Switch to trait for checking iterator in repetition

dtolnay · dtolnay · commit bf48c854dad1 · 2025-09-28T17:40:29.000-07:00
diff --git a/src/lib.rs b/src/lib.rs
@@ -890,7 +890,7 @@ macro_rules! quote_token_with_context {
         use $crate::__private::ext::*;
         let has_iter = $crate::__private::ThereIsNoIteratorInRepetition;
         $crate::pounded_var_names!{quote_bind_into_iter!(has_iter) () $($inner)*}
-        let _: $crate::__private::HasIterator = has_iter;
+        <_ as $crate::__private::CheckHasIterator>::check(has_iter);
         // This is `while true` instead of `loop` because if there are no
         // iterators used inside of this repetition then the body would not
         // contain any `break`, so the compiler would emit unreachable code
@@ -913,7 +913,7 @@ macro_rules! quote_token_with_context {
         let mut _i = 0usize;
         let has_iter = $crate::__private::ThereIsNoIteratorInRepetition;
         $crate::pounded_var_names!{quote_bind_into_iter!(has_iter) () $($inner)*}
-        let _: $crate::__private::HasIterator = has_iter;
+        <_ as $crate::__private::CheckHasIterator>::check(has_iter);
         while true {
             $crate::pounded_var_names!{quote_bind_next_or_break!() () $($inner)*}
             if _i > 0 {
@@ -960,7 +960,7 @@ macro_rules! quote_token_with_context_spanned {
         use $crate::__private::ext::*;
         let has_iter = $crate::__private::ThereIsNoIteratorInRepetition;
         $crate::pounded_var_names!{quote_bind_into_iter!(has_iter) () $($inner)*}
-        let _: $crate::__private::HasIterator = has_iter;
+        <_ as $crate::__private::CheckHasIterator>::check(has_iter);
         while true {
             $crate::pounded_var_names!{quote_bind_next_or_break!() () $($inner)*}
             $crate::quote_each_token_spanned!{$tokens $span $($inner)*}
@@ -974,7 +974,7 @@ macro_rules! quote_token_with_context_spanned {
         let mut _i = 0usize;
         let has_iter = $crate::__private::ThereIsNoIteratorInRepetition;
         $crate::pounded_var_names!{quote_bind_into_iter!(has_iter) () $($inner)*}
-        let _: $crate::__private::HasIterator = has_iter;
+        <_ as $crate::__private::CheckHasIterator>::check(has_iter);
         while true {
             $crate::pounded_var_names!{quote_bind_next_or_break!() () $($inner)*}
             if _i > 0 {
diff --git a/src/runtime.rs b/src/runtime.rs
@@ -50,6 +50,13 @@ impl BitOr<HasIterator> for HasIterator {
     }
 }
 
