From 619c6055e71a5a202cba3f5cb979f17c3581faa3 Mon Sep 17 00:00:00 2001
From: Matthew Jasper <mjjasper1@gmail.com>
Date: Fri, 8 May 2020 17:25:18 +0100
Subject: [PATCH] Fix debug assertion in error code

---
 .../traits/error_reporting/suggestions.rs      |  9 +++++++++
 ...m-ref-trait-object-literal-bound-regions.rs | 18 ++++++++++++++++++
 ...f-trait-object-literal-bound-regions.stderr | 18 ++++++++++++++++++
 3 files changed, 45 insertions(+)
 create mode 100644 src/test/ui/suggestions/imm-ref-trait-object-literal-bound-regions.rs
 create mode 100644 src/test/ui/suggestions/imm-ref-trait-object-literal-bound-regions.stderr

diff --git a/src/librustc_trait_selection/traits/error_reporting/suggestions.rs b/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
index ce7b1390d46..3c8aa08bdca 100644
--- a/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
+++ b/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
@@ -691,6 +691,15 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             }
 
             if let ty::Ref(region, t_type, mutability) = trait_ref.skip_binder().self_ty().kind {
+                if region.is_late_bound() || t_type.has_escaping_bound_vars() {
+                    // Avoid debug assertion in `mk_obligation_for_def_id`.
+                    //
+                    // If the self type has escaping bound vars then it's not
+                    // going to be the type of an expression, so the suggestion
+                    // probably won't apply anyway.
+                    return;
+                }
+
                 let trait_type = match mutability {
                     hir::Mutability::Mut => self.tcx.mk_imm_ref(region, t_type),
                     hir::Mutability::Not => self.tcx.mk_mut_ref(region, t_type),
diff --git a/src/test/ui/suggestions/imm-ref-trait-object-literal-bound-regions.rs b/src/test/ui/suggestions/imm-ref-trait-object-literal-bound-regions.rs
new file mode 100644
index 00000000000..319789c4ec2
--- /dev/null
+++ b/src/test/ui/suggestions/imm-ref-trait-object-literal-bound-regions.rs
@@ -0,0 +1,18 @@
+// Regression test for #70813 (this used to trigger a debug assertion)
+
+trait Trait {}
+
+struct S;
+
+impl<'a> Trait for &'a mut S {}
+
+fn foo<X>(_: X)
+where
+    for<'b> &'b X: Trait,
+{
+}
+
+fn main() {
+    let s = S;
+    foo::<S>(s); //~ ERROR the trait bound `for<'b> &'b S: Trait` is not satisfied
+}
diff --git a/src/test/ui/suggestions/imm-ref-trait-object-literal-bound-regions.stderr b/src/test/ui/suggestions/imm-ref-trait-object-literal-bound-regions.stderr
new file mode 100644
index 00000000000..83de3c4cfe0
--- /dev/null
+++ b/src/test/ui/suggestions/imm-ref-trait-object-literal-bound-regions.stderr
@@ -0,0 +1,18 @@
+error[E0277]: the trait bound `for<'b> &'b S: Trait` is not satisfied
+  --> $DIR/imm-ref-trait-object-literal-bound-regions.rs:17:5
+   |
+LL | fn foo<X>(_: X)
+   |    --- required by a bound in this
+LL | where
+LL |     for<'b> &'b X: Trait,
+   |                    ----- required by this bound in `foo`
+...
+LL |     foo::<S>(s);
+   |     ^^^^^^^^ the trait `for<'b> Trait` is not implemented for `&'b S`
+   |
+   = help: the following implementations were found:
+             <&'a mut S as Trait>
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.