[PATCH v2] rust: macros: vtable: fix `HAS_*` redefinition (`gen_const_name`)

[Qingsong Chen]

If we define the same function name twice in a trait (using `#[cfg]`),
the `vtable` macro will redefine its `gen_const_name`, e.g. this will
define `HAS_BAR` twice:

```rust
    #[vtable]
    pub trait Foo {
        #[cfg(CONFIG_X)]
        fn bar();

        #[cfg(not(CONFIG_X))]
        fn bar(x: usize);
    }
```

Changelog:
----------
v1 -> v2:
- Use `BTreeSet` and existing `consts` as suggested by Alice and Gary.
- Reword commit messages as suggested by Miguel.
====================

Fixes: b44becc5ee80 ("rust: macros: add `#[vtable]` proc macro")
Signed-off-by: Qingsong Chen <changxian.cqs@xxxxxxxxxxxx>
---
 rust/macros/vtable.rs | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/rust/macros/vtable.rs b/rust/macros/vtable.rs
index 34d5e7fb5768..8a1baedcc280 100644
--- a/rust/macros/vtable.rs
+++ b/rust/macros/vtable.rs
@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 
 use proc_macro::{Delimiter, Group, TokenStream, TokenTree};
-use std::collections::HashSet;
+use std::collections::BTreeSet;
 use std::fmt::Write;
 
 pub(crate) fn vtable(_attr: TokenStream, ts: TokenStream) -> TokenStream {
@@ -28,7 +28,7 @@ pub(crate) fn vtable(_attr: TokenStream, ts: TokenStream) -> TokenStream {
 
     let mut body_it = body.stream().into_iter();
     let mut functions = Vec::new();
-    let mut consts = HashSet::new();
+    let mut consts = BTreeSet::new();
     while let Some(token) = body_it.next() {
         match token {
             TokenTree::Ident(ident) if ident.to_string() == "fn" => {
@@ -74,6 +74,7 @@ pub(crate) fn vtable(_attr: TokenStream, ts: TokenStream) -> TokenStream {
                 const {gen_const_name}: bool = false;",
             )
             .unwrap();
+            consts.insert(gen_const_name);
         }
     } else {
         const_items = "const USE_VTABLE_ATTR: () = ();".to_owned();
-- 
2.40.1