Ensure inferred let pattern types are well-formed

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -902,6 +902,21 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Type check the pattern. Override if necessary to avoid knock-on errors.
         self.check_pat_top(decl.pat, decl_ty, ty_span, origin_expr, Some(decl.origin));
+        let mut pat_has_ref_binding = false;
+        decl.pat.walk_short(|pat| {
+            pat_has_ref_binding |= matches!(
+                pat.kind,
+                hir::PatKind::Binding(hir::BindingMode(hir::ByRef::Yes(..), _), ..)
+            );
+            !pat_has_ref_binding
+        });
+        if decl.ty.is_none() && pat_has_ref_binding {
+            self.register_wf_obligation(
+                decl_ty.into(),
+                decl.pat.span,
+                ObligationCauseCode::WellFormed(None),
+            );
+        }
         let pat_ty = self.node_ty(decl.pat.hir_id);
         self.overwrite_local_ty_if_err(decl.hir_id, decl.pat, pat_ty);
 

tests/ui/wf/let-pat-inferred-non-wf.rs

@@ -0,0 +1,53 @@
+#![allow(unused)]
+
+struct S<T: ?Sized>(T);
+
+fn should_fail_1() {
+    let ref y @ [ref x, _]; //~ ERROR E0277
+    x = "";
+}
+
+fn should_fail_2() {
+    let [ref x]; //~ ERROR E0277
+    x = "";
+}
+
+fn should_fail_3() {
+    let [[ref x], [_, y @ ..]]; //~ ERROR E0277
+    x = "";
+    y = [];
+}
+
+fn should_fail_4() {
+    let [(ref a, b), x]; //~ ERROR E0277
+    a = "";
+    b = 5;
+}
+
+fn should_fail_5() {
+    let (ref a, b); //~ ERROR E0277
+    a = "";
+    b = 5;
+}
+
+fn should_fail_6() {
+    let [S(ref x)]; //~ ERROR E0277
+    x = "";
+}
+
+fn should_pass_1() {
+    let ref x;
+    x = "";
+}
+
+fn should_pass_2() {
+    let ref y @ (ref x,);
+    x = "";
+}
+
+fn should_pass_3() {
+    let S(ref x);
+    x = "";
+}
+
+fn main() {}

tests/ui/wf/let-pat-inferred-non-wf.stderr

@@ -0,0 +1,62 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/let-pat-inferred-non-wf.rs:6:9
+   |
+LL |     let ref y @ [ref x, _];
+   |         ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = note: slice and array elements must have `Sized` type
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/let-pat-inferred-non-wf.rs:11:9
+   |
+LL |     let [ref x];
+   |         ^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = note: slice and array elements must have `Sized` type
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/let-pat-inferred-non-wf.rs:16:9
+   |
+LL |     let [[ref x], [_, y @ ..]];
+   |         ^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = note: slice and array elements must have `Sized` type
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/let-pat-inferred-non-wf.rs:22:9
+   |
+LL |     let [(ref a, b), x];
+   |         ^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = note: only the last element of a tuple may have a dynamically sized type
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/let-pat-inferred-non-wf.rs:28:9
+   |
+LL |     let (ref a, b);
+   |         ^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = note: only the last element of a tuple may have a dynamically sized type
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/let-pat-inferred-non-wf.rs:34:9
+   |
+LL |     let [S(ref x)];
+   |         ^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: within `S<str>`, the trait `Sized` is not implemented for `str`
+note: required because it appears within the type `S<str>`
+  --> $DIR/let-pat-inferred-non-wf.rs:3:8
+   |
+LL | struct S<T: ?Sized>(T);
+   |        ^
+   = note: slice and array elements must have `Sized` type
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0277`.