Attribute paths are not checked for absence of generic arguments

[petrochenkov]

E.g. this successfully compiles on stable 1.19 / beta 1.20 /nightly

#![allow(unused)]

macro_rules! m {
    ($attr_path: path) => {
        #[$attr_path]
        fn f() {}
    }
}

m!(inline<'nonexistent_lifetime, u8, NonExistentType>);

fn main() {}

There are too many kinds of attributes - built in, custom, derives, legacy derives, procedural macros, legacy procedural macros, maybe something else - I'm not sure which of them are affected. Many of these attributes are removed from AST during expansion, so simply adding a check into AST validation pass won't be enough, the checking needs to be done during expansion, like for macro paths (

rust/src/librustc_resolve/macros.rs

Line 391 in 3cf2c04

let msg = format!("type parameters are not allowed on {}s", kind);

).

cc @jseyfried

[petrochenkov]

This is a regression from Rust 1.18, before that paths weren't accepted in attributes.

[Mark-Simulacrum]

Nominating for prioritization.

[alexcrichton]

cc @rust-lang/compiler