Fix discriminant size of enums/variants (#12066)

* Fix discriminant size of enums/variants

This fixes an off-by-one-error that's been present for lifting/lowering
enums/variants for... forever I think? This means that precisely
256-variant-enums and 65536-variant-enums have been incorrectly loaded
from memory historically (or stored). Discovered via the new fuzz target
for roundtripping values and I expect the practical impact of this
bugfix to be negligible.

* Fix generation of `#[repr(..)]` in fuzzing

* Fix 32-bit compile

* Fix fuzzing

Author: alexcrichton

crates/component-util/src/lib.rs

@@ -21,11 +21,11 @@ pub enum DiscriminantSize {
 impl DiscriminantSize {
     /// Calculate the size of discriminant needed to represent a variant with the specified number of cases.
     pub const fn from_count(count: usize) -> Option<Self> {
-        if count <= 0xFF {
+        if count <= 1 << 8 {
             Some(Self::Size1)
-        } else if count <= 0xFFFF {
+        } else if count <= 1 << 16 {
             Some(Self::Size2)
-        } else if count <= 0xFFFF_FFFF {
+        } else if count as u64 <= 1 << 32 {
             Some(Self::Size4)
         } else {
             None

crates/test-util/src/component_fuzz.rs

@@ -1234,10 +1234,10 @@ pub fn rust_type(ty: &Type, name_counter: &mut u32, declarations: &mut TokenStre
                 .collect::<TokenStream>();
 
             let name = make_rust_name(name_counter);
-            let repr = match count.ilog2() {
-                0..=7 => quote!(u8),
-                8..=15 => quote!(u16),
-                _ => quote!(u32),
+            let repr = match DiscriminantSize::from_count(*count as usize).unwrap() {
+                DiscriminantSize::Size1 => quote!(u8),
+                DiscriminantSize::Size2 => quote!(u16),
+                DiscriminantSize::Size4 => quote!(u32),
             };
 
             declarations.extend(quote! {

tests/misc_testsuite/component-model/enums.wast

@@ -0,0 +1,55 @@
+(component
+
+  (core module $m
+    (memory (export "mem") 1)
+    (func (export "return-two") (param i32) (result i32)
+      i32.const 100
+      local.get 0
+      i32.store8 offset=0
+      i32.const 100
+      local.get 0
+      i32.store8 offset=1
+      i32.const 100)
+  )
+  (core instance $i (instantiate $m))
+  (type $e' (enum
+    "E0" "E1" "E2" "E3" "E4" "E5" "E6" "E7"
+    "E8" "E9" "E10" "E11" "E12" "E13" "E14" "E15"
+    "E16" "E17" "E18" "E19" "E20" "E21" "E22" "E23"
+    "E24" "E25" "E26" "E27" "E28" "E29" "E30" "E31"
+    "E32" "E33" "E34" "E35" "E36" "E37" "E38" "E39"
+    "E40" "E41" "E42" "E43" "E44" "E45" "E46" "E47"
+    "E48" "E49" "E50" "E51" "E52" "E53" "E54" "E55"
+    "E56" "E57" "E58" "E59" "E60" "E61" "E62" "E63"
+    "E64" "E65" "E66" "E67" "E68" "E69" "E70" "E71"
+    "E72" "E73" "E74" "E75" "E76" "E77" "E78" "E79"
+    "E80" "E81" "E82" "E83" "E84" "E85" "E86" "E87"
+    "E88" "E89" "E90" "E91" "E92" "E93" "E94" "E95"
+    "E96" "E97" "E98" "E99" "E100" "E101" "E102" "E103"
+    "E104" "E105" "E106" "E107" "E108" "E109" "E110" "E111"
+    "E112" "E113" "E114" "E115" "E116" "E117" "E118" "E119"
+    "E120" "E121" "E122" "E123" "E124" "E125" "E126" "E127"
+    "E128" "E129" "E130" "E131" "E132" "E133" "E134" "E135"
+    "E136" "E137" "E138" "E139" "E140" "E141" "E142" "E143"
+    "E144" "E145" "E146" "E147" "E148" "E149" "E150" "E151"
+    "E152" "E153" "E154" "E155" "E156" "E157" "E158" "E159"
+    "E160" "E161" "E162" "E163" "E164" "E165" "E166" "E167"
+    "E168" "E169" "E170" "E171" "E172" "E173" "E174" "E175"
+    "E176" "E177" "E178" "E179" "E180" "E181" "E182" "E183"
+    "E184" "E185" "E186" "E187" "E188" "E189" "E190" "E191"
+    "E192" "E193" "E194" "E195" "E196" "E197" "E198" "E199"
+    "E200" "E201" "E202" "E203" "E204" "E205" "E206" "E207"
+    "E208" "E209" "E210" "E211" "E212" "E213" "E214" "E215"
+    "E216" "E217" "E218" "E219" "E220" "E221" "E222" "E223"
+    "E224" "E225" "E226" "E227" "E228" "E229" "E230" "E231"
+    "E232" "E233" "E234" "E235" "E236" "E237" "E238" "E239"
+    "E240" "E241" "E242" "E243" "E244" "E245" "E246" "E247"
+    "E248" "E249" "E250" "E251" "E252" "E253" "E254" "E255"
+  ))
+  (export $e "e" (type $e'))
+  (func (export "return-two") (param "e" $e) (result (tuple $e $e))
+    (canon lift (core func $i "return-two") (memory $i "mem")))
+)
+
+(assert_return (invoke "return-two" (enum.const "E1"))
+  (tuple.const (enum.const "E1") (enum.const "E1")))

tests/misc_testsuite/component-model/resources.wast

@@ -1,3 +1,5 @@
+;;! component_model_async = true
+
 ;; bare bones "intrinsics work"
 (component
   (type $r (resource (rep i32)))