Adding upstream version 110.0.1.upstream/110.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 893 insertions, 0 deletions

diff --git a/third_party/rust/base64/src/decode.rs b/third_party/rust/base64/src/decode.rs
new file mode 100644
index 0000000000..e349240ac7
--- /dev/null
+++ b/third_party/rust/base64/src/decode.rs
@@ -0,0 +1,893 @@
+use crate::{tables, Config, PAD_BYTE};
+
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use crate::STANDARD;
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use alloc::vec::Vec;
+use core::fmt;
+#[cfg(any(feature = "std", test))]
+use std::error;
+
+// decode logic operates on chunks of 8 input bytes without padding
+const INPUT_CHUNK_LEN: usize = 8;
+const DECODED_CHUNK_LEN: usize = 6;
+// we read a u64 and write a u64, but a u64 of input only yields 6 bytes of output, so the last
+// 2 bytes of any output u64 should not be counted as written to (but must be available in a
+// slice).
+const DECODED_CHUNK_SUFFIX: usize = 2;
+
+// how many u64's of input to handle at a time
+const CHUNKS_PER_FAST_LOOP_BLOCK: usize = 4;
+const INPUT_BLOCK_LEN: usize = CHUNKS_PER_FAST_LOOP_BLOCK * INPUT_CHUNK_LEN;
+// includes the trailing 2 bytes for the final u64 write
+const DECODED_BLOCK_LEN: usize =
+    CHUNKS_PER_FAST_LOOP_BLOCK * DECODED_CHUNK_LEN + DECODED_CHUNK_SUFFIX;
+
+/// Errors that can occur while decoding.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum DecodeError {
+    /// An invalid byte was found in the input. The offset and offending byte are provided.
+    InvalidByte(usize, u8),
+    /// The length of the input is invalid.
+    /// A typical cause of this is stray trailing whitespace or other separator bytes.
+    /// In the case where excess trailing bytes have produced an invalid length *and* the last byte
+    /// is also an invalid base64 symbol (as would be the case for whitespace, etc), `InvalidByte`
+    /// will be emitted instead of `InvalidLength` to make the issue easier to debug.
+    InvalidLength,
+    /// The last non-padding input symbol's encoded 6 bits have nonzero bits that will be discarded.
+    /// This is indicative of corrupted or truncated Base64.
+    /// Unlike InvalidByte, which reports symbols that aren't in the alphabet, this error is for
+    /// symbols that are in the alphabet but represent nonsensical encodings.
+    InvalidLastSymbol(usize, u8),
+}
+
+impl fmt::Display for DecodeError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            DecodeError::InvalidByte(index, byte) => {
+                write!(f, "Invalid byte {}, offset {}.", byte, index)
+            }
+            DecodeError::InvalidLength => write!(f, "Encoded text cannot have a 6-bit remainder."),
+            DecodeError::InvalidLastSymbol(index, byte) => {
+                write!(f, "Invalid last symbol {}, offset {}.", byte, index)
+            }
+        }
+    }
+}
+
+#[cfg(any(feature = "std", test))]
+impl error::Error for DecodeError {
+    fn description(&self) -> &str {
+        match *self {
+            DecodeError::InvalidByte(_, _) => "invalid byte",
+            DecodeError::InvalidLength => "invalid length",
+            DecodeError::InvalidLastSymbol(_, _) => "invalid last symbol",
+        }
+    }
+
+    fn cause(&self) -> Option<&dyn error::Error> {
+        None
+    }
+}
+
+///Decode from string reference as octets.
+///Returns a Result containing a Vec<u8>.
+///Convenience `decode_config(input, base64::STANDARD);`.
+///
+///# Example
+///
+///```rust
+///extern crate base64;
+///
+///fn main() {
+///    let bytes = base64::decode("aGVsbG8gd29ybGQ=").unwrap();
+///    println!("{:?}", bytes);
+///}
+///```
+#[cfg(any(feature = "alloc", feature = "std", test))]
+pub fn decode<T: AsRef<[u8]>>(input: T) -> Result<Vec<u8>, DecodeError> {
+    decode_config(input, STANDARD)
+}
+
+///Decode from string reference as octets.
+///Returns a Result containing a Vec<u8>.
