2 files changed, 19 insertions, 2 deletions
diff --git a/src/encoding/base32/base32.go b/src/encoding/base32/base32.go
index de95df0043..e921887285 100644
--- a/src/encoding/base32/base32.go
+++ b/src/encoding/base32/base32.go
@@ -402,7 +402,13 @@ func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
 // and returns the extended buffer.
 // If the input is malformed, it returns the partially decoded src and an error.
 func (enc *Encoding) AppendDecode(dst, src []byte) ([]byte, error) {
-	n := enc.DecodedLen(len(src))
+	// Compute the output size without padding to avoid over allocating.
+	n := len(src)
+	for n > 0 && rune(src[n-1]) == enc.padChar {
+		n--
+	}
+	n = decodedLen(n, NoPadding)
+
 	dst = slices.Grow(dst, n)
 	n, err := enc.Decode(dst[len(dst):][:n], src)
 	return dst[:len(dst)+n], err
@@ -567,7 +573,11 @@ func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
 // DecodedLen returns the maximum length in bytes of the decoded data
 // corresponding to n bytes of base32-encoded data.
 func (enc *Encoding) DecodedLen(n int) int {
-	if enc.padChar == NoPadding {
+	return decodedLen(n, enc.padChar)
+}
+
+func decodedLen(n int, padChar rune) int {
+	if padChar == NoPadding {
 		return n/8*5 + n%8*5/8
 	}
 	return n / 8 * 5
diff --git a/src/encoding/base32/base32_test.go b/src/encoding/base32/base32_test.go
index 0132744507..33638adeac 100644
--- a/src/encoding/base32/base32_test.go
+++ b/src/encoding/base32/base32_test.go
@@ -110,6 +110,13 @@ func TestDecode(t *testing.T) {
 		dst, err := StdEncoding.AppendDecode([]byte("lead"), []byte(p.encoded))
 		testEqual(t, "AppendDecode(%q) = error %v, want %v", p.encoded, err, error(nil))
 		testEqual(t, `AppendDecode("lead", %q) = %q, want %q`, p.encoded, string(dst), "lead"+p.decoded)
+
+		dst2, err := StdEncoding.AppendDecode(dst[:0:len(p.decoded)], []byte(p.encoded))
+		testEqual(t, "AppendDecode(%q) = error %v, want %v", p.encoded, err, error(nil))
+		testEqual(t, `AppendDecode("", %q) = %q, want %q`, p.encoded, string(dst2), p.decoded)
+		if len(dst) > 0 && len(dst2) > 0 && &dst[0] != &dst2[0] {
+			t.Errorf("unexpected capacity growth: got %d, want %d", cap(dst2), cap(dst))
+		}
 	}
 }