Improve error message on incorrectly typed span

src/lib.rs

@@ -619,28 +619,28 @@ macro_rules! quote_spanned {
 #[macro_export]
 macro_rules! quote_spanned {
     ($span:expr=>) => {{
-        let _ = $crate::__private::get_span($span).__into_span();
+        let _: $crate::__private::Span = $crate::__private::get_span($span).__into_span();
         $crate::__private::TokenStream::new()
     }};
 
     // Special case rule for a single tt, for performance.
     ($span:expr=> $tt:tt) => {{
         let mut _s = $crate::__private::TokenStream::new();
-        let _span = $crate::__private::get_span($span).__into_span();
+        let _span: $crate::__private::Span = $crate::__private::get_span($span).__into_span();
         $crate::quote_token_spanned!{$tt _s _span}
         _s
     }};
 
     // Special case rules for two tts, for performance.
     ($span:expr=> # $var:ident) => {{
         let mut _s = $crate::__private::TokenStream::new();
-        let _ = $crate::__private::get_span($span).__into_span();
+        let _: $crate::__private::Span = $crate::__private::get_span($span).__into_span();
         $crate::ToTokens::to_tokens(&$var, &mut _s);
         _s
     }};
     ($span:expr=> $tt1:tt $tt2:tt) => {{
         let mut _s = $crate::__private::TokenStream::new();
-        let _span = $crate::__private::get_span($span).__into_span();
+        let _span: $crate::__private::Span = $crate::__private::get_span($span).__into_span();
         $crate::quote_token_spanned!{$tt1 _s _span}
         $crate::quote_token_spanned!{$tt2 _s _span}
         _s
@@ -649,7 +649,7 @@ macro_rules! quote_spanned {
     // Rule for any other number of tokens.
     ($span:expr=> $($tt:tt)*) => {{
         let mut _s = $crate::__private::TokenStream::new();
-        let _span = $crate::__private::get_span($span).__into_span();
+        let _span: $crate::__private::Span = $crate::__private::get_span($span).__into_span();
         $crate::quote_each_token_spanned!{_s _span $($tt)*}
         _s
     }};

src/runtime.rs

@@ -1,4 +1,4 @@
-use self::get_span::{GetSpanInner, GetSpan};
+use self::get_span::{GetSpan, GetSpanBase, GetSpanInner};
 use crate::{IdentFragment, ToTokens, TokenStreamExt};
 use core::fmt;
 use core::iter;
@@ -168,7 +168,7 @@ impl<T: ToTokens> ToTokens for RepInterp<T> {
 
 #[inline]
 pub fn get_span<T>(span: T) -> GetSpan<T> {
-    GetSpan(GetSpanInner(span))
+    GetSpan(GetSpanInner(GetSpanBase(span)))
 }
 
 mod get_span {
@@ -178,24 +178,41 @@ mod get_span {
 
     pub struct GetSpan<T>(pub(crate) GetSpanInner<T>);
 
-    pub struct GetSpanInner<T>(pub(crate) T);
+    pub struct GetSpanInner<T>(pub(crate) GetSpanBase<T>);
+
+    pub struct GetSpanBase<T>(pub(crate) T);
 
     impl GetSpan<Span> {
         #[inline]
         pub fn __into_span(self) -> Span {
-            (self.0).0
+            ((self.0).0).0
         }
     }
 
     impl GetSpanInner<DelimSpan> {
         #[inline]
         pub fn __into_span(&self) -> Span {
-            self.0.join()
+            (self.0).0.join()
+        }
+    }
+
+    impl<T> GetSpanBase<T> {
+        pub fn __into_span(&self) -> T {
+            unreachable!()
+        }
+    }
+
+    impl<T> Deref for GetSpan<T> {
+        type Target = GetSpanInner<T>;
+
+        #[inline]
+        fn deref(&self) -> &Self::Target {
+            &self.0
         }
     }
 
-    impl Deref for GetSpan<DelimSpan> {
-        type Target = GetSpanInner<DelimSpan>;
+    impl<T> Deref for GetSpanInner<T> {
+        type Target = GetSpanBase<T>;
 
         #[inline]
         fn deref(&self) -> &Self::Target {

tests/ui/wrong-type-span.stderr

@@ -1,9 +1,10 @@
-error[E0599]: no method named `__into_span` found for struct `__private::get_span::GetSpan<&str>` in the current scope
+error[E0308]: mismatched types
  --> tests/ui/wrong-type-span.rs:6:5
   |
 6 |     quote_spanned!(span=> #x);
-  |     ^^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `GetSpan<&str>`
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+  |     |
+  |     expected `Span`, found `&str`
+  |     expected due to this
   |
-  = note: the method was found for
-          - `__private::get_span::GetSpan<Span>`
   = note: this error originates in the macro `quote_spanned` (in Nightly builds, run with -Z macro-backtrace for more info)