How do I write a declarative macro that takes an expression with a free variable?

[jwodder]

I'm trying to write a declarative macro for generating trait implementations that differ only in type, a literal, and the expression used to retrieve a value from an argument. My code at the moment is:

use bytes::{Buf, Bytes};
use std::net::Ipv4Addr;

pub trait TryFromBuf: Sized {
    fn try_from_buf(buf: &mut Bytes) -> Result<Self, PacketError>;
}

macro_rules! impl_tryfrombuf {
    ($t:ty, $len:literal, $get:expr) => {
        impl TryFromBuf for $t {
            fn try_from_buf(buf: &mut Bytes) -> Result<Self, PacketError> {
                if buf.remaining() >= $len {
                    Ok($get)
                } else {
                    Err(PacketError)
                }
            }
        }
    };
}

impl_tryfrombuf!(u32, 4, buf.get_u32());
impl_tryfrombuf!(Ipv4Addr, 4, buf.get_u32().into());

pub struct PacketError;

Rust Playground

However, this fails to compile, apparently because buf is not defined at the point where the macro is invoked:

error[E0425]: cannot find value `buf` in this scope
  --> src/lib.rs:22:26
   |
22 | impl_tryfrombuf!(u32, 4, buf.get_u32());
   |                          ^^^ not found in this scope

error[E0425]: cannot find value `buf` in this scope
  --> src/lib.rs:23:31
   |
23 | impl_tryfrombuf!(Ipv4Addr, 4, buf.get_u32().into());
   |                               ^^^ not found in this scope

For more information about this error, try `rustc --explain E0425`.

I've tried changing :expr to :stmnt, changing :expr to :block and adding braces around the expressions, and changing $get:expr & Ok($get) to $($get:tt)+ & Ok($( $get )+), but none have compiled. How can I make this work?

[H2CO3]

Declarative macros are hygienic, so an identifier that is declared inside the macro cannot be used outside of it. No amount of token matcher declarations can change this.

You have a couple of options:

1. switch to a procedural macro
2. accept an explicit name/identifier instead of naming it buf and pass it to the macro
3. use an immediately invoked closure

Approach #2 is demonstrated here. And approach #3 is here.

[Kromgart]

Another option is to match the function/method calls into token-trees and keep the buf variable completely inside the macro:

use bytes::{Buf, Bytes};
use std::net::Ipv4Addr;

pub trait TryFromBuf: Sized {
    fn try_from_buf(buf: &mut Bytes) -> Result<Self, PacketError>;
}

macro_rules! impl_tryfrombuf {
    ($t:ty, $len:literal, $($get:tt)+) => {
        impl TryFromBuf for $t {
            fn try_from_buf(buf: &mut Bytes) -> Result<Self, PacketError> {
                if buf.remaining() >= $len {
                    Ok(buf.$($get)+)
                } else {
                    Err(PacketError)
                }
            }
        }
    };
}

impl_tryfrombuf!(u32, 4, get_u32());
impl_tryfrombuf!(Ipv4Addr, 4, get_u32().into());

pub struct PacketError;