Rollup merge of #92118 - jackh726:type-alias-position-error, r=petrochenkov

Parse and suggest moving where clauses after equals for type aliases

~Mostly the same as #90076, but doesn't make any syntax changes.~ Whether or not we want to land the syntax changes, we should  parse the invalid where clause position and suggest moving.

r? `@nikomatsakis`
cc `@petrochenkov` you might have thoughts on implementation

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -2780,34 +2780,34 @@ impl<'a> State<'a> {
                 self.word_space(",");
             }
 
-            match *predicate {
-                ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
-                    ref bound_generic_params,
-                    ref bounded_ty,
-                    ref bounds,
-                    ..
-                }) => {
-                    self.print_formal_generic_params(bound_generic_params);
-                    self.print_type(bounded_ty);
-                    self.print_type_bounds(":", bounds);
-                }
-                ast::WherePredicate::RegionPredicate(ast::WhereRegionPredicate {
-                    ref lifetime,
-                    ref bounds,
-                    ..
-                }) => {
-                    self.print_lifetime_bounds(*lifetime, bounds);
-                }
-                ast::WherePredicate::EqPredicate(ast::WhereEqPredicate {
-                    ref lhs_ty,
-                    ref rhs_ty,
-                    ..
-                }) => {
-                    self.print_type(lhs_ty);
-                    self.space();
-                    self.word_space("=");
-                    self.print_type(rhs_ty);
-                }
+            self.print_where_predicate(predicate);
+        }
+    }
+
+    pub fn print_where_predicate(&mut self, predicate: &ast::WherePredicate) {
+        match predicate {
+            ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
+                bound_generic_params,
+                bounded_ty,
+                bounds,
+                ..
+            }) => {
+                self.print_formal_generic_params(bound_generic_params);
+                self.print_type(bounded_ty);
+                self.print_type_bounds(":", bounds);
+            }
+            ast::WherePredicate::RegionPredicate(ast::WhereRegionPredicate {
+                lifetime,
+                bounds,
+                ..
+            }) => {
+                self.print_lifetime_bounds(*lifetime, bounds);
+            }
+            ast::WherePredicate::EqPredicate(ast::WhereEqPredicate { lhs_ty, rhs_ty, .. }) => {
+                self.print_type(lhs_ty);
+                self.space();
+                self.word_space("=");
+                self.print_type(rhs_ty);
             }
         }
     }

compiler/rustc_parse/src/parser/item.rs

@@ -794,6 +794,44 @@ impl<'a> Parser<'a> {
         ))
     }
 
+    /// Emits an error that the where clause at the end of a type alias is not
+    /// allowed and suggests moving it.
+    fn error_ty_alias_where(
+        &self,
+        before_where_clause_present: bool,
+        before_where_clause_span: Span,
+        after_predicates: &[WherePredicate],
+        after_where_clause_span: Span,
+    ) {
+        let mut err =
+            self.struct_span_err(after_where_clause_span, "where clause not allowed here");
+        if !after_predicates.is_empty() {
+            let mut state = crate::pprust::State::new();
+            if !before_where_clause_present {
+                state.space();
+                state.word_space("where");
+            } else {
+                state.word_space(",");
+            }
+            let mut first = true;
+            for p in after_predicates.iter() {
+                if !first {
+                    state.word_space(",");
+                }
+                first = false;
+                state.print_where_predicate(p);
+            }
+            let suggestion = state.s.eof();
+            err.span_suggestion(
+                before_where_clause_span.shrink_to_hi(),
+                "move it here",
+                suggestion,
+                Applicability::MachineApplicable,
+            );
+        }
+        err.emit()
+    }
+
     /// Parses a `type` alias with the following grammar:
     /// ```
     /// TypeAlias = "type" Ident Generics {":" GenericBounds}? {"=" Ty}? ";" ;
@@ -806,9 +844,24 @@ impl<'a> Parser<'a> {
         // Parse optional colon and param bounds.
         let bounds =
             if self.eat(&token::Colon) { self.parse_generic_bounds(None)? } else { Vec::new() };
+
         generics.where_clause = self.parse_where_clause()?;
 
         let ty = if self.eat(&token::Eq) { Some(self.parse_ty()?) } else { None };
+
+        if self.token.is_keyword(kw::Where) {
+            let after_where_clause = self.parse_where_clause()?;
+
+            self.error_ty_alias_where(
+                generics.where_clause.has_where_token,
+                generics.where_clause.span,
+                &after_where_clause.predicates,
+                after_where_clause.span,
+            );
+
+            generics.where_clause.predicates.extend(after_where_clause.predicates.into_iter());
+        }
+
         self.expect_semi()?;
 
         Ok((ident, ItemKind::TyAlias(Box::new(TyAlias { defaultness, generics, bounds, ty }))))