Help span argument of quote_spanned get inferred to Span

src/lib.rs

@@ -619,28 +619,28 @@ macro_rules! quote_spanned {
 #[macro_export]
 macro_rules! quote_spanned {
     ($span:expr=>) => {{
-        let _ = $crate::__private::IntoSpan::into_span($span);
+        let _ = $crate::__private::GetSpan($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 = $crate::__private::IntoSpan::into_span($span);
+        let _span = $crate::__private::GetSpan($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 _ = $crate::__private::IntoSpan::into_span($span);
+        let _ = $crate::__private::GetSpan($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 = $crate::__private::IntoSpan::into_span($span);
+        let _span = $crate::__private::GetSpan($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 = $crate::__private::IntoSpan::into_span($span);
+        let _span = $crate::__private::GetSpan($span).__into_span();
         $crate::quote_each_token_spanned!{_s _span $($tt)*}
         _s
     }};

src/runtime.rs

@@ -1,7 +1,7 @@
 use crate::{IdentFragment, ToTokens, TokenStreamExt};
 use core::fmt;
 use core::iter;
-use core::ops::BitOr;
+use core::ops::{BitOr, Deref};
 use proc_macro2::extra::DelimSpan;
 use proc_macro2::{Group, Ident, Punct, Spacing, TokenTree};
 
@@ -166,21 +166,34 @@ impl<T: ToTokens> ToTokens for RepInterp<T> {
     }
 }
 
-pub trait IntoSpan {
-    fn into_span(self) -> Span;
+#[repr(transparent)]
+pub struct GetSpan<T>(pub T);
+
+#[repr(transparent)]
+pub struct WrapperDelimSpan {
+    span: DelimSpan,
+}
+
+impl GetSpan<Span> {
+    #[inline]
+    pub fn __into_span(self) -> Span {
+        self.0
+    }
 }
 
-impl IntoSpan for Span {
+impl WrapperDelimSpan {
     #[inline]
-    fn into_span(self) -> Span {
-        self
+    pub fn __into_span(&self) -> Span {
+        self.span.join()
     }
 }
 
-impl IntoSpan for DelimSpan {
+impl Deref for GetSpan<DelimSpan> {
+    type Target = WrapperDelimSpan;
+
     #[inline]
-    fn into_span(self) -> Span {
-        self.join()
+    fn deref(&self) -> &Self::Target {
+        unsafe { &*(self as *const GetSpan<DelimSpan> as *const WrapperDelimSpan) }
     }
 }
 

tests/test.rs

@@ -6,10 +6,12 @@
     clippy::used_underscore_binding
 )]
 
+extern crate proc_macro;
+
 use std::borrow::Cow;
 use std::collections::BTreeSet;
 
-use proc_macro2::{Ident, Span, TokenStream};
+use proc_macro2::{Delimiter, Group, Ident, Span, TokenStream};
 use quote::{format_ident, quote, quote_spanned, TokenStreamExt};
 
 struct X;
@@ -518,3 +520,30 @@ fn test_quote_raw_id() {
     let id = quote!(r#raw_id);
     assert_eq!(id.to_string(), "r#raw_id");
 }
+
+#[test]
+fn test_type_inference_for_span() {
+    trait CallSite {
+        fn get() -> Self;
+    }
+
+    impl CallSite for Span {
+        fn get() -> Self {
+            Span::call_site()
+        }
+    }
+
+    let span = Span::call_site();
+    let _ = quote_spanned!(span=> ...);
+
+    let delim_span = Group::new(Delimiter::Parenthesis, TokenStream::new()).delim_span();
+    let _ = quote_spanned!(delim_span=> ...);
+
+    let inferred = CallSite::get();
+    let _ = quote_spanned!(inferred=> ...);
+
+    if false {
+        let proc_macro_span = proc_macro::Span::call_site();
+        let _ = quote_spanned!(proc_macro_span.into()=> ...);
+    }
+}

tests/ui/wrong-type-span.stderr

@@ -1,12 +1,9 @@
-error[E0277]: the trait bound `&str: IntoSpan` is not satisfied
- --> tests/ui/wrong-type-span.rs:6:20
+error[E0599]: no method named `__into_span` found for struct `GetSpan<&str>` in the current scope
+ --> tests/ui/wrong-type-span.rs:6:5
   |
 6 |     quote_spanned!(span=> #x);
-  |     ---------------^^^^------
-  |     |              |
-  |     |              the trait `IntoSpan` is not implemented for `&str`
-  |     required by a bound introduced by this call
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `GetSpan<&str>`
   |
-  = help: the following other types implement trait `IntoSpan`:
-            Span
-            proc_macro2::extra::DelimSpan
+  = note: the method was found for
+          - `GetSpan<Span>`
+  = note: this error originates in the macro `quote_spanned` (in Nightly builds, run with -Z macro-backtrace for more info)