[librsvg: 1/2] Box a big variant in CascadedValues to use less stack space

[Federico Mena Quintero <federico src gnome org>]

commit e500e03a054a853fd6befd00b27b720b7a6d3171
Author: Federico Mena Quintero <federico gnome org>
Date:   Thu Feb 25 17:52:22 2021 -0600

    Box a big variant in CascadedValues to use less stack space
    
    CascadedInner has two variants, and the second is a big struct:
    
        #[allow(clippy::large_enum_variant)]
        enum CascadedInner<'a> {
            FromNode(Ref<'a, Element>),
            FromValues(ComputedValues),
        }
    
    (Yes, clippy warned us...)
    
    It turns out that deeply-nested <use> elements can consume a lot of
    stack space, since every nested call to
    DrawingCtx::draw_from_use_node() will create a
    CascadedInner::FromValues - it is specifically to handle the
    independent cascading of nodes referenced from <use>.
    
    This commit just boxes the payload of that variant, so it should
    shrink the stack space of draw_from_use_node() by approximately 700
    bytes.
    
    Old size_of::<CascadedValues<'_>>() = 776
    New size_of::<CascadedValues<'_>>() = 24 (plus the rest in the heap)
    
    https://gitlab.gnome.org/GNOME/librsvg/-/issues/686

 src/node.rs | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)
---
diff --git a/src/node.rs b/src/node.rs
index 5c9ddef2..862d2328 100644
--- a/src/node.rs
+++ b/src/node.rs
@@ -106,10 +106,9 @@ pub struct CascadedValues<'a> {
     inner: CascadedInner<'a>,
 }
 
-#[allow(clippy::large_enum_variant)]
 enum CascadedInner<'a> {
     FromNode(Ref<'a, Element>),
-    FromValues(ComputedValues),
+    FromValues(Box<ComputedValues>),
 }
 
 impl<'a> CascadedValues<'a> {
@@ -124,7 +123,7 @@ impl<'a> CascadedValues<'a> {
                 inner: CascadedInner::FromNode(node.borrow_element()),
             },
 
-            CascadedInner::FromValues(ref v) => CascadedValues::new_from_values(node, v),
+            CascadedInner::FromValues(ref v) => CascadedValues::new_from_values(node, &*v),
         }
     }
 
@@ -145,7 +144,7 @@ impl<'a> CascadedValues<'a> {
     /// This is for the `<use>` element, which draws the element which it references with the
     /// `<use>`'s own cascade, not with the element's original cascade.
     pub fn new_from_values(node: &'a Node, values: &ComputedValues) -> CascadedValues<'a> {
-        let mut v = values.clone();
+        let mut v = Box::new(values.clone());
         node.borrow_element()
             .get_specified_values()
             .to_computed_values(&mut v);
@@ -162,7 +161,7 @@ impl<'a> CascadedValues<'a> {
     pub fn get(&'a self) -> &'a ComputedValues {
         match self.inner {
             CascadedInner::FromNode(ref e) => e.get_computed_values(),
-            CascadedInner::FromValues(ref v) => v,
+            CascadedInner::FromValues(ref v) => &*v,
         }
     }
 }