Rollup merge of #86206 - FabianWolff:issue-86188, r=Mark-Simulacrum

Fix type checking of return expressions outside of function bodies

This pull request fixes #86188. The problem is that the current code for type-checking `return` expressions stops if the `return` occurs outside of a function body, while the correct behavior is to continue type-checking the return value expression (otherwise an ICE happens later on because variables declared in the return value expression don't have a type).

Also, I have noticed that it is sometimes not obvious why a `return` is outside of a function body; for instance, in the example from #86188 (which currently causes an ICE):
```rust
fn main() {
    [(); return || {
        let tx;
    }]
}
```
I have changed the error message to also explain why the `return` is considered outside of the function body:
```
error[E0572]: return statement outside of function body
 --> ice0.rs:2:10
  |
1 |  / fn main() {
2 |  |     [(); return || {
  |  |__________^
3 | ||         let tx;
4 | ||     }]
  | ||_____^ the return is part of this body...
5 |  | }
  |  |_- ...not the enclosing function body
```

compiler/rustc_typeck/src/check/expr.rs

@@ -675,7 +675,39 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expr: &'tcx hir::Expr<'tcx>,
     ) -> Ty<'tcx> {
         if self.ret_coercion.is_none() {
-            self.tcx.sess.emit_err(ReturnStmtOutsideOfFnBody { span: expr.span });
+            let mut err = ReturnStmtOutsideOfFnBody {
+                span: expr.span,
+                encl_body_span: None,
+                encl_fn_span: None,
+            };
+
+            let encl_item_id = self.tcx.hir().get_parent_item(expr.hir_id);
+            let encl_item = self.tcx.hir().expect_item(encl_item_id);
+
+            if let hir::ItemKind::Fn(..) = encl_item.kind {
+                // We are inside a function body, so reporting "return statement
+                // outside of function body" needs an explanation.
+
+                let encl_body_owner_id = self.tcx.hir().enclosing_body_owner(expr.hir_id);
+
+                // If this didn't hold, we would not have to report an error in
+                // the first place.
+                assert_ne!(encl_item_id, encl_body_owner_id);
+
+                let encl_body_id = self.tcx.hir().body_owned_by(encl_body_owner_id);
+                let encl_body = self.tcx.hir().body(encl_body_id);
+
+                err.encl_body_span = Some(encl_body.value.span);
+                err.encl_fn_span = Some(encl_item.span);
+            }
+
+            self.tcx.sess.emit_err(err);
+
+            if let Some(e) = expr_opt {
+                // We still have to type-check `e` (issue #86188), but calling
+                // `check_return_expr` only works inside fn bodies.
+                self.check_expr(e);
+            }
         } else if let Some(e) = expr_opt {
             if self.ret_coercion_span.get().is_none() {
                 self.ret_coercion_span.set(Some(e.span));

compiler/rustc_typeck/src/errors.rs

@@ -147,6 +147,10 @@ pub struct TypeofReservedKeywordUsed {
 pub struct ReturnStmtOutsideOfFnBody {
     #[message = "return statement outside of function body"]
     pub span: Span,
+    #[label = "the return is part of this body..."]
+    pub encl_body_span: Option<Span>,
+    #[label = "...not the enclosing function body"]
+    pub encl_fn_span: Option<Span>,
 }
 
 #[derive(SessionDiagnostic)]

src/test/ui/issues/issue-51714.rs

@@ -1,13 +1,21 @@
 fn main() {
+//~^ NOTE: not the enclosing function body
+//~| NOTE: not the enclosing function body
+//~| NOTE: not the enclosing function body
+//~| NOTE: not the enclosing function body
     |_:  [_; return || {}] | {};
-    //~^ ERROR return statement outside of function body
+    //~^ ERROR: return statement outside of function body [E0572]
+    //~| NOTE: the return is part of this body...
 
     [(); return || {}];
-    //~^ ERROR return statement outside of function body
+    //~^ ERROR: return statement outside of function body [E0572]
+    //~| NOTE: the return is part of this body...
 
     [(); return |ice| {}];
-    //~^ ERROR return statement outside of function body
+    //~^ ERROR: return statement outside of function body [E0572]
+    //~| NOTE: the return is part of this body...
 
     [(); return while let Some(n) = Some(0) {}];
-    //~^ ERROR return statement outside of function body
+    //~^ ERROR: return statement outside of function body [E0572]
+    //~| NOTE: the return is part of this body...
 }

src/test/ui/issues/issue-51714.stderr

@@ -1,26 +1,62 @@
 error[E0572]: return statement outside of function body
-  --> $DIR/issue-51714.rs:2:14
+  --> $DIR/issue-51714.rs:6:14
    |
-LL |     |_:  [_; return || {}] | {};
-   |              ^^^^^^^^^^^^
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+LL | |
+LL | |     |_:  [_; return || {}] | {};
+   | |              ^^^^^^^^^^^^ the return is part of this body...
+...  |
+LL | |
+LL | | }
+   | |_- ...not the enclosing function body
 
 error[E0572]: return statement outside of function body
-  --> $DIR/issue-51714.rs:5:10
+  --> $DIR/issue-51714.rs:10:10
    |
-LL |     [(); return || {}];
-   |          ^^^^^^^^^^^^
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     [(); return || {}];
+   | |          ^^^^^^^^^^^^ the return is part of this body...
+...  |
+LL | |
+LL | | }
+   | |_- ...not the enclosing function body
 
