Rollup merge of #121326 - fmease:detect-empty-leading-where-clauses-on-ty-aliases, r=compiler-errors

Detect empty leading where clauses on type aliases

1. commit: refactor the AST of type alias where clauses
   * I could no longer bear the look of `.0.1` and `.1.0`
   * Arguably moving `split` out of `TyAlias` into a substruct might not make that much sense from a semantic standpoint since it reprs an index into `TyAlias.predicates` but it's alright and it cleans up the usage sites of `TyAlias`
2. commit: fix an oversight: An empty leading where clause is still a leading where clause
   * semantically reject empty leading where clauses on lazy type aliases
     * e.g., on `#![feature(lazy_type_alias)] type X where = ();`
   * make empty leading where clauses on assoc types trigger lint `deprecated_where_clause_location`
     * e.g., `impl Trait for () { type X where = (); }`

1 files changed, 16 insertions, 9 deletions

diff --git a/compiler/rustc_lint/src/context/diagnostics.rs b/compiler/rustc_lint/src/context/diagnostics.rs
index 71aef50391d..14e4c79563b 100644
--- a/compiler/rustc_lint/src/context/diagnostics.rs
+++ b/compiler/rustc_lint/src/context/diagnostics.rs
@@ -428,15 +428,22 @@ pub(super) fn builtin(sess: &Session, diagnostic: BuiltinLintDiagnostics, diag:
                 diag.note("see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration");
             }
         }
-        BuiltinLintDiagnostics::DeprecatedWhereclauseLocation(new_span, suggestion) => {
-            diag.multipart_suggestion(
-                "move it to the end of the type declaration",
-                vec![(diag.span.primary_span().unwrap(), "".to_string()), (new_span, suggestion)],
-                Applicability::MachineApplicable,
-            );
-            diag.note(
-                        "see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information",
-                    );
+        BuiltinLintDiagnostics::DeprecatedWhereclauseLocation(sugg) => {
+            let left_sp = diag.span.primary_span().unwrap();
+            match sugg {
+                Some((right_sp, sugg)) => diag.multipart_suggestion(
+                    "move it to the end of the type declaration",
+                    vec![(left_sp, String::new()), (right_sp, sugg)],
+                    Applicability::MachineApplicable,
+                ),
+                None => diag.span_suggestion(
+                    left_sp,
+                    "remove this `where`",
+                    "",
+                    Applicability::MachineApplicable,
+                ),
+            };
+            diag.note("see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information");
         }
         BuiltinLintDiagnostics::SingleUseLifetime {
             param_span,