Simplify use of nested variant discriminants

(by removing the need for passing an explicit type name to the `#[enumcapsulate(discriminant(nested = …))]` variant attribute)

Author: regexident

CHANGELOG.md

@@ -24,7 +24,7 @@ Please make sure to add your changes to the appropriate categories:
 
 ### Changed
 
-- n/a
+- Changed the `#[enumcapsulate(discriminant(nested))]` variant attribute into a value-less unit attribute (`nested`), removing the previously required explicit type argument (`nested = <Type>`).
 
 ### Deprecated
 

macros/README.md

@@ -288,7 +288,7 @@ Specify a nested sub-discriminant type for the generated discriminant variant.
 
 If you wish a discriminant variant generated by the `VariantDiscriminant`
 derive macro to be nested, then you can do so by use
-of an `#[enumcapsulate(discriminant(nested = …))]` attribute:
+of an `#[enumcapsulate(discriminant(nested))]` attribute:
 
 ```rust
 #[derive(VariantDiscriminant)]
@@ -299,8 +299,8 @@ enum VariantA {
 
 #[derive(VariantDiscriminant)]
 enum Enum {
-    #[enumcapsulate(discriminant(nested = VariantADiscriminant))]
-    VariantA(i32),
+    #[enumcapsulate(discriminant(nested))]
+    VariantA(VariantA),
     
     VariantB { b: u32 },
 }

macros/src/config/for_variant.rs

@@ -1,6 +1,4 @@
 mod config;
 mod discriminant;
 
-pub(crate) use self::config::VariantConfig;
-
-use self::discriminant::DiscriminantConfig;
+pub(crate) use self::{config::*, discriminant::*};

macros/src/config/for_variant/discriminant.rs

@@ -6,7 +6,7 @@ use crate::attr::{NAME, NESTED, VALUE};
 pub(crate) struct DiscriminantConfig {
     value: Option<syn::Expr>,
     name: Option<syn::Ident>,
-    nested: Option<syn::Path>,
+    nested: bool,
 }
 
 impl DiscriminantConfig {
@@ -19,8 +19,8 @@ impl DiscriminantConfig {
             if meta.path.is_ident(VALUE) {
                 if self.value.is_some() {
                     return Err(meta.error("`value = …` already specified"));
-                } else if self.nested.is_some() {
-                    return Err(meta.error("conflicting with use of `nesting = …`"));
+                } else if self.nested {
+                    return Err(meta.error("conflicting with use of `nesting`"));
                 }
 
                 self.value = Some(meta.value()?.parse()?);
@@ -33,13 +33,13 @@ impl DiscriminantConfig {
             } else if meta.path.is_ident(NESTED) {
                 if matches!(variant.fields, syn::Fields::Unit) {
                     return Err(meta.error("no field found on variant"));
-                } else if self.nested.is_some() {
-                    return Err(meta.error("`nested = …` already specified"));
+                } else if self.nested {
+                    return Err(meta.error("`nested` already specified"));
                 } else if self.value.is_some() {
                     return Err(meta.error("conflicting with use of `value = …`"));
                 }
 
-                self.nested = Some(meta.value()?.parse()?);
+                self.nested = true;
             } else {
                 return Err(meta.error("unsupported discriminant attribute"));
             }
@@ -56,7 +56,7 @@ impl DiscriminantConfig {
         self.name.as_ref()
     }
 
-    pub(crate) fn nested(&self) -> Option<&syn::Path> {
-        self.nested.as_ref()
+    pub(crate) fn nested(&self) -> bool {
+        self.nested
     }
 }

macros/src/enum_deriver.rs

@@ -1,6 +1,6 @@
 use proc_macro2::TokenStream as TokenStream2;
 use quote::quote;
-use syn::{parse_quote_spanned, visit::Visit as _, Fields, Type, Variant};
+use syn::{parse_quote, parse_quote_spanned, visit::Visit as _, Fields, Type, Variant};
 
 use crate::*;
 
@@ -562,10 +562,13 @@ impl EnumDeriver {
         for (variant, variant_config) in variants.into_iter().zip(variant_configs) {
             let variant_ident: &syn::Ident = &variant.ident;
 
+            let field_selection = variant_config.selected_field(&variant.fields)?;
