Allow stuct literals in if let guards

This is consistent with normal match guards.

Author: matthewjasper

Diff for: compiler/rustc_parse/src/parser/expr.rs

@@ -2477,9 +2477,7 @@ impl<'a> Parser<'a> {
         } else {
             self.expect(&token::Eq)?;
         }
-        let expr = self.with_res(self.restrictions | Restrictions::NO_STRUCT_LITERAL, |this| {
-            this.parse_expr_assoc_with(1 + prec_let_scrutinee_needs_par(), None.into())
-        })?;
+        let expr = self.parse_expr_assoc_with(1 + prec_let_scrutinee_needs_par(), None.into())?;
         let span = lo.to(expr.span);
         self.sess.gated_spans.gate(sym::let_chains, span);
         Ok(self.mk_expr(span, ExprKind::Let(pat, expr, span)))

Diff for: tests/ui/parser/struct-literal-in-match-guard.rs

@@ -3,6 +3,8 @@
 // Unlike `if` condition, `match` guards accept struct literals.
 // This is detected in <https://github.com/rust-lang/rust/pull/74566#issuecomment-663613705>.
 
+#![feature(if_let_guard)]
+
 #[derive(PartialEq)]
 struct Foo {
     x: isize,
@@ -11,6 +13,7 @@ struct Foo {
 fn foo(f: Foo) {
     match () {
         () if f == Foo { x: 42 } => {}
+        () if let Foo { x: 0.. } = Foo { x: 42 } => {}
         _ => {}
     }
 }