+///
+///# Example
+///
+///```rust
+///extern crate base64;
+///
+///fn main() {
+///    let bytes = base64::decode_config("aGVsbG8gd29ybGR+Cg==", base64::STANDARD).unwrap();
+///    println!("{:?}", bytes);
+///
+///    let bytes_url = base64::decode_config("aGVsbG8gaW50ZXJuZXR-Cg==", base64::URL_SAFE).unwrap();
+///    println!("{:?}", bytes_url);
+///}
+///```
+#[cfg(any(feature = "alloc", feature = "std", test))]
+pub fn decode_config<T: AsRef<[u8]>>(input: T, config: Config) -> Result<Vec<u8>, DecodeError> {
+    let decoded_length_estimate = (input
+        .as_ref()
+        .len()
+        .checked_add(3)
+        .expect("decoded length calculation overflow"))
+        / 4
+        * 3;
+    let mut buffer = Vec::<u8>::with_capacity(decoded_length_estimate);
+
+    decode_config_buf(input, config, &mut buffer).map(|_| buffer)
+}
+
+///Decode from string reference as octets.
+///Writes into the supplied buffer to avoid allocation.
+///Returns a Result containing an empty tuple, aka ().
+///
+///# Example
+///
+///```rust
+///extern crate base64;
+///
+///fn main() {
+///    let mut buffer = Vec::<u8>::new();
+///    base64::decode_config_buf("aGVsbG8gd29ybGR+Cg==", base64::STANDARD, &mut buffer).unwrap();
+///    println!("{:?}", buffer);
+///
+///    buffer.clear();
+///
+///    base64::decode_config_buf("aGVsbG8gaW50ZXJuZXR-Cg==", base64::URL_SAFE, &mut buffer)
+///        .unwrap();
+///    println!("{:?}", buffer);
+///}
+///```
+#[cfg(any(feature = "alloc", feature = "std", test))]
+pub fn decode_config_buf<T: AsRef<[u8]>>(
+    input: T,
+    config: Config,
+    buffer: &mut Vec<u8>,
+) -> Result<(), DecodeError> {
+    let input_bytes = input.as_ref();
+
+    let starting_output_len = buffer.len();
+
+    let num_chunks = num_chunks(input_bytes);
+    let decoded_len_estimate = num_chunks
+        .checked_mul(DECODED_CHUNK_LEN)
+        .and_then(|p| p.checked_add(starting_output_len))
+        .expect("Overflow when calculating output buffer length");
+    buffer.resize(decoded_len_estimate, 0);
+
+    let bytes_written;
+    {
+        let buffer_slice = &mut buffer.as_mut_slice()[starting_output_len..];
+        bytes_written = decode_helper(input_bytes, num_chunks, config, buffer_slice)?;
+    }
+
+    buffer.truncate(starting_output_len + bytes_written);
+
+    Ok(())
+}
+
+/// Decode the input into the provided output slice.
+///
+/// This will not write any bytes past exactly what is decoded (no stray garbage bytes at the end).
+///
+/// If you don't know ahead of time what the decoded length should be, size your buffer with a
+/// conservative estimate for the decoded length of an input: 3 bytes of output for every 4 bytes of
+/// input, rounded up, or in other words `(input_len + 3) / 4 * 3`.
+///
+/// If the slice is not large enough, this will panic.
+pub fn decode_config_slice<T: AsRef<[u8]>>(
+    input: T,
+    config: Config,
+    output: &mut [u8],
+) -> Result<usize, DecodeError> {
+    let input_bytes = input.as_ref();
+
+    decode_helper(input_bytes, num_chunks(input_bytes), config, output)
+}
+
+/// Return the number of input chunks (including a possibly partial final chunk) in the input
+fn num_chunks(input: &[u8]) -> usize {
+    input
+        .len()
+        .checked_add(INPUT_CHUNK_LEN - 1)
+        .expect("Overflow when calculating number of chunks in input")
+        / INPUT_CHUNK_LEN
+}
+
+/// Helper to avoid duplicating num_chunks calculation, which is costly on short inputs.
+/// Returns the number of bytes written, or an error.
+// We're on the fragile edge of compiler heuristics here. If this is not inlined, slow. If this is
+// inlined(always), a different slow. plain ol' inline makes the benchmarks happiest at the moment,
+// but this is fragile and the best setting changes with only minor code modifications.
