From f93cbedadec03e5d4d6733e0756d87b6eb706a4c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Fri, 6 Aug 2021 02:36:54 -0700
Subject: [PATCH] Do not ICE on HIR based WF check when involving lifetimes

Fix #87549.
---
 .../src/traits/error_reporting/mod.rs         |  7 +++--
 src/test/ui/wf/hir-wf-check-erase-regions.rs  | 14 +++++++++
 .../ui/wf/hir-wf-check-erase-regions.stderr   | 31 +++++++++++++++++++
 3 files changed, 49 insertions(+), 3 deletions(-)
 create mode 100644 src/test/ui/wf/hir-wf-check-erase-regions.rs
 create mode 100644 src/test/ui/wf/hir-wf-check-erase-regions.stderr

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 3f713ce3c3914..ac07cc1f03439 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -245,9 +245,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
                     root_obligation.cause.code.peel_derives()
                 {
-                    if let Some(cause) =
-                        self.tcx.diagnostic_hir_wf_check((obligation.predicate, wf_loc.clone()))
-                    {
+                    if let Some(cause) = self.tcx.diagnostic_hir_wf_check((
+                        tcx.erase_regions(obligation.predicate),
+                        wf_loc.clone(),
+                    )) {
                         obligation.cause = cause;
                         span = obligation.cause.span;
                     }
diff --git a/src/test/ui/wf/hir-wf-check-erase-regions.rs b/src/test/ui/wf/hir-wf-check-erase-regions.rs
new file mode 100644
index 0000000000000..bb398e5698a80
--- /dev/null
+++ b/src/test/ui/wf/hir-wf-check-erase-regions.rs
@@ -0,0 +1,14 @@
+// Regression test for #87549.
+// compile-flags: -C incremental=tmp/wf/hir-wf-check-erase-regions
+
+pub struct Table<T, const N: usize>([Option<T>; N]);
+
+impl<'a, T, const N: usize> IntoIterator for &'a Table<T, N> {
+    type IntoIter = std::iter::Flatten<std::slice::Iter<'a, T>>; //~ ERROR `&T` is not an iterator
+    type Item = &'a T;
+
+    fn into_iter(self) -> Self::IntoIter { //~ ERROR `&T` is not an iterator
+        unimplemented!()
+    }
+}
+fn main() {}
diff --git a/src/test/ui/wf/hir-wf-check-erase-regions.stderr b/src/test/ui/wf/hir-wf-check-erase-regions.stderr
new file mode 100644
index 0000000000000..a704754e82a92
--- /dev/null
+++ b/src/test/ui/wf/hir-wf-check-erase-regions.stderr
@@ -0,0 +1,31 @@
+error[E0277]: `&T` is not an iterator
+  --> $DIR/hir-wf-check-erase-regions.rs:7:5
+   |
+LL |     type IntoIter = std::iter::Flatten<std::slice::Iter<'a, T>>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `&T` is not an iterator
+   |
+  ::: $SRC_DIR/core/src/iter/adapters/flatten.rs:LL:COL
+   |
+LL | pub struct Flatten<I: Iterator<Item: IntoIterator>> {
+   |                                      ------------ required by this bound in `Flatten`
+   |
+   = help: the trait `Iterator` is not implemented for `&T`
+   = note: required because of the requirements on the impl of `IntoIterator` for `&T`
+
+error[E0277]: `&T` is not an iterator
+  --> $DIR/hir-wf-check-erase-regions.rs:10:27
+   |
+LL |     fn into_iter(self) -> Self::IntoIter {
+   |                           ^^^^^^^^^^^^^^ `&T` is not an iterator
+   |
+  ::: $SRC_DIR/core/src/iter/adapters/flatten.rs:LL:COL
+   |
+LL | pub struct Flatten<I: Iterator<Item: IntoIterator>> {
+   |                                      ------------ required by this bound in `Flatten`
+   |
+   = help: the trait `Iterator` is not implemented for `&T`
+   = note: required because of the requirements on the impl of `IntoIterator` for `&T`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.