enum_visitor_derive/
lib.rs

1use proc_macro::TokenStream;
2use quote::{format_ident, quote};
3use syn::{parse_macro_input, Data, DeriveInput, Fields};
4
5/// Derive macro to generate helper macros for a given enum.
6///
7/// It generates two local macros in the same module as the enum:
8/// - `visit_<enum_snake>!(expr, |v| body)` — unique name, avoids conflicts;
9/// - `visit!(expr, |v| body)` — convenient short name for use in the
10///   same module/impl; note: if you derive this on multiple enums in the same
11///   module，该宏名会发生冲突，请将枚举分到不同 `mod` 中或使用带枚举名的宏。
12///
13/// 限制：当前仅支持“单元素元组变体”（如 `Enum::Variant(T)`）。
14#[proc_macro_derive(VisitEnum)]
15pub fn derive_visit_enum(input: TokenStream) -> TokenStream {
16    let input = parse_macro_input!(input as DeriveInput);
17
18    let enum_ident = input.ident;
19    let enum_name = enum_ident.to_string();
20    let macro_ident = format_ident!("visit_{}", to_snake_case(&enum_name));
21
22    let data_enum = match input.data {
23        Data::Enum(e) => e,
24        _ => {
25            return syn::Error::new_spanned(enum_ident, "VisitEnum can only be derived for enums")
26                .to_compile_error()
27                .into();
28        }
29    };
30
31    // Collect variant idents and ensure single unnamed field
32    let mut arms_expr = Vec::new();
33    let mut arms_block = Vec::new();
34
35    for variant in data_enum.variants.iter() {
36        let v_ident = &variant.ident;
37        match &variant.fields {
38            Fields::Unnamed(unnamed) if unnamed.unnamed.len() == 1 => {
39                // ok
40            }
41            _ => {
42                return syn::Error::new_spanned(
43                    &variant.ident,
44                    "VisitEnum only supports tuple variants with exactly 1 field",
45                )
46                .to_compile_error()
47                .into();
48            }
49        }
50
51        // Expression-body arm: use `$v` and `$body` from the macro invocation site.
52        arms_expr.push(quote! { #enum_ident::#v_ident($v) => { $body } });
53        // Block-body arm: use `$v` and `$($tt)*` from the invocation site.
54        arms_block.push(quote! { #enum_ident::#v_ident($v) => { $($tt)* } });
55    }
56
57    // Build the macro definition. We create a local macro (not exported) so its
58    // name stays within the user's crate namespace. The name encodes the enum
59    // name to avoid collisions.
60    let gen = quote! {
61        // A helper macro to visit all variants of this enum.
62        // Usage: `visit_<enum_snake>!(expr, |v| <use v> )`
63        #[allow(non_snake_case, unused_macros)]
64        macro_rules! #macro_ident {
65            ($expr:expr, |$v:pat_param| $body:expr $(,)?) => {{
66                match $expr {
67                    #( #arms_expr ),*
68                }
69            }};
70            ($expr:expr, |$v:pat_param| { $($tt:tt)* } $(,)?) => {{
71                match $expr {
72                    #( #arms_block ),*
73                }
74            }};
75        }
76
77        // Convenience short name within the same module.
78        #[allow(unused_macros)]
79        macro_rules! visit {
80            ($expr:expr, |$v:pat_param| $body:expr $(,)?) => { #macro_ident!($expr, |$v| $body) };
81            ($expr:expr, |$v:pat_param| { $($tt:tt)* } $(,)?) => { #macro_ident!($expr, |$v| { $($tt)* }) };
82        }
83    };
84
85    gen.into()
86}
87
88fn to_snake_case(name: &str) -> String {
89    // minimal snake_case converter: FooBar -> foo_bar
90    let mut out = String::new();
91    for (i, ch) in name.chars().enumerate() {
92        if ch.is_uppercase() {
93            if i != 0 {
94                out.push('_');
95            }
96            for c in ch.to_lowercase() {
97                out.push(c);
98            }
99        } else {
100            out.push(ch);
101        }
102    }
103    out
104}
enum_visitor_derive/lib.rs

enum_visitor_derive/
lib.rs