+#[inline]
+fn decode_helper(
+    input: &[u8],
+    num_chunks: usize,
+    config: Config,
+    output: &mut [u8],
+) -> Result<usize, DecodeError> {
+    let char_set = config.char_set;
+    let decode_table = char_set.decode_table();
+
+    let remainder_len = input.len() % INPUT_CHUNK_LEN;
+
+    // Because the fast decode loop writes in groups of 8 bytes (unrolled to
+    // CHUNKS_PER_FAST_LOOP_BLOCK times 8 bytes, where possible) and outputs 8 bytes at a time (of
+    // which only 6 are valid data), we need to be sure that we stop using the fast decode loop
+    // soon enough that there will always be 2 more bytes of valid data written after that loop.
+    let trailing_bytes_to_skip = match remainder_len {
+        // if input is a multiple of the chunk size, ignore the last chunk as it may have padding,
+        // and the fast decode logic cannot handle padding
+        0 => INPUT_CHUNK_LEN,
+        // 1 and 5 trailing bytes are illegal: can't decode 6 bits of input into a byte
+        1 | 5 => {
+            // trailing whitespace is so common that it's worth it to check the last byte to
+            // possibly return a better error message
+            if let Some(b) = input.last() {
+                if *b != PAD_BYTE && decode_table[*b as usize] == tables::INVALID_VALUE {
+                    return Err(DecodeError::InvalidByte(input.len() - 1, *b));
+                }
+            }
+
+            return Err(DecodeError::InvalidLength);
+        }
+        // This will decode to one output byte, which isn't enough to overwrite the 2 extra bytes
+        // written by the fast decode loop. So, we have to ignore both these 2 bytes and the
+        // previous chunk.
+        2 => INPUT_CHUNK_LEN + 2,
+        // If this is 3 unpadded chars, then it would actually decode to 2 bytes. However, if this
+        // is an erroneous 2 chars + 1 pad char that would decode to 1 byte, then it should fail
+        // with an error, not panic from going past the bounds of the output slice, so we let it
+        // use stage 3 + 4.
+        3 => INPUT_CHUNK_LEN + 3,
+        // This can also decode to one output byte because it may be 2 input chars + 2 padding
+        // chars, which would decode to 1 byte.
+        4 => INPUT_CHUNK_LEN + 4,
+        // Everything else is a legal decode len (given that we don't require padding), and will
+        // decode to at least 2 bytes of output.
+        _ => remainder_len,
+    };
+
+    // rounded up to include partial chunks
+    let mut remaining_chunks = num_chunks;
+
+    let mut input_index = 0;
+    let mut output_index = 0;
+
+    {
+        let length_of_fast_decode_chunks = input.len().saturating_sub(trailing_bytes_to_skip);
+
+        // Fast loop, stage 1
+        // manual unroll to CHUNKS_PER_FAST_LOOP_BLOCK of u64s to amortize slice bounds checks
+        if let Some(max_start_index) = length_of_fast_decode_chunks.checked_sub(INPUT_BLOCK_LEN) {
+            while input_index <= max_start_index {
+                let input_slice = &input[input_index..(input_index + INPUT_BLOCK_LEN)];
+                let output_slice = &mut output[output_index..(output_index + DECODED_BLOCK_LEN)];
+
+                decode_chunk(
+                    &input_slice[0..],
+                    input_index,
+                    decode_table,
+                    &mut output_slice[0..],
+                )?;
+                decode_chunk(
+                    &input_slice[8..],
+                    input_index + 8,
+                    decode_table,
+                    &mut output_slice[6..],
+                )?;
+                decode_chunk(
+                    &input_slice[16..],
+                    input_index + 16,
+                    decode_table,
+                    &mut output_slice[12..],
+                )?;
+                decode_chunk(
+                    &input_slice[24..],
+                    input_index + 24,
+                    decode_table,
+                    &mut output_slice[18..],
+                )?;
+
+                input_index += INPUT_BLOCK_LEN;
+                output_index += DECODED_BLOCK_LEN - DECODED_CHUNK_SUFFIX;
+                remaining_chunks -= CHUNKS_PER_FAST_LOOP_BLOCK;
+            }
+        }
+
+        // Fast loop, stage 2 (aka still pretty fast loop)
+        // 8 bytes at a time for whatever we didn't do in stage 1.
