From e2ba9d1e0daaf7c0f8b1d2564ab7977dfaf3e582 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Tue, 23 Nov 2021 16:08:18 -0800
Subject: [PATCH 1/2] Add test for using $:literal to index a tuple

Currently fails to build:

    error: unexpected token: `0`
      --> src/test/ui/macros/macro-interpolation.rs:19:16
       |
    LL |         $var . $field
       |                ^^^^^^
    ...
    LL |     let _ = field!(tuple.0);
       |             --------------- in this macro invocation
       |
       = note: this error originates in the macro `field` (in Nightly builds, run with -Z macro-backtrace for more info)

    error: macro expansion ignores token `0` and any following
      --> src/test/ui/macros/macro-interpolation.rs:19:16
       |
    LL |         $var . $field
       |                ^^^^^^
    ...
    LL |     let _ = field!(tuple.0);
       |             --------------- caused by the macro expansion here
       |
       = note: the usage of `field!` is likely invalid in expression context

    error: aborting due to 2 previous errors
---
 src/test/ui/macros/macro-interpolation.rs | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/test/ui/macros/macro-interpolation.rs b/src/test/ui/macros/macro-interpolation.rs
index 35003a79ad703..d6cc682797647 100644
--- a/src/test/ui/macros/macro-interpolation.rs
+++ b/src/test/ui/macros/macro-interpolation.rs
@@ -24,10 +24,19 @@ macro_rules! qpath {
     };
 }
 
+macro_rules! field {
+    ($var:ident . $field:literal) => {
+        $var . $field
+    };
+}
+
 pub fn main() {
     let _: qpath!(path, <str as ToOwned>::Owned);
     let _: qpath!(ty, <str as ToOwned>::Owned);
 
+    let tuple = ('x',);
+    let _ = field!(tuple.0);
+
     assert!(overly_complicated!(f, x, Option<usize>, { return Some(x); },
                                Some(8), Some(y), y) == 8)
 }

From aadf994c80c5abe963816ddea1bcca51edcaf803 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Tue, 23 Nov 2021 16:12:31 -0800
Subject: [PATCH 2/2] Uninterpolate literal to use as field index

---
 compiler/rustc_ast/src/token.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_ast/src/token.rs b/compiler/rustc_ast/src/token.rs
index db066d7c6a519..1ca7281619edc 100644
--- a/compiler/rustc_ast/src/token.rs
+++ b/compiler/rustc_ast/src/token.rs
@@ -452,8 +452,9 @@ impl Token {
     }
 
     // A convenience function for matching on identifiers during parsing.
-    // Turns interpolated identifier (`$i: ident`) or lifetime (`$l: lifetime`) token
-    // into the regular identifier or lifetime token it refers to,
+    // Turns interpolated identifier ($i:ident), lifetime ($l:lifetime), and
+    // integer literal ($l:literal) into the regular identifier or lifetime or
+    // literal token it refers to,
     // otherwise returns the original token.
     pub fn uninterpolate(&self) -> Cow<'_, Token> {
         match &self.kind {
@@ -462,6 +463,11 @@ impl Token {
                     Cow::Owned(Token::new(Ident(ident.name, is_raw), ident.span))
                 }
                 NtLifetime(ident) => Cow::Owned(Token::new(Lifetime(ident.name), ident.span)),
+                NtLiteral(ref literal)
+                    if let ast::ExprKind::Lit(ast::Lit { token, kind: ast::LitKind::Int(..), .. }) = literal.kind =>
+                {
+                    Cow::Owned(Token::new(Literal(token), literal.span))
+                }
                 _ => Cow::Borrowed(self),
             },
             _ => Cow::Borrowed(self),