTemporary) {
+            return;
+        }
 
-                debug!("warn_if_unreachable: id={:?} span={:?} kind={}", id, span, kind);
+        // Don't lint if the result of an async block or async function is `!`.
+        // This does not affect the unreachable lints *within* the body.
+        if span.is_desugaring(DesugaringKind::Async) {
+            return;
+        }
 
-                let msg = format!("unreachable {kind}");
-                self.tcx().node_span_lint(lint::builtin::UNREACHABLE_CODE, id, span, |lint| {
-                    lint.primary_message(msg.clone());
-                    lint.span_label(span, msg).span_label(
-                        orig_span,
-                        custom_note.unwrap_or("any code following this expression is unreachable"),
-                    );
-                })
-            }
+        // Don't lint *within* the `.await` operator, since that's all just desugaring junk.
+        // We only want to lint if there is a subsequent expression after the `.await`.
+        if span.is_desugaring(DesugaringKind::Await) {
+            return;
         }
+
+        let Diverges::Always { span: orig_span, custom_note } = self.diverges.get() else {
+            return;
+        };
+
+        // Don't warn twice.
+        self.diverges.set(Diverges::WarnedAlways);
+
+        debug!("warn_if_unreachable: id={:?} span={:?} kind={}", id, span, kind);
+
+        let msg = format!("unreachable {kind}");
+        self.tcx().node_span_lint(lint::builtin::UNREACHABLE_CODE, id, span, |lint| {
+            lint.primary_message(msg.clone());
+            lint.span_label(span, msg).span_label(
+                orig_span,
+                custom_note.unwrap_or("any code following this expression is unreachable"),
+            );
+        })
     }
 
     /// Resolves type and const variables in `ty` if possible. Unlike the infcx

tests/ui/async-await/unreachable-lint-2.rs

@@ -0,0 +1,15 @@
+//@ edition:2018
+
+#![deny(unreachable_code)]
+
+async fn foo() {
+    endless().await;
+    println!("this is unreachable!");
+    //~^ ERROR unreachable statement
+}
+
+async fn endless() -> ! {
+    loop {}
+}
+
+fn main() { }

tests/ui/async-await/unreachable-lint-2.stderr

@@ -0,0 +1,17 @@
+error: unreachable statement
+  --> $DIR/unreachable-lint-2.rs:7:5
+   |
+LL |     endless().await;
+   |               ----- any code following this expression is unreachable
+LL |     println!("this is unreachable!");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable statement
+   |
+note: the lint level is defined here
+  --> $DIR/unreachable-lint-2.rs:3:9
+   |
+LL | #![deny(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+   = note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+