 error[E0572]: return statement outside of function body
-  --> $DIR/issue-51714.rs:8:10
+  --> $DIR/issue-51714.rs:14:10
    |
-LL |     [(); return |ice| {}];
-   |          ^^^^^^^^^^^^^^^
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     [(); return |ice| {}];
+   | |          ^^^^^^^^^^^^^^^ the return is part of this body...
+...  |
+LL | |
+LL | | }
+   | |_- ...not the enclosing function body
 
 error[E0572]: return statement outside of function body
-  --> $DIR/issue-51714.rs:11:10
+  --> $DIR/issue-51714.rs:18:10
    |
-LL |     [(); return while let Some(n) = Some(0) {}];
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     [(); return while let Some(n) = Some(0) {}];
+   | |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the return is part of this body...
+LL | |
+LL | |
+LL | | }
+   | |_- ...not the enclosing function body
 
 error: aborting due to 4 previous errors
 

src/test/ui/return/issue-86188-return-not-in-fn-body.rs

@@ -0,0 +1,22 @@
+// Due to a compiler bug, if a return occurs outside of a function body
+// (e.g. in an AnonConst body), the return value expression would not be
+// type-checked, leading to an ICE. This test checks that the ICE no
+// longer happens, and that an appropriate error message is issued that
+// also explains why the return is considered "outside of a function body"
+// if it seems to be inside one, as in the main function below.
+
+const C: [(); 42] = {
+    [(); return || {
+    //~^ ERROR: return statement outside of function body [E0572]
+        let tx;
+    }]
+};
+
+fn main() {
+//~^ NOTE: ...not the enclosing function body
+    [(); return || {
+    //~^ ERROR: return statement outside of function body [E0572]
+    //~| NOTE: the return is part of this body...
+        let tx;
+    }];
+}

src/test/ui/return/issue-86188-return-not-in-fn-body.stderr

@@ -0,0 +1,28 @@
+error[E0572]: return statement outside of function body
+  --> $DIR/issue-86188-return-not-in-fn-body.rs:9:10
+   |
+LL |       [(); return || {
+   |  __________^
+LL | |
+LL | |         let tx;
+LL | |     }]
+   | |_____^
+
+error[E0572]: return statement outside of function body
+  --> $DIR/issue-86188-return-not-in-fn-body.rs:17:10
+   |
+LL |  / fn main() {
+LL |  |
+LL |  |     [(); return || {
+   |  |__________^
+LL | ||
+LL | ||
+LL | ||         let tx;
+LL | ||     }];
+   | ||_____^ the return is part of this body...
+LL |  | }
+   |  |_- ...not the enclosing function body
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0572`.

src/test/ui/return/return-match-array-const.rs

@@ -1,10 +1,19 @@
 fn main() {
+//~^ NOTE: not the enclosing function body
+//~| NOTE: not the enclosing function body
+//~| NOTE: not the enclosing function body
     [(); return match 0 { n => n }];
-    //~^ ERROR: return statement outside of function body
+    //~^ ERROR: return statement outside of function body [E0572]
+    //~| NOTE: the return is part of this body...
 
     [(); return match 0 { 0 => 0 }];
-    //~^ ERROR: return statement outside of function body
+    //~^ ERROR: return statement outside of function body [E0572]
+    //~| NOTE: the return is part of this body...
 
     [(); return match () { 'a' => 0, _ => 0 }];
-    //~^ ERROR: return statement outside of function body
+    //~^ ERROR: return statement outside of function body [E0572]
+    //~| NOTE: the return is part of this body...
+    //~| ERROR: mismatched types [E0308]
+    //~| NOTE: expected `()`, found `char`
+    //~| NOTE: this expression has type `()`
 }

src/test/ui/return/return-match-array-const.stderr

@@ -1,21 +1,56 @@
-error[E0572]: return statement outside of function body
-  --> $DIR/return-match-array-const.rs:2:10
-   |
-LL |     [(); return match 0 { n => n }];
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0572]: return statement outside of function body
   --> $DIR/return-match-array-const.rs:5:10
    |
-LL |     [(); return match 0 { 0 => 0 }];
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+LL | |     [(); return match 0 { n => n }];
+   | |          ^^^^^^^^^^^^^^^^^^^^^^^^^ the return is part of this body...
+...  |
+LL | |
+LL | | }
+   | |_- ...not the enclosing function body
 
 error[E0572]: return statement outside of function body
-  --> $DIR/return-match-array-const.rs:8:10
+  --> $DIR/return-match-array-const.rs:9:10
+   |
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     [(); return match 0 { 0 => 0 }];
+   | |          ^^^^^^^^^^^^^^^^^^^^^^^^^ the return is part of this body...
+...  |
+LL | |
+LL | | }
+   | |_- ...not the enclosing function body
+
+error[E0572]: return statement outside of function body
+  --> $DIR/return-match-array-const.rs:13:10
+   |
+LL | / fn main() {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     [(); return match () { 'a' => 0, _ => 0 }];
+   | |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the return is part of this body...
+...  |
+LL | |
+LL | | }
+   | |_- ...not the enclosing function body
+
+error[E0308]: mismatched types
+  --> $DIR/return-match-array-const.rs:13:28
    |
 LL |     [(); return match () { 'a' => 0, _ => 0 }];
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                       --   ^^^ expected `()`, found `char`
+   |                       |
+   |                       this expression has type `()`
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0572`.
+Some errors have detailed explanations: E0308, E0572.
+For more information about an error, try `rustc --explain E0308`.