Increase `Span` from 4 bytes to 8 bytes.

This increases the size of some important types, such as `ast::Expr` and
`mir::Statement`. However, it drastically reduces how much the interner
is used, and the fields are more natural sizes that don't require bit
operations to extract.

As a result, instruction counts drop across a range of workloads, by as
much as 12% for incremental "check" builds of `script-servo`.

Peak memory usage goes up a little for some cases, but down by more for
some other cases -- as much as 18% for non-incremental builds of
`packed-simd`.

The commit also:
- removes the `repr(packed)`, because it has negligible effect, but can
  cause undefined behaviour;
- replaces explicit impls of common traits (`Copy`, `PartialEq`, etc.)
  with derived ones.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index bcc8fdf8cd4..cf909e30e32 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -946,7 +946,7 @@ pub struct Expr {
 
 // `Expr` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(target_arch = "x86_64")]
-static_assert!(MEM_SIZE_OF_EXPR: std::mem::size_of::<Expr>() == 88);
+static_assert!(MEM_SIZE_OF_EXPR: std::mem::size_of::<Expr>() == 96);
 
 impl Expr {
     /// Whether this expression would be valid somewhere that expects a value; for example, an `if`