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 rust-lang#13998 was closed some time ago.

Author: nnethercote

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 })

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);