From 2884a74d083cc6d601e5f1e53cc33b60e9655cfc Mon Sep 17 00:00:00 2001
From: Fabian Wolff <fabian.wolff@alumni.ethz.ch>
Date: Thu, 19 Aug 2021 01:01:49 +0200
Subject: [PATCH 1/2] Fix non-capturing closure return type coercion

---
 compiler/rustc_typeck/src/check/coercion.rs |  4 +++
 src/test/ui/coercion/issue-88097.rs         | 31 +++++++++++++++++++++
 2 files changed, 35 insertions(+)
 create mode 100644 src/test/ui/coercion/issue-88097.rs

diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index 208eb27c84406..e3c43aa8b8482 100644
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/compiler/rustc_typeck/src/check/coercion.rs
@@ -941,6 +941,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             exprs.len()
         );
 
+        if prev_ty == new_ty {
+            return Ok(prev_ty);
+        }
+
         // Special-case that coercion alone cannot handle:
         // Function items or non-capturing closures of differing IDs or InternalSubsts.
         let (a_sig, b_sig) = {
diff --git a/src/test/ui/coercion/issue-88097.rs b/src/test/ui/coercion/issue-88097.rs
new file mode 100644
index 0000000000000..e543e1bae9239
--- /dev/null
+++ b/src/test/ui/coercion/issue-88097.rs
@@ -0,0 +1,31 @@
+// In #88097, the compiler attempted to coerce a closure type to itself via
+// a function pointer, which caused an unnecessary error. Check that this
+// behavior has been fixed.
+
+// check-pass
+
+fn peculiar() -> impl Fn(u8) -> u8 {
+    return |x| x + 1
+}
+
+fn peculiar2() -> impl Fn(u8) -> u8 {
+    return |x| x + 1;
+}
+
+fn peculiar3() -> impl Fn(u8) -> u8 {
+    let f = |x| x + 1;
+    return f
+}
+
+fn peculiar4() -> impl Fn(u8) -> u8 {
+    let f = |x| x + 1;
+    f
+}
+
+fn peculiar5() -> impl Fn(u8) -> u8 {
+    let f = |x| x + 1;
+    let g = |x| x + 2;
+    return if true { f } else { g }
+}
+
+fn main() {}

From bbe3be9bf8d8e8503b8effbe392d0e449224b406 Mon Sep 17 00:00:00 2001
From: Fabian Wolff <fabian.wolff@alumni.ethz.ch>
Date: Tue, 31 Aug 2021 23:44:15 +0200
Subject: [PATCH 2/2] Add explanatory comment

---
 compiler/rustc_typeck/src/check/coercion.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index e3c43aa8b8482..e05d14289c348 100644
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/compiler/rustc_typeck/src/check/coercion.rs
@@ -941,6 +941,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             exprs.len()
         );
 
+        // The following check fixes #88097, where the compiler erroneously
+        // attempted to coerce a closure type to itself via a function pointer.
         if prev_ty == new_ty {
             return Ok(prev_ty);
         }