rust: qemu-api-macros: add automatic TryFrom/TryInto derivation

This is going to be fairly common. Using a custom procedural macro
provides better error messages and automatically finds the right
type.

Note that this is different from the same-named macro in the
derive_more crate.  That one provides conversion from e.g. tuples
to enums with tuple variants, not from integers to enums.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api-macros/src/lib.rs

@@ -5,8 +5,8 @@
 use proc_macro::TokenStream;
 use quote::quote;
 use syn::{
-    parse_macro_input, parse_quote, punctuated::Punctuated, token::Comma, Data, DeriveInput, Field,
-    Fields, Ident, Type, Visibility,
+    parse_macro_input, parse_quote, punctuated::Punctuated, spanned::Spanned, token::Comma, Data,
+    DeriveInput, Field, Fields, Ident, Meta, Path, Token, Type, Variant, Visibility,
 };
 
 mod utils;
@@ -98,3 +98,73 @@ pub fn derive_offsets(input: TokenStream) -> TokenStream {
 
     TokenStream::from(expanded)
 }
+
+#[allow(non_snake_case)]
+fn get_repr_uN(input: &DeriveInput, msg: &str) -> Result<Path, MacroError> {
+    let repr = input.attrs.iter().find(|attr| attr.path().is_ident("repr"));
+    if let Some(repr) = repr {
+        let nested = repr.parse_args_with(Punctuated::<Meta, Token![,]>::parse_terminated)?;
+        for meta in nested {
+            match meta {
+                Meta::Path(path) if path.is_ident("u8") => return Ok(path),
+                Meta::Path(path) if path.is_ident("u16") => return Ok(path),
+                Meta::Path(path) if path.is_ident("u32") => return Ok(path),
+                Meta::Path(path) if path.is_ident("u64") => return Ok(path),
+                _ => {}
+            }
+        }
+    }
+
+    Err(MacroError::Message(
+        format!("#[repr(u8/u16/u32/u64) required for {}", msg),
+        input.ident.span(),
+    ))
+}
+
+fn get_variants(input: &DeriveInput) -> Result<&Punctuated<Variant, Comma>, MacroError> {
+    if let Data::Enum(e) = &input.data {
+        if let Some(v) = e.variants.iter().find(|v| v.fields != Fields::Unit) {
+            return Err(MacroError::Message(
+                "Cannot derive TryInto for enum with non-unit variants.".to_string(),
+                v.fields.span(),
+            ));
+        }
+        Ok(&e.variants)
+    } else {
+        Err(MacroError::Message(
+            "Cannot derive TryInto for union or struct.".to_string(),
+            input.ident.span(),
+        ))
+    }
+}
+
+#[rustfmt::skip::macros(quote)]
+fn derive_tryinto_or_error(input: DeriveInput) -> Result<proc_macro2::TokenStream, MacroError> {
+    let repr = get_repr_uN(&input, "#[derive(TryInto)]")?;
+
+    let name = &input.ident;
+    let variants = get_variants(&input)?;
+    let discriminants: Vec<&Ident> = variants.iter().map(|f| &f.ident).collect();
+
+    Ok(quote! {
+        impl core::convert::TryFrom<#repr> for #name {
+            type Error = #repr;
+
+            fn try_from(value: #repr) -> Result<Self, Self::Error> {
+                #(const #discriminants: #repr = #name::#discriminants as #repr;)*;
+                match value {
+                    #(#discriminants => Ok(Self::#discriminants),)*
+                    _ => Err(value),
+                }
+            }
+        }
+    })
+}
+
+#[proc_macro_derive(TryInto)]
+pub fn derive_tryinto(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    let expanded = derive_tryinto_or_error(input).unwrap_or_else(Into::into);
+
+    TokenStream::from(expanded)
+}