+        if let Some(max_start_index) = length_of_fast_decode_chunks.checked_sub(INPUT_CHUNK_LEN) {
+            while input_index < max_start_index {
+                decode_chunk(
+                    &input[input_index..(input_index + INPUT_CHUNK_LEN)],
+                    input_index,
+                    decode_table,
+                    &mut output
+                        [output_index..(output_index + DECODED_CHUNK_LEN + DECODED_CHUNK_SUFFIX)],
+                )?;
+
+                output_index += DECODED_CHUNK_LEN;
+                input_index += INPUT_CHUNK_LEN;
+                remaining_chunks -= 1;
+            }
+        }
+    }
+
+    // Stage 3
+    // If input length was such that a chunk had to be deferred until after the fast loop
+    // because decoding it would have produced 2 trailing bytes that wouldn't then be
+    // overwritten, we decode that chunk here. This way is slower but doesn't write the 2
+    // trailing bytes.
+    // However, we still need to avoid the last chunk (partial or complete) because it could
+    // have padding, so we always do 1 fewer to avoid the last chunk.
+    for _ in 1..remaining_chunks {
+        decode_chunk_precise(
+            &input[input_index..],
+            input_index,
+            decode_table,
+            &mut output[output_index..(output_index + DECODED_CHUNK_LEN)],
+        )?;
+
+        input_index += INPUT_CHUNK_LEN;
+        output_index += DECODED_CHUNK_LEN;
+    }
+
+    // always have one more (possibly partial) block of 8 input
+    debug_assert!(input.len() - input_index > 1 || input.is_empty());
+    debug_assert!(input.len() - input_index <= 8);
+
+    // Stage 4
+    // Finally, decode any leftovers that aren't a complete input block of 8 bytes.
+    // Use a u64 as a stack-resident 8 byte buffer.
+    let mut leftover_bits: u64 = 0;
+    let mut morsels_in_leftover = 0;
+    let mut padding_bytes = 0;
+    let mut first_padding_index: usize = 0;
+    let mut last_symbol = 0_u8;
+    let start_of_leftovers = input_index;
+    for (i, b) in input[start_of_leftovers..].iter().enumerate() {
+        // '=' padding
+        if *b == PAD_BYTE {
+            // There can be bad padding in a few ways:
+            // 1 - Padding with non-padding characters after it
+            // 2 - Padding after zero or one non-padding characters before it
+            //     in the current quad.
+            // 3 - More than two characters of padding. If 3 or 4 padding chars
+            //     are in the same quad, that implies it will be caught by #2.
+            //     If it spreads from one quad to another, it will be caught by
+            //     #2 in the second quad.
+
+            if i % 4 < 2 {
+                // Check for case #2.
+                let bad_padding_index = start_of_leftovers
+                    + if padding_bytes > 0 {
+                        // If we've already seen padding, report the first padding index.
+                        // This is to be consistent with the faster logic above: it will report an
+                        // error on the first padding character (since it doesn't expect to see
+                        // anything but actual encoded data).
+                        first_padding_index
+                    } else {
+                        // haven't seen padding before, just use where we are now
+                        i
+                    };
+                return Err(DecodeError::InvalidByte(bad_padding_index, *b));
+            }
+
+            if padding_bytes == 0 {
+                first_padding_index = i;
+            }
+
+            padding_bytes += 1;
+            continue;
+        }
+
+        // Check for case #1.
+        // To make '=' handling consistent with the main loop, don't allow
+        // non-suffix '=' in trailing chunk either. Report error as first
+        // erroneous padding.
+        if padding_bytes > 0 {
+            return Err(DecodeError::InvalidByte(
+                start_of_leftovers + first_padding_index,
+                PAD_BYTE,
+            ));
+        }
+        last_symbol = *b;
+
+        // can use up to 8 * 6 = 48 bits of the u64, if last chunk has no padding.
+        // To minimize shifts, pack the leftovers from left to right.
