Recognise nested tuples/arrays/structs

src/librustc_typeck/check/expr.rs

@@ -723,6 +723,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         );
     }
 
+    fn is_destructuring_place_expr(&self, expr: &'tcx hir::Expr) -> bool {
+        match &expr.kind {
+            ExprKind::Array(comps) | ExprKind::Tup(comps) => {
+                comps.iter().all(|e| self.is_destructuring_place_expr(e))
+            }
+            ExprKind::Struct(_path, fields, rest) => {
+                rest.as_ref().map(|e| self.is_destructuring_place_expr(e)).unwrap_or(true) &&
+                    fields.iter().all(|f| self.is_destructuring_place_expr(&f.expr))
+            }
+            _ => expr.is_syntactic_place_expr(),
+        }
+    }
+
     pub(crate) fn check_lhs_assignable(
         &self,
         lhs: &'tcx hir::Expr,
@@ -736,17 +749,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 DiagnosticId::Error(err_code.into()),
             );
             err.span_label(lhs.span, "cannot assign to this expression");
-            let destructuring_assignment = match &lhs.kind {
-                ExprKind::Array(comps) | ExprKind::Tup(comps) => {
-                    comps.iter().all(|e| e.is_syntactic_place_expr())
-                }
-                ExprKind::Struct(_path, fields, rest) => {
-                    rest.as_ref().map(|e| e.is_syntactic_place_expr()).unwrap_or(true) &&
-                        fields.iter().all(|f| f.expr.is_syntactic_place_expr())
-                }
-                _ => false,
-            };
-            if destructuring_assignment {
+            if self.is_destructuring_place_expr(lhs) {
                 err.note("destructuring assignments are not yet supported");
                 err.note(
                     "for more information, see https://github.com/rust-lang/rfcs/issues/372",

src/test/ui/bad/destructuring-assignment.rs

@@ -18,4 +18,8 @@ fn main() {
     //~^ ERROR binary assignment operation `+=` cannot be applied
 
     S { x: a, ..s } = S { x: 3, y: 4 }; //~ ERROR invalid left-hand side of assignment
+
+    let c = 3;
+
+    ((a, b), c) = ((3, 4), 5); //~ ERROR invalid left-hand side of assignment
 }

src/test/ui/bad/destructuring-assignment.stderr

@@ -105,7 +105,18 @@ LL |     S { x: a, ..s } = S { x: 3, y: 4 };
    = note: destructuring assignments are not yet supported
    = note: for more information, see https://github.com/rust-lang/rfcs/issues/372
 
-error: aborting due to 10 previous errors
+error[E0070]: invalid left-hand side of assignment
+  --> $DIR/destructuring-assignment.rs:24:5
+   |
+LL |     ((a, b), c) = ((3, 4), 5);
+   |     -----------^^^^^^^^^^^^^^
+   |     |
+   |     cannot assign to this expression
+   |
+   = note: destructuring assignments are not yet supported
+   = note: for more information, see https://github.com/rust-lang/rfcs/issues/372
+
+error: aborting due to 11 previous errors
 
 Some errors have detailed explanations: E0067, E0070, E0368.
 For more information about an error, try `rustc --explain E0067`.