From d6c63bdb2188d4c90cf0e9958d7744834a132072 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Thu, 9 May 2024 11:56:38 +1000
Subject: [PATCH] Fix up `DescriptionCtx::new`.

The comment mentions that `ReBound` and `ReVar` aren't expected here.
Experimentation with the full test suite indicates this is true, and
that `ReErased` also doesn't occur. So the commit introduces `bug!` for
those cases. (If any of them show up later on, at least we'll have a
test case.)

The commit also remove the first sentence in the comment.
`RePlaceholder` is now handled in the match arm above this comment and
nothing is printed for it, so that sentence is just wrong. Furthermore,
issue #13998 was closed some time ago.
---
 compiler/rustc_infer/src/errors/note_and_explain.rs   | 10 +---------
 compiler/rustc_infer/src/infer/error_reporting/mod.rs |  8 +++-----
 2 files changed, 4 insertions(+), 14 deletions(-)

diff --git a/compiler/rustc_infer/src/errors/note_and_explain.rs b/compiler/rustc_infer/src/errors/note_and_explain.rs
index 70d94873530e..c7f07ebed973 100644
--- a/compiler/rustc_infer/src/errors/note_and_explain.rs
+++ b/compiler/rustc_infer/src/errors/note_and_explain.rs
@@ -69,16 +69,8 @@ impl<'a> DescriptionCtx<'a> {
 
             ty::RePlaceholder(_) | ty::ReError(_) => return None,
 
-            // FIXME(#13998) RePlaceholder should probably print like
-            // ReLateParam rather than dumping Debug output on the user.
-            //
-            // We shouldn't really be having unification failures with ReVar
-            // and ReBound though.
-            //
-            // FIXME(@lcnr): figure out why we have to handle `ReBound`
-            // here, this feels somewhat off.
             ty::ReVar(_) | ty::ReBound(..) | ty::ReErased => {
-                (alt_span, "revar", format!("{region:?}"))
+                bug!("unexpected region for DescriptionCtx: {:?}", region);
             }
         };
         Some(DescriptionCtx { span, kind, arg })
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 635bbca37ecc..cb5be47e3046 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -172,11 +172,9 @@ pub(super) fn note_and_explain_region<'tcx>(
 
         ty::ReError(_) => return,
 
-        // We shouldn't really be having unification failures with ReVar
-        // and ReBound though.
-        //
-        // FIXME(@lcnr): Figure out whether this is reachable and if so, why.
-        ty::ReVar(_) | ty::ReBound(..) | ty::ReErased => (format!("lifetime `{region}`"), alt_span),
+        ty::ReVar(_) | ty::ReBound(..) | ty::ReErased => {
+            bug!("unexpected region for note_and_explain_region: {:?}", region);
+        }
     };
 
     emit_msg_span(err, prefix, description, span, suffix);