libsyntax_pos: Don't use packed attribute for Span on sparc64/v9

Due the limitation that #[derive(...)] on #[repr(packed)] structs
does not guarantee proper alignment of the compiler-generated
impls is not guaranteed (#39696), the change in #44646 to compress
Spans results in the compiler generating code with unaligned access.

Until #39696 has been fixed, the issue can be worked around by
not using the packed attribute on sparc64 and sparcv9 on the
Span struct.

Fixes: #45509

src/libsyntax_pos/span_encoding.rs

@@ -25,8 +25,10 @@ use std::cell::RefCell;
 /// The primary goal of `Span` is to be as small as possible and fit into other structures
 /// (that's why it uses `packed` as well). Decoding speed is the second priority.
 /// See `SpanData` for the info on span fields in decoded representation.
+
+/// FIXME: Don't use packed attribute on sparc64/v9, see: #45509
 #[derive(Clone, Copy, PartialEq, Eq, Hash)]
-#[repr(packed)]
+#[cfg_attr(not(any(target_arch = "sparc64", target_arch = "sparcv9")), repr(packed))]
 pub struct Span(u32);
 
 /// Dummy span, both position and length are zero, syntax context is zero as well.