add parentheses when unboxing suggestion needed

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -2648,15 +2648,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     }
 
                     let make_sugg = |expr: &Expr<'_>, span: Span, sugg: &str| {
-                        let needs_parens = match expr.kind {
-                            // parenthesize if needed (Issue #46756)
-                            hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
-                            // parenthesize borrows of range literals (Issue #54505)
-                            _ if is_range_literal(expr) => true,
-                            _ => false,
-                        };
-
-                        if needs_parens {
+                        if self.needs_parentheses(expr) {
                             (
                                 vec![
                                     (span.shrink_to_lo(), format!("{prefix}{sugg}(")),
@@ -2869,6 +2861,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             return None;
                         }
 
+                        if self.needs_parentheses(expr) {
+                            return Some((
+                                vec![
+                                    (span, format!("{suggestion}(")),
+                                    (expr.span.shrink_to_hi(), ")".to_string()),
+                                ],
+                                message,
+                                Applicability::MachineApplicable,
+                                true,
+                                false,
+                            ));
+                        }
+
                         return Some((
                             vec![(span, suggestion)],
                             message,
@@ -2897,6 +2902,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         false
     }
 
+    fn needs_parentheses(&self, expr: &hir::Expr<'_>) -> bool {
+        match expr.kind {
+            // parenthesize if needed (Issue #46756)
+            hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
+            // parenthesize borrows of range literals (Issue #54505)
+            _ if is_range_literal(expr) => true,
+            _ => false,
+        }
+    }
+
     pub(crate) fn suggest_cast(
         &self,
         err: &mut Diag<'_>,

tests/ui/coercion/unboxing-needing-parenthases-issue-132924.rs

@@ -0,0 +1,18 @@
+//@ check-fail
+fn main() {
+    let x = Box::new(Some(1));
+
+    let test: Option<i32> = x;
+    //~^ ERROR mismatched types
+    let x = Box::new(Some(1));
+    let test: Option<i32> = { x as Box<Option<i32>> };
+    //~^ ERROR mismatched types
+
+    let x = Box::new(Some(1));
+    let test: Option<i32> = if true { x as Box<Option<i32>> } else { None };
+    //~^ ERROR mismatched types
+
+    let x = std::rc::Rc::new(Some(1));
+    let test: Option<i32> = x as std::rc::Rc<Option<i32>>;
+    //~^ ERROR mismatched types
+}

tests/ui/coercion/unboxing-needing-parenthases-issue-132924.stderr

@@ -0,0 +1,59 @@
+error[E0308]: mismatched types
+  --> $DIR/unboxing-needing-parenthases-issue-132924.rs:5:29
+   |
+LL |     let test: Option<i32> = x;
+   |               -----------   ^ expected `Option<i32>`, found `Box<Option<{integer}>>`
+   |               |
+   |               expected due to this
+   |
+   = note: expected enum `Option<i32>`
+            found struct `Box<Option<{integer}>>`
+help: consider unboxing the value
+   |
+LL |     let test: Option<i32> = *x;
+   |                             +
+
+error[E0308]: mismatched types
+  --> $DIR/unboxing-needing-parenthases-issue-132924.rs:8:31
+   |
+LL |     let test: Option<i32> = { x as Box<Option<i32>> };
+   |                               ^^^^^^^^^^^^^^^^^^^^^ expected `Option<i32>`, found `Box<Option<i32>>`
+   |
+   = note: expected enum `Option<_>`
+            found struct `Box<Option<_>>`
+help: consider unboxing the value
+   |
+LL |     let test: Option<i32> = { *(x as Box<Option<i32>>) };
+   |                               ++                     +
+
+error[E0308]: mismatched types
+  --> $DIR/unboxing-needing-parenthases-issue-132924.rs:12:39
+   |
+LL |     let test: Option<i32> = if true { x as Box<Option<i32>> } else { None };
+   |                                       ^^^^^^^^^^^^^^^^^^^^^ expected `Option<i32>`, found `Box<Option<i32>>`
+   |
+   = note: expected enum `Option<_>`
+            found struct `Box<Option<_>>`
+help: consider unboxing the value
+   |
+LL |     let test: Option<i32> = if true { *(x as Box<Option<i32>>) } else { None };
+   |                                       ++                     +
+
+error[E0308]: mismatched types
+  --> $DIR/unboxing-needing-parenthases-issue-132924.rs:16:29
+   |
+LL |     let test: Option<i32> = x as std::rc::Rc<Option<i32>>;
+   |               -----------   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Option<i32>`, found `Rc<Option<i32>>`
+   |               |
+   |               expected due to this
+   |
+   = note: expected enum `Option<_>`
+            found struct `Rc<Option<_>>`
+help: consider dereferencing the type
+   |
+LL |     let test: Option<i32> = *(x as std::rc::Rc<Option<i32>>);
+   |                             ++                             +
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.