Rollup merge of rust-lang#115313 - gurry:issue-114918-cycle-detected,…

… r=compiler-errors

Make `get_return_block()` return `Some` only for HIR nodes in body

Fixes rust-lang#114918

The issue occurred while compiling the following input:

```rust
fn uwu() -> [(); { () }] {
    loop {}
}
```

It was caused by the code below trying to suggest a missing return type which resulted in a const eval cycle: https://github.com/rust-lang/rust/blob/1bd043098e05839afb557bd7a2858cb09a4054ca/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs#L68-L75

The root cause was `get_return_block()` returning an `Fn` node for a node in the return type (i.e. the second `()` in the return type `[(); { () }]` of the input) although it is supposed to do so only for nodes that lie in the body of the function and return `None` otherwise (at least as per my understanding).

The PR fixes the issue by fixing this behaviour of `get_return_block()`.

compiler/rustc_middle/src/hir/map/mod.rs

@@ -701,6 +701,8 @@ impl<'hir> Map<'hir> {
             // expressions.
             ignore_tail = true;
         }
+
+        let mut prev_hir_id = None;
         while let Some((hir_id, node)) = iter.next() {
             if let (Some((_, next_node)), false) = (iter.peek(), ignore_tail) {
                 match next_node {
@@ -715,14 +717,23 @@ impl<'hir> Map<'hir> {
                 | Node::ForeignItem(_)
                 | Node::TraitItem(_)
                 | Node::Expr(Expr { kind: ExprKind::Closure { .. }, .. })
-                | Node::ImplItem(_) => return Some(hir_id),
+                | Node::ImplItem(_)
+                    // The input node `id` must be enclosed in the method's body as opposed
+                    // to some other place such as its return type (fixes #114918).
+                    // We verify that indirectly by checking that the previous node is the
+                    // current node's body
+                    if node.body_id().map(|b| b.hir_id) == prev_hir_id =>  {
+                        return Some(hir_id)
+                }
                 // Ignore `return`s on the first iteration
                 Node::Expr(Expr { kind: ExprKind::Loop(..) | ExprKind::Ret(..), .. })
                 | Node::Local(_) => {
                     return None;
                 }
                 _ => {}
             }
+
+            prev_hir_id = Some(hir_id);
         }
         None
     }

tests/ui/typeck/issue-114918/const-in-fn-return-type.rs

@@ -0,0 +1,10 @@
+// Regression test for #114918
+// Test that a const generic enclosed in a block within a return type
+// produces a type mismatch error instead of triggering a const eval cycle
+
+#[allow(unused_braces)]
+fn func() -> [u8; { () } ] { //~ ERROR mismatched types
+    loop {}
+}
+
+fn main() {}

tests/ui/typeck/issue-114918/const-in-fn-return-type.stderr

@@ -0,0 +1,9 @@
+error[E0308]: mismatched types
+  --> $DIR/const-in-fn-return-type.rs:6:21
+   |
+LL | fn func() -> [u8; { () } ] {
+   |                     ^^ expected `usize`, found `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/typeck/issue-114918/const-in-impl-fn-return-type.rs

@@ -0,0 +1,20 @@
+// Regression test for #114918
+// Test that a const generic enclosed in a block within the return type
+// of an impl fn produces a type mismatch error instead of triggering
+// a const eval cycle
+
+
+trait Trait {
+    fn func<const N: u32>() -> [ (); N ];
+}
+
+struct S {}
+
+#[allow(unused_braces)]
+impl Trait for S {
+    fn func<const N: u32>() -> [ (); { () }] { //~ ERROR mismatched types
+        N
+    }
+}
+
+fn main() {}

tests/ui/typeck/issue-114918/const-in-impl-fn-return-type.stderr

@@ -0,0 +1,9 @@
+error[E0308]: mismatched types
+  --> $DIR/const-in-impl-fn-return-type.rs:15:40
+   |
+LL |     fn func<const N: u32>() -> [ (); { () }] {
+   |                                        ^^ expected `usize`, found `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/typeck/issue-114918/const-in-struct-type-arg.rs

@@ -0,0 +1,12 @@
+// Regression test for #114918
+// Test that a const generic enclosed in a block in a struct's type arg
+// produces a type mismatch error instead of triggering a const eval cycle
+
+#[allow(unused_braces)]
+struct S<const N: usize> {
+        arr: [u8; N]
+}
+
+fn main() {
+    let s = S::<{ () }> { arr: [5, 6, 7]}; //~ ERROR mismatched types
+}

tests/ui/typeck/issue-114918/const-in-struct-type-arg.stderr

@@ -0,0 +1,9 @@
+error[E0308]: mismatched types
+  --> $DIR/const-in-struct-type-arg.rs:11:19
+   |
+LL |     let s = S::<{ () }> { arr: [5, 6, 7]};
+   |                   ^^ expected `usize`, found `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/typeck/issue-114918/const-in-trait-fn-return-type.rs

@@ -0,0 +1,13 @@
+// Regression test for #114918
+// Test that a const generic enclosed in a block within the return type
+// of a trait method produces a type mismatch error instead of triggering
+// a const eval cycle
+
+#[allow(unused_braces)]
+trait Trait {
+    fn func<const N: u32>() -> [ (); { () }] { //~ ERROR mismatched types
+        N
+    }
+}
+
+fn main() {}

tests/ui/typeck/issue-114918/const-in-trait-fn-return-type.stderr

@@ -0,0 +1,9 @@
+error[E0308]: mismatched types
+  --> $DIR/const-in-trait-fn-return-type.rs:8:40
+   |
+LL |     fn func<const N: u32>() -> [ (); { () }] {
+   |                                        ^^ expected `usize`, found `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.