Incorrect base64 implementation?

[david-perez]

Consider this patch:

diff --git a/rust-runtime/aws-smithy-types/src/base64.rs b/rust-runtime/aws-smithy-types/src/base64.rs
index 846c7eb1..5c49b708 100644
--- a/rust-runtime/aws-smithy-types/src/base64.rs
+++ b/rust-runtime/aws-smithy-types/src/base64.rs
@@ -196,6 +196,19 @@ mod test {
         assert_eq!(encode("anything you wan"), "YW55dGhpbmcgeW91IHdhbg==");
     }

+    #[test]
+    fn test_base64_misc() {
+        let us_1 = decode("xyz");
+        let them_1 = base64::decode("xyz");
+        dbg!(&us_1);
+        dbg!(&them_1);
+
+        let us_2 = decode("YmxvYg=");
+        let them_2 = base64::decode("YmxvYg=");
+        dbg!(&us_2);
+        dbg!(&them_2);
+    }
+
     #[test]
     fn test_invalid_padding() {
         // no internal padding

cargo test test_base64_misc -- --nocapture yields:

running 1 test
[aws-smithy-types/src/base64.rs:203] &us_1 = Ok(
    [
        199,
        44,
    ],
)
[aws-smithy-types/src/base64.rs:204] &them_1 = Err(
    InvalidLastSymbol(
        2,
        122,
    ),
)
[aws-smithy-types/src/base64.rs:208] &us_2 = Ok(
    [
        98,
        108,
        111,
        98,
    ],
)
[aws-smithy-types/src/base64.rs:209] &them_2 = Ok(
    [
        98,
        108,
        111,
        98,
    ],
)
test base64::test::test_base64_misc ... ok

So the first case seems to be invalid but our implementation is accepting it. The second case is technically not valid base64 (note code point length is not divisible by 4), since it's missing an extra padding = character, but I'm guessing both implementations can unambiguously decode it. However, other implementations are stricter:

>> atob("YmxvYg=");
Uncaught DOMException: String contains an invalid character

These two cases are deemed as invalid base64 according to Smithy protocol tests.