Do not emit E0277 on incorrect tuple destructured binding

src/librustc_typeck/check/_match.rs

@@ -298,11 +298,19 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  TypeVariableOrigin::TypeInference(pat.span)));
  let element_tys = tcx.mk_type_list(element_tys_iter);
  let pat_ty = tcx.mk_ty(ty::Tuple(element_tys));
- self.demand_eqtype(pat.span, expected, pat_ty);
- for (i, elem) in elements.iter().enumerate_and_adjust(max_len, ddpos) {
- self.check_pat_walk(elem, &element_tys[i], def_bm, true);
+ if let Some(mut err) = self.demand_eqtype_diag(pat.span, expected, pat_ty) {
+ err.emit();
+ let element_tys_iter = (0..max_len).map(|_| tcx.types.err);
+ for (_, elem) in elements.iter().enumerate_and_adjust(max_len, ddpos) {
+ self.check_pat_walk(elem, &tcx.types.err, def_bm, true);
+ }
+ tcx.mk_tup(element_tys_iter)
+ } else {
+ for (i, elem) in elements.iter().enumerate_and_adjust(max_len, ddpos) {
+ self.check_pat_walk(elem, &element_tys[i], def_bm, true);
+ }
+ pat_ty
  }
- pat_ty
  }
  PatKind::Box(ref inner) => {
  let inner_ty = self.next_ty_var(TypeVariableOrigin::TypeInference(inner.span));

src/test/ui/elide-errors-on-mismatched-tuple.rs

@@ -0,0 +1,28 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Hide irrelevant E0277 errors (#50333)
+
+trait T {}
+
+struct A;
+impl T for A {}
+impl A {
+ fn new() -> Self {
+ Self {}
+ }
+}
+
+fn main() {
+ let (a, b, c) = (A::new(), A::new()); // This tuple is 2 elements, should be three
+ //~^ ERROR mismatched types
+ let ts: Vec<&T> = vec![&a, &b, &c];
+ // There is no E0277 error above, as `a`, `b` and `c` are `TyErr`
+}

src/test/ui/elide-errors-on-mismatched-tuple.stderr

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+ --> $DIR/elide-errors-on-mismatched-tuple.rs:24:9
+ |
+LL | let (a, b, c) = (A::new(), A::new()); // This tuple is 2 elements, should be three
+ | ^^^^^^^^^ expected a tuple with 2 elements, found one with 3 elements
+ |
+ = note: expected type `(A, A)`
+ found type `(_, _, _)`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.