Defer stabilizing span hygiene until after 1.45 release

Author: dtolnay

src/fallback.rs

@@ -374,6 +374,7 @@ impl Span {
         Span { lo: 0, hi: 0 }
     }
 
+    #[cfg(procmacro2_semver_exempt)]
     #[cfg(hygiene)]
     pub fn mixed_site() -> Span {
         Span::call_site()
@@ -384,13 +385,15 @@ impl Span {
         Span::call_site()
     }
 
+    #[cfg(procmacro2_semver_exempt)]
     pub fn resolved_at(&self, _other: Span) -> Span {
         // Stable spans consist only of line/column information, so
         // `resolved_at` and `located_at` only select which span the
         // caller wants line/column information from.
         *self
     }
 
+    #[cfg(procmacro2_semver_exempt)]
     pub fn located_at(&self, other: Span) -> Span {
         other
     }

src/lib.rs

@@ -353,6 +353,7 @@ impl Span {
     /// of the macro. This is the same hygiene behavior as `macro_rules`.
     ///
     /// This function requires Rust 1.45 or later.
+    #[cfg(procmacro2_semver_exempt)]
     #[cfg(hygiene)]
     pub fn mixed_site() -> Span {
         Span::_new(imp::Span::mixed_site())
@@ -368,12 +369,14 @@ impl Span {
 
     /// Creates a new span with the same line/column information as `self` but
     /// that resolves symbols as though it were at `other`.
+    #[cfg(procmacro2_semver_exempt)]
     pub fn resolved_at(&self, other: Span) -> Span {
         Span::_new(self.inner.resolved_at(other.inner))
     }
 
     /// Creates a new span with the same name resolution behavior as `self` but
     /// with the line/column information of `other`.
+    #[cfg(procmacro2_semver_exempt)]
     pub fn located_at(&self, other: Span) -> Span {
         Span::_new(self.inner.located_at(other.inner))
     }

src/wrapper.rs

@@ -376,6 +376,7 @@ impl Span {
         }
     }
 
+    #[cfg(procmacro2_semver_exempt)]
     #[cfg(hygiene)]
     pub fn mixed_site() -> Span {
         if inside_proc_macro() {
@@ -394,6 +395,7 @@ impl Span {
         }
     }
 
+    #[cfg(procmacro2_semver_exempt)]
     pub fn resolved_at(&self, other: Span) -> Span {
         match (self, other) {
             #[cfg(hygiene)]
@@ -408,6 +410,7 @@ impl Span {
         }
     }
 
+    #[cfg(procmacro2_semver_exempt)]
     pub fn located_at(&self, other: Span) -> Span {
         match (self, other) {
             #[cfg(hygiene)]