perf: only store start offsets for attribute spans

This spares us two usizes per AttrInternal<Range<usize>>.
So on a 64 bit target where a usize is 8 bytes this spares
us 16 bytes of memory per attribute (if spans are enabled,
... for Token<()> this obviously doesn't change anything).

And the DefaultEmitter now also no longer has to update the
spans on each Emitter::push_attribute_(name|value) call.

The spans are now calculated on demand by the Attribute methods,
which is fine since the assumption is that API users are only
interested in a few specific spans (rather than all spans).

1 files changed, 16 insertions, 8 deletions

diff --git a/src/attr.rs b/src/attr.rs
index a56eb95..4c7e330 100644
--- a/src/attr.rs
+++ b/src/attr.rs
@@ -30,8 +30,10 @@ pub struct AttributeMap<O> {
 #[derive(Debug, Eq, PartialEq)]
 pub(crate) struct AttrInternal<O> {
     pub value: String,
-    pub name_span: Range<O>,
-    pub value_span: Range<O>,
+    /// The start offset of the attribute name.
+    pub name_offset: O,
+    /// The start offset of the attribute value.
+    pub value_offset: O,
 }
 
 /// An HTML attribute borrowed from an [`AttributeMap`].
@@ -74,14 +76,14 @@ impl<'a, O: Offset> Attribute<'a, O> {
         &self.map_val.value
     }
 
-    /// Returns the span of the attribute name.
+    /// Calculates the span of the attribute name and returns it.
     pub fn name_span(&self) -> Range<O> {
-        self.map_val.name_span.clone()
+        self.map_val.name_offset..self.map_val.name_offset + self.name.len()
     }
 
-    /// Returns the span of the attribute value.
+    /// Calculates the span of the attribute value and returns it.
     pub fn value_span(&self) -> Range<O> {
-        self.map_val.value_span.clone()
+        self.map_val.value_offset..self.map_val.value_offset + self.map_val.value.len()
     }
 }
 
@@ -154,8 +156,8 @@ impl<O: Default> FromIterator<(String, String)> for AttributeMap<O> {
                         name,
                         AttrInternal {
                             value,
-                            name_span: O::default()..O::default(),
-                            value_span: O::default()..O::default(),
+                            name_offset: O::default(),
+                            value_offset: O::default(),
                         },
                     )
                 })
@@ -163,3 +165,9 @@ impl<O: Default> FromIterator<(String, String)> for AttributeMap<O> {
         }
     }
 }
+
+impl<O: Offset> AttrInternal<O> {
+    pub(crate) fn name_span(&self, name_len: usize) -> Range<O> {
+        self.name_offset..self.name_offset + name_len
+    }
+}