+        let shift = 64 - (morsels_in_leftover + 1) * 6;
+        // tables are all 256 elements, lookup with a u8 index always succeeds
+        let morsel = decode_table[*b as usize];
+        if morsel == tables::INVALID_VALUE {
+            return Err(DecodeError::InvalidByte(start_of_leftovers + i, *b));
+        }
+
+        leftover_bits |= (morsel as u64) << shift;
+        morsels_in_leftover += 1;
+    }
+
+    let leftover_bits_ready_to_append = match morsels_in_leftover {
+        0 => 0,
+        2 => 8,
+        3 => 16,
+        4 => 24,
+        6 => 32,
+        7 => 40,
+        8 => 48,
+        _ => unreachable!(
+            "Impossible: must only have 0 to 8 input bytes in last chunk, with no invalid lengths"
+        ),
+    };
+
+    // if there are bits set outside the bits we care about, last symbol encodes trailing bits that
+    // will not be included in the output
+    let mask = !0 >> leftover_bits_ready_to_append;
+    if !config.decode_allow_trailing_bits && (leftover_bits & mask) != 0 {
+        // last morsel is at `morsels_in_leftover` - 1
+        return Err(DecodeError::InvalidLastSymbol(
+            start_of_leftovers + morsels_in_leftover - 1,
+            last_symbol,
+        ));
+    }
+
+    let mut leftover_bits_appended_to_buf = 0;
+    while leftover_bits_appended_to_buf < leftover_bits_ready_to_append {
+        // `as` simply truncates the higher bits, which is what we want here
+        let selected_bits = (leftover_bits >> (56 - leftover_bits_appended_to_buf)) as u8;
+        output[output_index] = selected_bits;
+        output_index += 1;
+
+        leftover_bits_appended_to_buf += 8;
+    }
+
+    Ok(output_index)
+}
+
+#[inline]
+fn write_u64(output: &mut [u8], value: u64) {
+    output[..8].copy_from_slice(&value.to_be_bytes());
+}
+
+/// Decode 8 bytes of input into 6 bytes of output. 8 bytes of output will be written, but only the
+/// first 6 of those contain meaningful data.
+///
+/// `input` is the bytes to decode, of which the first 8 bytes will be processed.
+/// `index_at_start_of_input` is the offset in the overall input (used for reporting errors
+/// accurately)
+/// `decode_table` is the lookup table for the particular base64 alphabet.
+/// `output` will have its first 8 bytes overwritten, of which only the first 6 are valid decoded
+/// data.
+// yes, really inline (worth 30-50% speedup)
+#[inline(always)]
+fn decode_chunk(
+    input: &[u8],
+    index_at_start_of_input: usize,
+    decode_table: &[u8; 256],
+    output: &mut [u8],
+) -> Result<(), DecodeError> {
+    let mut accum: u64;
+
+    let morsel = decode_table[input[0] as usize];
+    if morsel == tables::INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(index_at_start_of_input, input[0]));
+    }
+    accum = (morsel as u64) << 58;
+
+    let morsel = decode_table[input[1] as usize];
+    if morsel == tables::INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 1,
+            input[1],
+        ));
+    }
+    accum |= (morsel as u64) << 52;
+
+    let morsel = decode_table[input[2] as usize];
+    if morsel == tables::INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 2,
+            input[2],
+        ));
+    }
+    accum |= (morsel as u64) << 46;
+
+    let morsel = decode_table[input[3] as usize];
+    if morsel == tables::INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 3,
+            input[3],
+        ));
+    }
+    accum |= (morsel as u64) << 40;
+
+    let morsel = decode_table[input[4] as usize];
+    if morsel == tables::INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 4,
+            input[4],
+        ));
+    }
+    accum |= (morsel as u64) << 34;
+
+    let morsel = decode_table[input[5] as usize];
+    if morsel == tables::INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 5,
+            input[5],
+        ));
+    }
+    accum |= (morsel as u64) << 28;
+
+    let morsel = decode_table[input[6] as usize];
+    if morsel == tables::INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 6,
+            input[6],
+        ));
+    }
+    accum |= (morsel as u64) << 22;
+
+    let morsel = decode_table[input[7] as usize];
+    if morsel == tables::INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 7,
+            input[7],
+        ));
+    }
+    accum |= (morsel as u64) << 16;
+
+    write_u64(output, accum);
+
+    Ok(())
+}
+
+/// Decode an 8-byte chunk, but only write the 6 bytes actually decoded instead of including 2
+/// trailing garbage bytes.
