Add user seed to -Z randomize-layout

compiler/rustc_middle/src/ty/layout.rs

@@ -347,10 +347,6 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
 
         let mut inverse_memory_index: Vec<u32> = (0..fields.len() as u32).collect();
 
-        // `ReprOptions.layout_seed` is a deterministic seed that we can use to
-        // randomize field ordering with
-        let mut rng = Xoshiro128StarStar::seed_from_u64(repr.field_shuffle_seed);
-
         let optimize = !repr.inhibit_struct_field_reordering_opt();
         if optimize {
             let end =
@@ -364,6 +360,10 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
             // the field ordering to try and catch some code making assumptions about layouts
             // we don't guarantee
             if repr.can_randomize_type_layout() {
+                // `ReprOptions.layout_seed` is a deterministic seed that we can use to
+                // randomize field ordering with
+                let mut rng = Xoshiro128StarStar::seed_from_u64(repr.field_shuffle_seed);
+
                 // Shuffle the ordering of the fields
                 optimizing.shuffle(&mut rng);
 

compiler/rustc_middle/src/ty/mod.rs

@@ -1608,9 +1608,9 @@ bitflags! {
         // the seed stored in `ReprOptions.layout_seed`
         const RANDOMIZE_LAYOUT   = 1 << 5;
         // Any of these flags being set prevent field reordering optimisation.
-        const IS_UNOPTIMISABLE   = ReprFlags::IS_C.bits |
-                                   ReprFlags::IS_SIMD.bits |
-                                   ReprFlags::IS_LINEAR.bits;
+        const IS_UNOPTIMISABLE   = ReprFlags::IS_C.bits
+                                 | ReprFlags::IS_SIMD.bits
+                                 | ReprFlags::IS_LINEAR.bits;
     }
 }
 
@@ -1640,7 +1640,14 @@ impl ReprOptions {
 
         // Generate a deterministically-derived seed from the item's path hash
         // to allow for cross-crate compilation to actually work
-        let field_shuffle_seed = tcx.def_path_hash(did).0.to_smaller_hash();
+        let mut field_shuffle_seed = tcx.def_path_hash(did).0.to_smaller_hash();
+
+        // If the user defined a custom seed for layout randomization, xor the item's
+        // path hash with the user defined seed, this will allowing determinism while
+        // still allowing users to further randomize layout generation for e.g. fuzzing
+        if let Some(user_seed) = tcx.sess.opts.debugging_opts.layout_seed {
+            field_shuffle_seed ^= user_seed;
+        }
 
         for attr in tcx.get_attrs(did).iter() {
             for r in attr::find_repr_attrs(&tcx.sess, attr) {

compiler/rustc_session/src/options.rs

@@ -1321,6 +1321,8 @@ options! {
         "print some statistics about the query system (default: no)"),
     randomize_layout: bool = (false, parse_bool, [TRACKED],
         "randomize the layout of types (default: no)"),
+    layout_seed: Option<u64> = (None, parse_opt_number, [TRACKED],
+        "seed layout randomization"),
     relax_elf_relocations: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "whether ELF relocations can be relaxed"),
     relro_level: Option<RelroLevel> = (None, parse_relro_level, [TRACKED],