support enums with more than 256 variants in derive macro (#4370)

* support enums with more than 256 variants in derive macro

This addresses #4361.  Technically, we now support up to 2^32 variants, which is
the maximum for the canonical ABI.  In practice, though, the derived code for
enums with even just 2^16 variants takes a prohibitively long time to compile.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* simplify `LowerExpander::expand_variant` code

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

crates/misc/component-macro-test/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "component-macro-test"
+authors = ["The Wasmtime Project Developers"]
+license = "Apache-2.0 WITH LLVM-exception"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[lib]
+proc-macro = true
+
+[dependencies]
+proc-macro2 = "1.0"
+quote = "1.0"
+syn = { version = "1.0", features = ["full"] }

crates/misc/component-macro-test/src/lib.rs

@@ -0,0 +1,34 @@
+use proc_macro2::{Span, TokenStream};
+use quote::quote;
+use syn::parse_macro_input;
+
+#[proc_macro_attribute]
+pub fn add_variants(
+    attr: proc_macro::TokenStream,
+    item: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+    expand_variants(
+        &parse_macro_input!(attr as syn::LitInt),
+        parse_macro_input!(item as syn::ItemEnum),
+    )
+    .unwrap_or_else(syn::Error::into_compile_error)
+    .into()
+}
+
+fn expand_variants(count: &syn::LitInt, mut ty: syn::ItemEnum) -> syn::Result<TokenStream> {
+    let count = count
+        .base10_digits()
+        .parse::<usize>()
+        .map_err(|_| syn::Error::new(count.span(), "expected unsigned integer"))?;
+
+    ty.variants = (0..count)
+        .map(|index| syn::Variant {
+            attrs: Vec::new(),
+            ident: syn::Ident::new(&format!("V{}", index), Span::call_site()),
+            fields: syn::Fields::Unit,
+            discriminant: None,
+        })
+        .collect();
+
+    Ok(quote!(#ty))
+}