+#[inline]
+fn decode_chunk_precise(
+    input: &[u8],
+    index_at_start_of_input: usize,
+    decode_table: &[u8; 256],
+    output: &mut [u8],
+) -> Result<(), DecodeError> {
+    let mut tmp_buf = [0_u8; 8];
+
+    decode_chunk(
+        input,
+        index_at_start_of_input,
+        decode_table,
+        &mut tmp_buf[..],
+    )?;
+
+    output[0..6].copy_from_slice(&tmp_buf[0..6]);
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{
+        encode::encode_config_buf,
+        encode::encode_config_slice,
+        tests::{assert_encode_sanity, random_config},
+    };
+
+    use rand::{
+        distributions::{Distribution, Uniform},
+        FromEntropy, Rng,
+    };
+
+    #[test]
+    fn decode_chunk_precise_writes_only_6_bytes() {
+        let input = b"Zm9vYmFy"; // "foobar"
+        let mut output = [0_u8, 1, 2, 3, 4, 5, 6, 7];
+        decode_chunk_precise(&input[..], 0, tables::STANDARD_DECODE, &mut output).unwrap();
+        assert_eq!(&vec![b'f', b'o', b'o', b'b', b'a', b'r', 6, 7], &output);
+    }
+
+    #[test]
+    fn decode_chunk_writes_8_bytes() {
+        let input = b"Zm9vYmFy"; // "foobar"
+        let mut output = [0_u8, 1, 2, 3, 4, 5, 6, 7];
+        decode_chunk(&input[..], 0, tables::STANDARD_DECODE, &mut output).unwrap();
+        assert_eq!(&vec![b'f', b'o', b'o', b'b', b'a', b'r', 0, 0], &output);
+    }
+
+    #[test]
+    fn decode_into_nonempty_vec_doesnt_clobber_existing_prefix() {
+        let mut orig_data = Vec::new();
+        let mut encoded_data = String::new();
+        let mut decoded_with_prefix = Vec::new();
+        let mut decoded_without_prefix = Vec::new();
+        let mut prefix = Vec::new();
+
+        let prefix_len_range = Uniform::new(0, 1000);
+        let input_len_range = Uniform::new(0, 1000);
+
+        let mut rng = rand::rngs::SmallRng::from_entropy();
+
+        for _ in 0..10_000 {
+            orig_data.clear();
+            encoded_data.clear();
+            decoded_with_prefix.clear();
+            decoded_without_prefix.clear();
+            prefix.clear();
+
+            let input_len = input_len_range.sample(&mut rng);
+
+            for _ in 0..input_len {
+                orig_data.push(rng.gen());
+            }
+
+            let config = random_config(&mut rng);
+            encode_config_buf(&orig_data, config, &mut encoded_data);
+            assert_encode_sanity(&encoded_data, config, input_len);
+
+            let prefix_len = prefix_len_range.sample(&mut rng);
+
+            // fill the buf with a prefix
+            for _ in 0..prefix_len {
+                prefix.push(rng.gen());
+            }
+
+            decoded_with_prefix.resize(prefix_len, 0);
+            decoded_with_prefix.copy_from_slice(&prefix);
+
+            // decode into the non-empty buf
+            decode_config_buf(&encoded_data, config, &mut decoded_with_prefix).unwrap();
+            // also decode into the empty buf
+            decode_config_buf(&encoded_data, config, &mut decoded_without_prefix).unwrap();
+
+            assert_eq!(
+                prefix_len + decoded_without_prefix.len(),
+                decoded_with_prefix.len()
+            );
+            assert_eq!(orig_data, decoded_without_prefix);
+
+            // append plain decode onto prefix
+            prefix.append(&mut decoded_without_prefix);
+
+            assert_eq!(prefix, decoded_with_prefix);
+        }
+    }
+
+    #[test]
+    fn decode_into_slice_doesnt_clobber_existing_prefix_or_suffix() {
+        let mut orig_data = Vec::new();
+        let mut encoded_data = String::new();
+        let mut decode_buf = Vec::new();
+        let mut decode_buf_copy: Vec<u8> = Vec::new();
+
+        let input_len_range = Uniform::new(0, 1000);
+
+        let mut rng = rand::rngs::SmallRng::from_entropy();
+
+        for _ in 0..10_000 {
+            orig_data.clear();
+            encoded_data.clear();
+            decode_buf.clear();
+            decode_buf_copy.clear();
+
+            let input_len = input_len_range.sample(&mut rng);
+
