Reserve 8 bytes per format placeholder

Author: thaliaarchi

library/core/src/fmt/mod.rs

@@ -652,8 +652,9 @@ impl<'a> Arguments<'a> {
             // There are some arguments, so any additional push
             // will reallocate the string. To avoid that,
             // we're "pre-doubling" the capacity here.
-            // It cannot overflow, because the maximum length is `isize::MAX`.
-            pieces_length * 2
+            pieces_length
+                .checked_add(self.args.len().checked_mul(8).unwrap_or(0))
+                .unwrap_or(pieces_length)
         }
     }
 }

library/coretests/tests/fmt/mod.rs

@@ -26,7 +26,7 @@ fn test_estimated_capacity() {
     assert_eq!(format_args!("Hello").estimated_capacity(), 5);
     assert_eq!(format_args!("Hello, {}!", { "" }).estimated_capacity(), 16);
     assert_eq!(format_args!("{}, hello!", { "World" }).estimated_capacity(), 0);
-    assert_eq!(format_args!("{}. 16-bytes piece", { "World" }).estimated_capacity(), 32);
+    assert_eq!(format_args!("{}. 16-bytes piece", { "World" }).estimated_capacity(), 24);
 }
 
 #[test]