Rename procmacro2_unstable to procmacro2_semver_exempt
diff --git a/src/unstable.rs b/src/unstable.rs
index 56df0dd..f8cd68c 100644
--- a/src/unstable.rs
+++ b/src/unstable.rs
@@ -159,11 +159,11 @@
     }
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 #[derive(Clone, PartialEq, Eq)]
 pub struct FileName(String);
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 impl fmt::Display for FileName {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         self.0.fmt(f)
@@ -172,11 +172,11 @@
 
 // NOTE: We have to generate our own filename object here because we can't wrap
 // the one provided by proc_macro.
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 #[derive(Clone, PartialEq, Eq)]
 pub struct SourceFile(proc_macro::SourceFile, FileName);
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 impl SourceFile {
     fn new(sf: proc_macro::SourceFile) -> Self {
         let filename = FileName(sf.path().to_string());
@@ -193,21 +193,21 @@
     }
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 impl AsRef<FileName> for SourceFile {
     fn as_ref(&self) -> &FileName {
         self.path()
     }
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 impl fmt::Debug for SourceFile {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         self.0.fmt(f)
     }
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 pub struct LineColumn {
     pub line: usize,
     pub column: usize,
@@ -229,24 +229,24 @@
         self.0
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn source_file(&self) -> SourceFile {
         SourceFile::new(self.0.source_file())
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn start(&self) -> LineColumn {
         let proc_macro::LineColumn{ line, column } = self.0.start();
         LineColumn { line, column }
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn end(&self) -> LineColumn {
         let proc_macro::LineColumn{ line, column } = self.0.end();
         LineColumn { line, column }
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn join(&self, other: Span) -> Option<Span> {
         self.0.join(other.0).map(Span)
     }