Add a note with a link to explain empty types

compiler/rustc_mir_build/messages.ftl

@@ -332,6 +332,7 @@ mir_build_unreachable_matches_same_values = matches some of the same values
 mir_build_unreachable_pattern = unreachable pattern
     .label = no value can reach this
     .unreachable_matches_no_values = matches no values because `{$ty}` is uninhabited
+    .unreachable_uninhabited_note = to learn more about uninhabited types, see https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types
     .unreachable_covered_by_catchall = matches any value
     .unreachable_covered_by_one = matches all the relevant values
     .unreachable_covered_by_many = multiple earlier patterns match some of the same values

compiler/rustc_mir_build/src/errors.rs

@@ -588,6 +588,8 @@ pub(crate) struct UnreachablePattern<'tcx> {
     pub(crate) span: Option<Span>,
     #[subdiagnostic]
     pub(crate) matches_no_values: Option<UnreachableMatchesNoValues<'tcx>>,
+    #[note(mir_build_unreachable_uninhabited_note)]
+    pub(crate) uninhabited_note: Option<()>,
     #[label(mir_build_unreachable_covered_by_catchall)]
     pub(crate) covered_by_catchall: Option<Span>,
     #[label(mir_build_unreachable_covered_by_one)]

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -921,6 +921,7 @@ fn report_unreachable_pattern<'p, 'tcx>(
     let mut lint = UnreachablePattern {
         span: Some(pat_span),
         matches_no_values: None,
+        uninhabited_note: None,
         covered_by_catchall: None,
         covered_by_one: None,
         covered_by_many: None,
@@ -929,6 +930,7 @@ fn report_unreachable_pattern<'p, 'tcx>(
         [] => {
             // Empty pattern; we report the uninhabited type that caused the emptiness.
             lint.span = None; // Don't label the pattern itself
+            lint.uninhabited_note = Some(()); // Give a link about empty types
             pat.walk(&mut |subpat| {
                 let ty = **subpat.ty();
                 if cx.is_uninhabited(ty) {