1 files changed, 99 insertions, 0 deletions
diff --git a/syn-mid/src/arg.rs b/syn-mid/src/arg.rs
new file mode 100644
index 0000000..593a1ac
--- /dev/null
+++ b/syn-mid/src/arg.rs
@@ -0,0 +1,99 @@
+use syn::{Attribute, Lifetime, Token};
+
+use super::PatType;
+
+ast_enum_of_structs! {
+    /// An argument in a function signature: the `n: usize` in `fn f(n: usize)`.
+    pub enum FnArg {
+        /// The `self` argument of an associated method, whether taken by value
+        /// or by reference.
+        Receiver(Receiver),
+
+        /// A function argument accepted by pattern and type.
+        Typed(PatType),
+    }
+}
+
+ast_struct! {
+    /// The `self` argument of an associated method, whether taken by value
+    /// or by reference.
+    pub struct Receiver {
+        pub attrs: Vec<Attribute>,
+        pub reference: Option<(Token![&], Option<Lifetime>)>,
+        pub mutability: Option<Token![mut]>,
+        pub self_token: Token![self],
+    }
+}
+
+mod parsing {
+    use syn::{
+        parse::{discouraged::Speculative, Parse, ParseStream, Result},
+        Attribute, Token,
+    };
+
+    use super::{FnArg, PatType, Receiver};
+
+    impl Parse for FnArg {
+        fn parse(input: ParseStream<'_>) -> Result<Self> {
+            let attrs = input.call(Attribute::parse_outer)?;
+
+            let ahead = input.fork();
+            if let Ok(mut receiver) = ahead.parse::<Receiver>() {
+                if !ahead.peek(Token![:]) {
+                    input.advance_to(&ahead);
+                    receiver.attrs = attrs;
+                    return Ok(FnArg::Receiver(receiver));
+                }
+            }
+
+            let mut typed = input.call(fn_arg_typed)?;
+            typed.attrs = attrs;
+            Ok(FnArg::Typed(typed))
+        }
+    }
+
+    impl Parse for Receiver {
+        fn parse(input: ParseStream<'_>) -> Result<Self> {
+            Ok(Self {
+                attrs: Vec::new(),
+                reference: {
+                    if input.peek(Token![&]) {
+                        Some((input.parse()?, input.parse()?))
+                    } else {
+                        None
+                    }
+                },
+                mutability: input.parse()?,
+                self_token: input.parse()?,
+            })
+        }
+    }
+
+    fn fn_arg_typed(input: ParseStream<'_>) -> Result<PatType> {
+        Ok(PatType {
+            attrs: Vec::new(),
+            pat: input.parse()?,
+            colon_token: input.parse()?,
+            ty: Box::new(input.parse()?),
+        })
+    }
+}
+
+mod printing {
+    use proc_macro2::TokenStream;
+    use quote::{ToTokens, TokenStreamExt};
+
+    use super::Receiver;
+
+    impl ToTokens for Receiver {
+        fn to_tokens(&self, tokens: &mut TokenStream) {
+            tokens.append_all(&self.attrs);
+            if let Some((ampersand, lifetime)) = &self.reference {
+                ampersand.to_tokens(tokens);
+                lifetime.to_tokens(tokens);
+            }
+            self.mutability.to_tokens(tokens);
+            self.self_token.to_tokens(tokens);
+        }
+    }
+}