+            for _ in 0..input_len {
+                orig_data.push(rng.gen());
+            }
+
+            let config = random_config(&mut rng);
+            encode_config_buf(&orig_data, config, &mut encoded_data);
+            assert_encode_sanity(&encoded_data, config, input_len);
+
+            // fill the buffer with random garbage, long enough to have some room before and after
+            for _ in 0..5000 {
+                decode_buf.push(rng.gen());
+            }
+
+            // keep a copy for later comparison
+            decode_buf_copy.extend(decode_buf.iter());
+
+            let offset = 1000;
+
+            // decode into the non-empty buf
+            let decode_bytes_written =
+                decode_config_slice(&encoded_data, config, &mut decode_buf[offset..]).unwrap();
+
+            assert_eq!(orig_data.len(), decode_bytes_written);
+            assert_eq!(
+                orig_data,
+                &decode_buf[offset..(offset + decode_bytes_written)]
+            );
+            assert_eq!(&decode_buf_copy[0..offset], &decode_buf[0..offset]);
+            assert_eq!(
+                &decode_buf_copy[offset + decode_bytes_written..],
+                &decode_buf[offset + decode_bytes_written..]
+            );
+        }
+    }
+
+    #[test]
+    fn decode_into_slice_fits_in_precisely_sized_slice() {
+        let mut orig_data = Vec::new();
+        let mut encoded_data = String::new();
+        let mut decode_buf = Vec::new();
+
+        let input_len_range = Uniform::new(0, 1000);
+
+        let mut rng = rand::rngs::SmallRng::from_entropy();
+
+        for _ in 0..10_000 {
+            orig_data.clear();
+            encoded_data.clear();
+            decode_buf.clear();
+
+            let input_len = input_len_range.sample(&mut rng);
+
+            for _ in 0..input_len {
+                orig_data.push(rng.gen());
+            }
+
+            let config = random_config(&mut rng);
+            encode_config_buf(&orig_data, config, &mut encoded_data);
+            assert_encode_sanity(&encoded_data, config, input_len);
+
+            decode_buf.resize(input_len, 0);
+
+            // decode into the non-empty buf
+            let decode_bytes_written =
+                decode_config_slice(&encoded_data, config, &mut decode_buf[..]).unwrap();
+
+            assert_eq!(orig_data.len(), decode_bytes_written);
+            assert_eq!(orig_data, decode_buf);
+        }
+    }
+
+    #[test]
+    fn detect_invalid_last_symbol_two_bytes() {
+        let decode =
+            |input, forgiving| decode_config(input, STANDARD.decode_allow_trailing_bits(forgiving));
+
+        // example from https://github.com/marshallpierce/rust-base64/issues/75
+        assert!(decode("iYU=", false).is_ok());
+        // trailing 01
+        assert_eq!(
+            Err(DecodeError::InvalidLastSymbol(2, b'V')),
+            decode("iYV=", false)
+        );
+        assert_eq!(Ok(vec![137, 133]), decode("iYV=", true));
+        // trailing 10
+        assert_eq!(
+            Err(DecodeError::InvalidLastSymbol(2, b'W')),
+            decode("iYW=", false)
+        );
+        assert_eq!(Ok(vec![137, 133]), decode("iYV=", true));
+        // trailing 11
+        assert_eq!(
+            Err(DecodeError::InvalidLastSymbol(2, b'X')),
+            decode("iYX=", false)
+        );
+        assert_eq!(Ok(vec![137, 133]), decode("iYV=", true));
+
+        // also works when there are 2 quads in the last block
+        assert_eq!(
+            Err(DecodeError::InvalidLastSymbol(6, b'X')),
+            decode("AAAAiYX=", false)
+        );
+        assert_eq!(Ok(vec![0, 0, 0, 137, 133]), decode("AAAAiYX=", true));
+    }
+
+    #[test]
+    fn detect_invalid_last_symbol_one_byte() {
+        // 0xFF -> "/w==", so all letters > w, 0-9, and '+', '/' should get InvalidLastSymbol
+
+        assert!(decode("/w==").is_ok());
+        // trailing 01
+        assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'x')), decode("/x=="));
+        assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'z')), decode("/z=="));
