Add Span::mixed_site() for Rust 1.45+

Fixes dtolnay#210

This was stabilized in Rust in
rust-lang/rust#68716

build.rs

@@ -57,6 +57,10 @@ fn main() {
         println!("cargo:rustc-cfg=span_locations");
     }
 
+    if version.minor >= 45 {
+        println!("cargo:rustc-cfg=hygiene");
+    }
+
     let target = env::var("TARGET").unwrap();
     if !enable_use_proc_macro(&target) {
         return;

src/fallback.rs

@@ -374,6 +374,11 @@ impl Span {
         Span { lo: 0, hi: 0 }
     }
 
+    #[cfg(hygiene)]
+    pub fn mixed_site() -> Span {
+        Span::call_site()
+    }
+
     #[cfg(procmacro2_semver_exempt)]
     pub fn def_site() -> Span {
         Span::call_site()

src/lib.rs

@@ -348,6 +348,16 @@ impl Span {
         Span::_new(imp::Span::call_site())
     }
 
+    /// The span located at the invocation of the procedural macro, but with
+    /// local variables, labels, and `$crate` resolved at the definition site
+    /// of the macro. This is the same hygiene behavior as `macro_rules`.
+    ///
+    /// This function requires Rust 1.45 or later.
+    #[cfg(hygiene)]
+    pub fn mixed_site() -> Span {
+        Span::_new(imp::Span::mixed_site())
+    }
+
     /// A span that resolves at the macro definition site.
     ///
     /// This method is semver exempt and not exposed by default.

src/wrapper.rs

@@ -376,6 +376,15 @@ impl Span {
         }
     }
 
+    #[cfg(hygiene)]
+    pub fn mixed_site() -> Span {
+        if inside_proc_macro() {
+            Span::Compiler(proc_macro::Span::mixed_site())
+        } else {
+            Span::Fallback(fallback::Span::mixed_site())
+        }
+    }
+
     #[cfg(super_unstable)]
     pub fn def_site() -> Span {
         if inside_proc_macro() {