+#[doc(hidden)]
+pub trait CheckHasIterator: Sized {
+    fn check(self) {}
+}
+
+impl CheckHasIterator for HasIterator {}
+
 /// Extension traits used by the implementation of `quote!`. These are defined
 /// in separate traits, rather than as a single trait due to ambiguity issues.
 ///
diff --git a/tests/ui/does-not-have-iter-interpolated-dup.stderr b/tests/ui/does-not-have-iter-interpolated-dup.stderr
@@ -1,11 +1,8 @@
-error[E0308]: mismatched types
+error[E0277]: the trait bound `ThereIsNoIteratorInRepetition: CheckHasIterator` is not satisfied
  --> tests/ui/does-not-have-iter-interpolated-dup.rs:8:5
   |
 8 |     quote!(#(#nonrep #nonrep)*);
-  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-  |     |
-  |     expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
-  |     expected due to this
-  |     here the type of `has_iter` is inferred to be `ThereIsNoIteratorInRepetition`
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `CheckHasIterator` is not implemented for `ThereIsNoIteratorInRepetition`
   |
+  = help: the trait `CheckHasIterator` is implemented for `HasIterator`
   = note: this error originates in the macro `$crate::quote_token_with_context` which comes from the expansion of the macro `quote` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/tests/ui/does-not-have-iter-interpolated.stderr b/tests/ui/does-not-have-iter-interpolated.stderr
@@ -1,11 +1,8 @@
-error[E0308]: mismatched types
+error[E0277]: the trait bound `ThereIsNoIteratorInRepetition: CheckHasIterator` is not satisfied
  --> tests/ui/does-not-have-iter-interpolated.rs:8:5
   |
 8 |     quote!(#(#nonrep)*);
-  |     ^^^^^^^^^^^^^^^^^^^
-  |     |
-  |     expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
-  |     expected due to this
-  |     here the type of `has_iter` is inferred to be `ThereIsNoIteratorInRepetition`
+  |     ^^^^^^^^^^^^^^^^^^^ the trait `CheckHasIterator` is not implemented for `ThereIsNoIteratorInRepetition`
   |
+  = help: the trait `CheckHasIterator` is implemented for `HasIterator`
   = note: this error originates in the macro `$crate::quote_token_with_context` which comes from the expansion of the macro `quote` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/tests/ui/does-not-have-iter-separated.stderr b/tests/ui/does-not-have-iter-separated.stderr
@@ -1,10 +1,8 @@
-error[E0308]: mismatched types
+error[E0277]: the trait bound `ThereIsNoIteratorInRepetition: CheckHasIterator` is not satisfied
  --> tests/ui/does-not-have-iter-separated.rs:4:5
   |
 4 |     quote!(#(a b),*);
-  |     ^^^^^^^^^^^^^^^^
-  |     |
-  |     expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
-  |     expected due to this
+  |     ^^^^^^^^^^^^^^^^ the trait `CheckHasIterator` is not implemented for `ThereIsNoIteratorInRepetition`
   |
+  = help: the trait `CheckHasIterator` is implemented for `HasIterator`
   = note: this error originates in the macro `$crate::quote_token_with_context` which comes from the expansion of the macro `quote` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/tests/ui/does-not-have-iter.stderr b/tests/ui/does-not-have-iter.stderr
@@ -1,10 +1,8 @@
-error[E0308]: mismatched types
+error[E0277]: the trait bound `ThereIsNoIteratorInRepetition: CheckHasIterator` is not satisfied
  --> tests/ui/does-not-have-iter.rs:4:5
   |
 4 |     quote!(#(a b)*);
-  |     ^^^^^^^^^^^^^^^
-  |     |
-  |     expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
-  |     expected due to this
+  |     ^^^^^^^^^^^^^^^ the trait `CheckHasIterator` is not implemented for `ThereIsNoIteratorInRepetition`
   |
+  = help: the trait `CheckHasIterator` is implemented for `HasIterator`
   = note: this error originates in the macro `$crate::quote_token_with_context` which comes from the expansion of the macro `quote` (in Nightly builds, run with -Z macro-backtrace for more info)

Original file line number	Diff line number	Diff line change
`@@ -50,6 +50,13 @@ impl BitOr<HasIterator> for HasIterator {`
`50`	`50`	`}`
`51`	`51`	`}`
`52`	`52`
	`53`	`+#[doc(hidden)]`
	`54`	`+pub trait CheckHasIterator: Sized {`
	`55`	`+ fn check(self) {}`
	`56`	`+}`
	`57`	`+`
	`58`	`+impl CheckHasIterator for HasIterator {}`
	`59`	`+`
`53`	`60`	/// Extension traits used by the implementation of `quote!`. These are defined
`54`	`61`	`/// in separate traits, rather than as a single trait due to ambiguity issues.`
`55`	`62`	`///`
Original file line number	Diff line number	Diff line change
`@@ -1,10 +1,8 @@`
`1`		`-error[E0308]: mismatched types`
	`1`	+error[E0277]: the trait bound `ThereIsNoIteratorInRepetition: CheckHasIterator` is not satisfied
`2`	`2`	`--> tests/ui/does-not-have-iter-separated.rs:4:5`
`3`	`3`	`\|`
`4`	`4`	`4 \| quote!(#(a b),*);`
`5`		`- \| ^^^^^^^^^^^^^^^^`
`6`		`- \| \|`
`7`		- \| expected `HasIterator`, found `ThereIsNoIteratorInRepetition`
`8`		`- \| expected due to this`
	`5`	+ \| ^^^^^^^^^^^^^^^^ the trait `CheckHasIterator` is not implemented for `ThereIsNoIteratorInRepetition`
`9`	`6`	`\|`
	`7`	+ = help: the trait `CheckHasIterator` is implemented for `HasIterator`
`10`	`8`	= note: this error originates in the macro `$crate::quote_token_with_context` which comes from the expansion of the macro `quote` (in Nightly builds, run with -Z macro-backtrace for more info)