Provide lengths for all types (#558)

Author: alex

README.md

@@ -29,7 +29,8 @@ $ cargo add asn1 --no-default-features
 
 - Added `Asn1Writable::encoded_length`, `SimpleAsn1Writable::data_length`, and
   `Asn1DefinedByWritable::encoded_length`. Implementing these functions reduces
-  the number of re-allocations required when writing.
+  the number of re-allocations required when writing. `None` can be returned if
+  it is not possible to provide an efficient implementation.
 
 #### Changes
 

src/types.rs

@@ -96,12 +96,10 @@ pub trait SimpleAsn1Writable: Sized {
     /// Get the length of the data content (without tag and length bytes) if it
     /// can be calculated efficiently.
     ///
-    /// The default implementation returns None, meaning the length is unknown.
-    /// Providing this method reduces the number of re-allocations required in
-    /// writing.
-    fn data_length(&self) -> Option<usize> {
-        None
-    }
+    /// It is always safe to return `None`, which indicates the length is
+    /// unknown. Returning `Some(...)` from this method reduces the number of
+    /// re-allocations required in writing.
+    fn data_length(&self) -> Option<usize>;
 }
 
 /// A trait for types that can be parsed based on a `DEFINED BY` value.
@@ -1608,6 +1606,10 @@ impl SimpleAsn1Writable for SequenceWriter<'_> {
     fn write_data(&self, dest: &mut WriteBuf) -> WriteResult {
         (self.f)(&mut Writer::new(dest))
     }
+
+    fn data_length(&self) -> Option<usize> {
+        None
+    }
 }
 
 /// Represents an ASN.1 `SEQUENCE OF`. This is an `Iterator` over values that
@@ -1753,6 +1755,11 @@ impl<
 
         Ok(())
     }
+
+    fn data_length(&self) -> Option<usize> {
+        let iter = self.clone();
+        iter.map(|el| el.encoded_length()).sum()
+    }
 }
 
 /// Writes a `SEQUENCE OF` ASN.1 structure from a slice of `T`.
@@ -1898,6 +1905,10 @@ impl<'a, T: Asn1Readable<'a> + Asn1Writable> SimpleAsn1Writable for SetOf<'a, T>
 
         Ok(())
     }
+    fn data_length(&self) -> Option<usize> {
+        let iter = self.clone();
+        iter.map(|el| el.encoded_length()).sum()
+    }
 }
 
 /// Writes an ASN.1 `SET OF` whose contents is a slice of `T`. This type handles