+        assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'0')), decode("/0=="));
+        assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'9')), decode("/9=="));
+        assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'+')), decode("/+=="));
+        assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'/')), decode("//=="));
+
+        // also works when there are 2 quads in the last block
+        assert_eq!(
+            Err(DecodeError::InvalidLastSymbol(5, b'x')),
+            decode("AAAA/x==")
+        );
+    }
+
+    #[test]
+    fn detect_invalid_last_symbol_every_possible_three_symbols() {
+        let mut base64_to_bytes = ::std::collections::HashMap::new();
+
+        let mut bytes = [0_u8; 2];
+        for b1 in 0_u16..256 {
+            bytes[0] = b1 as u8;
+            for b2 in 0_u16..256 {
+                bytes[1] = b2 as u8;
+                let mut b64 = vec![0_u8; 4];
+                assert_eq!(4, encode_config_slice(&bytes, STANDARD, &mut b64[..]));
+                let mut v = ::std::vec::Vec::with_capacity(2);
+                v.extend_from_slice(&bytes[..]);
+
+                assert!(base64_to_bytes.insert(b64, v).is_none());
+            }
+        }
+
+        // every possible combination of symbols must either decode to 2 bytes or get InvalidLastSymbol
+
+        let mut symbols = [0_u8; 4];
+        for &s1 in STANDARD.char_set.encode_table().iter() {
+            symbols[0] = s1;
+            for &s2 in STANDARD.char_set.encode_table().iter() {
+                symbols[1] = s2;
+                for &s3 in STANDARD.char_set.encode_table().iter() {
+                    symbols[2] = s3;
+                    symbols[3] = PAD_BYTE;
+
+                    match base64_to_bytes.get(&symbols[..]) {
+                        Some(bytes) => {
+                            assert_eq!(Ok(bytes.to_vec()), decode_config(&symbols, STANDARD))
+                        }
+                        None => assert_eq!(
+                            Err(DecodeError::InvalidLastSymbol(2, s3)),
+                            decode_config(&symbols[..], STANDARD)
+                        ),
+                    }
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn detect_invalid_last_symbol_every_possible_two_symbols() {
+        let mut base64_to_bytes = ::std::collections::HashMap::new();
+
+        for b in 0_u16..256 {
+            let mut b64 = vec![0_u8; 4];
+            assert_eq!(4, encode_config_slice(&[b as u8], STANDARD, &mut b64[..]));
+            let mut v = ::std::vec::Vec::with_capacity(1);
+            v.push(b as u8);
+
+            assert!(base64_to_bytes.insert(b64, v).is_none());
+        }
+
+        // every possible combination of symbols must either decode to 1 byte or get InvalidLastSymbol
+
+        let mut symbols = [0_u8; 4];
+        for &s1 in STANDARD.char_set.encode_table().iter() {
+            symbols[0] = s1;
+            for &s2 in STANDARD.char_set.encode_table().iter() {
+                symbols[1] = s2;
+                symbols[2] = PAD_BYTE;
+                symbols[3] = PAD_BYTE;
+
+                match base64_to_bytes.get(&symbols[..]) {
+                    Some(bytes) => {
+                        assert_eq!(Ok(bytes.to_vec()), decode_config(&symbols, STANDARD))
+                    }
+                    None => assert_eq!(
+                        Err(DecodeError::InvalidLastSymbol(1, s2)),
+                        decode_config(&symbols[..], STANDARD)
+                    ),
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn decode_config_estimation_works_for_various_lengths() {
+        for num_prefix_quads in 0..100 {
+            for suffix in &["AA", "AAA", "AAAA"] {
+                let mut prefix = "AAAA".repeat(num_prefix_quads);
+                prefix.push_str(suffix);
+                // make sure no overflow (and thus a panic) occurs
+                let res = decode_config(prefix, STANDARD);
+                assert!(res.is_ok());
+            }
+        }
+    }
+}