`#[deny(unreachable_patterns)]` does not yield an error with duplicated endpoints

[glaeqen]

To reproduce:

1. Apply patch

   diff --git a/example/firmware/src/bin/comms-02.rs b/example/firmware/src/bin/comms-02.rs
   index ad1efff..b7b4106 100644
   --- a/example/firmware/src/bin/comms-02.rs
   +++ b/example/firmware/src/bin/comms-02.rs
   @@ -68,6 +68,7 @@ define_dispatch! {
        Context = Context,
    >;
    PingEndpoint => blocking ping_handler,
   +    PingEndpoint => blocking ping_handler,
    GetUniqueIdEndpoint => blocking unique_id_handler,
    SetSingleLedEndpoint => async set_led_handler,
    SetAllLedEndpoint => async set_all_led_handler,

2. Try to build firmware crate
3. Observe success

Expected behaviour

Compilation failure

[jamesmunns]

This seems to be down to the fact that lints in expanded code doesn't fire.

If I make one crate that has this:

pub trait Val {
    const VAL: [u8; 8];
}

impl Val for u8 {
    const VAL: [u8; 8] = [0u8; 8];
}

impl Val for u16 {
    const VAL: [u8; 8] = [1u8; 8];
}

#[macro_export]
macro_rules! example {
    ($val: ident, $($name:ty),*) => {
        // We want this macro to fire if there are dupes
        #[deny(unreachable_patterns)]
        match $val {
            $(
                <$name as Val>::VAL => println!("$name"),
            )*
            _ => println!("Other"),
        }
    };
}

And another crate that does this:

#![deny(unreachable_patterns)]

use macro_source::Val;

fn main() {
    let val = [2u8; 8];
    macro_source::example!(val, u8, u8, u16);
}

I don't get the lint to fire at all, even though the expanded code looks like this:

#![feature(prelude_import)]
#![deny(unreachable_patterns)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
use macro_source::Val;
fn main() {
    let val = [2u8; 8];
    #[deny(unreachable_patterns)]
    match val {
        <u8 as Val>::VAL => {
            ::std::io::_print(format_args!("$name\n"));
        }
        <u8 as Val>::VAL => {
            ::std::io::_print(format_args!("$name\n"));
        }
        <u16 as Val>::VAL => {
            ::std::io::_print(format_args!("$name\n"));
        }
        _ => {
            ::std::io::_print(format_args!("Other\n"));
        }
    };
}

[jamesmunns]

Asked on T-Lang: https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/deny.20lint.20behavior.20in.20macro.20expansion/near/444722399

[jamesmunns]

Addressed by #35, released in v0.5.1