Prevent positional args from being named inside function body

build.rs

@@ -8,6 +8,10 @@ fn main() {
         None => return,
     };
 
+    if compiler < 45 {
+        println!("cargo:rustc-cfg=no_span_mixed_site");
+    }
+
     if compiler < 47 {
         println!("cargo:rustc-cfg=self_span_hack");
     }

src/expand.rs

@@ -1,7 +1,7 @@
 use crate::lifetime::{AddLifetimeToImplTrait, CollectLifetimes};
 use crate::parse::Item;
 use crate::receiver::{has_self_in_block, has_self_in_sig, mut_pat, ReplaceSelf};
-use proc_macro2::TokenStream;
+use proc_macro2::{Span, TokenStream};
 use quote::{format_ident, quote, quote_spanned, ToTokens};
 use std::collections::BTreeSet as Set;
 use std::mem;
@@ -399,8 +399,10 @@ fn transform_block(context: Context, sig: &mut Signature, block: &mut Block) {
 }
 
 fn positional_arg(i: usize, pat: &Pat) -> Ident {
-    use syn::spanned::Spanned;
-    format_ident!("__arg{}", i, span = pat.span())
+    let span: Span = syn::spanned::Spanned::span(pat);
+    #[cfg(not(no_span_mixed_site))]
+    let span = span.resolved_at(Span::mixed_site());
+    format_ident!("__arg{}", i, span = span)
 }
 
 fn has_bound(supertraits: &Supertraits, marker: &Ident) -> bool {

tests/ui/arg-implementation-detail.rs

@@ -0,0 +1,22 @@
+use async_trait::async_trait;
+
+pub struct Struct;
+
+#[async_trait]
+pub trait Trait {
+    async fn f((_a, _b): (Struct, Struct)) {
+        // Expands to something like:
+        //
+        //    fn f(__arg0: (Struct, Struct)) -> … {
+        //        Box::pin(async move {
+        //            let (_a, _b) = __arg0;
+        //            …
+        //        })
+        //    }
+        //
+        // but user's code must not be allowed to name that temporary argument:
+        let _ = __arg0;
+    }
+}
+
+fn main() {}

tests/ui/arg-implementation-detail.stderr

@@ -0,0 +1,5 @@
+error[E0425]: cannot find value `__arg0` in this scope
+  --> tests/ui/arg-implementation-detail.rs:18:17
+   |
+18 |         let _ = __arg0;
+   |                 ^^^^^^ not found in this scope