Fix the error of selecting obligation by not running the borrow checker.

src/librustc/ty/util.rs

@@ -522,6 +522,12 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.def_key(def_id).disambiguated_data.data == DefPathData::ClosureExpr
     }
 
+    /// True if this def-id refers to the implicit constructor for
+    /// a tuple struct like `struct Foo(u32)`.
+    pub fn is_struct_constructor(self, def_id: DefId) -> bool {
+        self.def_key(def_id).disambiguated_data.data == DefPathData::StructCtor
+    }
+
     /// Given the `DefId` of a fn or closure, returns the `DefId` of
     /// the innermost fn item that the closure is contained within.
     /// This is a significant def-id because, when we do

src/librustc_mir/borrow_check/mod.rs

@@ -76,7 +76,37 @@ fn mir_borrowck<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> BorrowC
     let input_mir = tcx.mir_validated(def_id);
     debug!("run query mir_borrowck: {}", tcx.item_path_str(def_id));
 
-    if !tcx.has_attr(def_id, "rustc_mir_borrowck") && !tcx.use_mir_borrowck() {
+    let mut return_early;
+
+    // Return early if we are not supposed to use MIR borrow checker for this function.
+    return_early = !tcx.has_attr(def_id, "rustc_mir_borrowck") && !tcx.use_mir_borrowck();
+
+    if tcx.is_struct_constructor(def_id) {
+        // We are not borrow checking the automatically generated struct constructors
+        // because we want to accept structs such as this (taken from the `linked-hash-map`
+        // crate):
+        // ```rust
+        // struct Qey<Q: ?Sized>(Q);
+        // ```
+        // MIR of this struct constructor looks something like this:
+        // ```rust
+        // fn Qey(_1: Q) -> Qey<Q>{
+        //     let mut _0: Qey<Q>;                  // return place
+        //
+        //     bb0: {
+        //         (_0.0: Q) = move _1;             // bb0[0]: scope 0 at src/main.rs:1:1: 1:26
+        //         return;                          // bb0[1]: scope 0 at src/main.rs:1:1: 1:26
+        //     }
+        // }
+        // ```
+        // The problem here is that `(_0.0: Q) = move _1;` is valid only if `Q` is
+        // of statically known size, which is not known to be true because of the
+        // `Q: ?Sized` constraint. However, it is true because the constructor can be
+        // called only when `Q` is of statically known size.
+        return_early = true;
+    }
+
+    if return_early {
         return BorrowCheckResult {
             closure_requirements: None,
             used_mut_upvars: SmallVec::new(),

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -1725,6 +1725,14 @@ impl MirPass for TypeckMir {
             // broken MIR, so try not to report duplicate errors.
             return;
         }
+
+        if tcx.is_struct_constructor(def_id) {
+            // We just assume that the automatically generated struct constructors are
+            // correct. See the comment in the `mir_borrowck` implementation for an
+            // explanation why we need this.
+            return;
+        }
+
         let param_env = tcx.param_env(def_id);
         tcx.infer_ctxt().enter(|infcx| {
             let _ = type_check_internal(

src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr

@@ -24,15 +24,16 @@ LL | |     });
    = note: where '_#1r: '_#0r
 
 error: free region `ReFree(DefId(0/0:6 ~ propagate_approximated_shorter_to_static_no_bound[317d]::supply[0]), BrNamed(crate0:DefIndex(1:16), 'a))` does not outlive free region `ReStatic`
-  --> $DIR/propagate-approximated-shorter-to-static-no-bound.rs:45:5
+  --> $DIR/propagate-approximated-shorter-to-static-no-bound.rs:45:47
    |
-LL | /     establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {
+LL |       establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {
+   |  _______________________________________________^
 LL | |         //~^ ERROR does not outlive free region
 LL | |
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get()) //~ WARNING not reporting region error due to nll
 LL | |     });
-   | |______^
+   | |_____^
 
 note: No external requirements
   --> $DIR/propagate-approximated-shorter-to-static-no-bound.rs:44:1

src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr

@@ -24,15 +24,16 @@ LL | |     });
    = note: where '_#1r: '_#0r
 
 error: free region `ReFree(DefId(0/0:6 ~ propagate_approximated_shorter_to_static_wrong_bound[317d]::supply[0]), BrNamed(crate0:DefIndex(1:16), 'a))` does not outlive free region `ReStatic`
-  --> $DIR/propagate-approximated-shorter-to-static-wrong-bound.rs:48:5
+  --> $DIR/propagate-approximated-shorter-to-static-wrong-bound.rs:48:47
    |
-LL | /     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
+LL |       establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
+   |  _______________________________________________^
 LL | |         //~^ ERROR does not outlive free region
 LL | |         // Only works if 'x: 'y:
 LL | |         demand_y(x, y, x.get())
 LL | |         //~^ WARNING not reporting region error due to nll
 LL | |     });
-   | |______^
+   | |_____^
 
 note: No external requirements
   --> $DIR/propagate-approximated-shorter-to-static-wrong-bound.rs:47:1

src/test/ui/nll/issue-50716-1.rs

@@ -0,0 +1,23 @@
+// Copyright 2012 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.
+//
+// An additional regression test for the issue #50716 “NLL ignores lifetimes
+// bounds derived from `Sized` requirements” that checks that the fixed compiler
+// accepts this code fragment with both AST and MIR borrow checkers.
+//
+// revisions: ast mir
+//
+// compile-pass
+
+#![cfg_attr(mir, feature(nll))]
+
+struct Qey<Q: ?Sized>(Q);
+
+fn main() {}