Don't suggest adding let in certain if conditions

compiler/rustc_ast/src/ast.rs

@@ -1278,6 +1278,22 @@ impl Expr {
             },
         )
     }
+
+    // To a first-order approximation, is this a pattern
+    pub fn is_approximately_pattern(&self) -> bool {
+        match &self.peel_parens().kind {
+            ExprKind::Box(_)
+            | ExprKind::Array(_)
+            | ExprKind::Call(_, _)
+            | ExprKind::Tup(_)
+            | ExprKind::Lit(_)
+            | ExprKind::Range(_, _, _)
+            | ExprKind::Underscore
+            | ExprKind::Path(_, _)
+            | ExprKind::Struct(_) => true,
+            _ => false,
+        }
+    }
 }
 
 /// Limit types of a range (inclusive or exclusive)