+            assert_eq!(field_selection.is_none(), variant.fields.is_empty());
+
             let mut discriminant_variant_ident: syn::Ident = variant_ident.clone();
             let mut discriminant_variant_expr: Option<&syn::Expr> =
                 variant.discriminant.as_ref().map(|(_, expr)| expr);
-            let mut discriminant_variant_nested: Option<&syn::Path> = None;
+            let mut discriminant_variant_nested: Option<syn::Type> = None;
 
             if let Some(discriminant_config) = variant_config.discriminant() {
                 if let Some(ident) = discriminant_config.ident() {
@@ -576,8 +579,13 @@ impl EnumDeriver {
                     discriminant_variant_expr = Some(expr);
                 }
 
-                if let Some(nested) = discriminant_config.nested() {
-                    discriminant_variant_nested = Some(nested);
+                if discriminant_config.nested() {
+                    let (field, _) = field_selection.expect("no selected field found");
+                    let ty = &field.ty;
+                    let nested_type = parse_quote! {
+                        <#ty as ::enumcapsulate::VariantDiscriminant>::Discriminant
+                    };
+                    discriminant_variant_nested = Some(nested_type);
                 }
             }
 
@@ -597,9 +605,6 @@ impl EnumDeriver {
                 #discriminant_variant_ident #variant_discriminant,
             });
 
-            let field_selection = variant_config.selected_field(&variant.fields)?;
-            assert_eq!(field_selection.is_none(), variant.fields.is_empty());
-
             let field_pattern = if variant_is_nested {
                 let (field, index) = field_selection.unwrap();
                 self.field_pattern(&variant.fields, (field, index))

tests/derive-tests/variant_discriminant/fail/nested_on_unit_variant.err.txt

@@ -1,6 +1,6 @@
 error: no field found on variant
  --> /tests/derive-tests/variant_discriminant/fail/nested_on_unit_variant.rs:5:34
   |
-5 |     #[enumcapsulate(discriminant(nested = VariantADiscriminant, value = 42))]
+5 |     #[enumcapsulate(discriminant(nested, value = 42))]
   |                                  ^^^^^^
 error: could not compile `<CRATE>` (bin "<BIN>") due to 1 previous error

tests/derive-tests/variant_discriminant/fail/nested_on_unit_variant.out.rs

@@ -1,6 +1,6 @@
 use enumcapsulate::VariantDiscriminant;
 pub enum Enum {
-    #[enumcapsulate(discriminant(nested = VariantADiscriminant, value = 42))]
+    #[enumcapsulate(discriminant(nested, value = 42))]
     VariantA,
 }
 fn main() {}

tests/derive-tests/variant_discriminant/fail/nested_on_unit_variant.rs

@@ -2,7 +2,7 @@ use enumcapsulate::VariantDiscriminant;
 
 #[derive(VariantDiscriminant)]
 pub enum Enum {
-    #[enumcapsulate(discriminant(nested = VariantADiscriminant, value = 42))]
+    #[enumcapsulate(discriminant(nested, value = 42))]
     VariantA,
 }
 

tests/derive-tests/variant_discriminant/fail/nested_with_value.err.txt

@@ -1,6 +1,6 @@
-error: conflicting with use of `nesting = …`
-  --> /tests/derive-tests/variant_discriminant/fail/nested_with_value.rs:11:65
+error: conflicting with use of `nesting`
+  --> /tests/derive-tests/variant_discriminant/fail/nested_with_value.rs:11:42
    |
-11 |     #[enumcapsulate(discriminant(nested = VariantADiscriminant, value = 42))]
-   |                                                                 ^^^^^
+11 |     #[enumcapsulate(discriminant(nested, value = 42))]
+   |                                          ^^^^^
 error: could not compile `<CRATE>` (bin "<BIN>") due to 1 previous error

tests/derive-tests/variant_discriminant/fail/nested_with_value.out.rs

@@ -87,7 +87,7 @@ impl ::enumcapsulate::VariantDiscriminant for VariantA {
     }
 }
 pub enum Enum {
-    #[enumcapsulate(discriminant(nested = VariantADiscriminant, value = 42))]
+    #[enumcapsulate(discriminant(nested, value = 42))]
     VariantA(VariantA),
 }
 fn main() {}