diff --git a/.travis.yml b/.travis.yml
index e5c59af..a524c01 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -11,16 +11,16 @@
         - pip install 'travis-cargo<0.2' --user && export PATH=$HOME/.local/bin:$PATH
       script:
         - cargo test
-        - cargo build --features unstable
-        - RUSTFLAGS='--cfg procmacro2_unstable' cargo test
-        - RUSTFLAGS='--cfg procmacro2_unstable' cargo build --features unstable
-        - RUSTFLAGS='--cfg procmacro2_unstable' cargo doc --no-deps
+        - cargo build --features nightly
+        - RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo test
+        - RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build --features nightly
+        - RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo doc --no-deps
       after_success:
         - travis-cargo --only nightly doc-upload
 
 script:
   - cargo test
-  - RUSTFLAGS='--cfg procmacro2_unstable' cargo test
+  - RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo test
 env:
   global:
     - TRAVIS_CARGO_NIGHTLY_FEATURE=""
diff --git a/Cargo.toml b/Cargo.toml
index 19e860f..ac33607 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -21,4 +21,13 @@
 unicode-xid = "0.1"
 
 [features]
-unstable = []
+
+# When enabled: act as a shim around the nightly compiler's proc_macro crate.
+# This requires a nightly compiler.
+#
+# When disabled: emulate the same API as the nightly compiler's proc_macro crate
+# but in a way that works on all stable compilers >=1.15.0.
+nightly = []
+
+# Deprecated; use "nightly" instead.
+unstable = ["nightly"]
diff --git a/README.md b/README.md
index c26ec75..576da8d 100644
--- a/README.md
+++ b/README.md
@@ -48,7 +48,7 @@
 }
 ```
 
-If you'd like you can enable the `unstable` feature in this crate. This will
+If you'd like you can enable the `nightly` feature in this crate. This will
 cause it to compile against the **unstable and nightly-only** features of the
 `proc_macro` crate. This in turn requires a nightly compiler. This should help
 preserve span information, however, coming in from the compiler itself.
@@ -57,7 +57,7 @@
 
 ```toml
 [dependencies]
-proc-macro2 = { version = "0.1", features = ["unstable"] }
+proc-macro2 = { version = "0.1", features = ["nightly"] }
 ```
 
 
@@ -65,11 +65,19 @@
 
 `proc-macro2` supports exporting some methods from `proc_macro` which are
 currently highly unstable, and may not be stabilized in the first pass of
-`proc_macro` stabilizations. These features are not exported by default.
+`proc_macro` stabilizations. These features are not exported by default. Minor
+versions of `proc-macro2` may make breaking changes to them at any time.
 
-To export these features, the `procmacro2_unstable` config flag must be passed
-to rustc. To pass this flag, run `cargo` with 
-`RUSTFLAGS='--cfg procmacro2_unstable' cargo build`.
+To enable these features, the `procmacro2_semver_exempt` config flag must be
+passed to rustc.
+
+```
+RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build
+```
+
+Note that this must not only be done for your crate, but for any crate that
+depends on your crate. This infectious nature is intentional, as it serves as a
+reminder that you are outside of the normal semver guarantees.
 
 
 # License
diff --git a/src/lib.rs b/src/lib.rs
index 26d3244..49e3d21 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -14,16 +14,16 @@
 //! to use this crate will be trivially able to switch to the upstream
 //! `proc_macro` crate once its API stabilizes.
 //!
-//! In the meantime this crate also has an `unstable` Cargo feature which
+//! In the meantime this crate also has a `nightly` Cargo feature which
 //! enables it to reimplement itself with the unstable API of `proc_macro`.
 //! This'll allow immediate usage of the beneficial upstream API, particularly
 //! around preserving span information.
 
-#![cfg_attr(feature = "unstable", feature(proc_macro))]
+#![cfg_attr(feature = "nightly", feature(proc_macro))]
 
 extern crate proc_macro;
 
-#[cfg(not(feature = "unstable"))]
+#[cfg(not(feature = "nightly"))]
 extern crate unicode_xid;
 
 use std::fmt;
@@ -31,14 +31,14 @@
 use std::iter::FromIterator;
 
 #[macro_use]
-#[cfg(not(feature = "unstable"))]
+#[cfg(not(feature = "nightly"))]
 mod strnom;
 
 #[path = "stable.rs"]
-#[cfg(not(feature = "unstable"))]
+#[cfg(not(feature = "nightly"))]
 mod imp;
 #[path = "unstable.rs"]
-#[cfg(feature = "unstable")]
+#[cfg(feature = "nightly")]
 mod imp;
 
 #[macro_use]
@@ -104,14 +104,14 @@
 }
 
 // Returned by reference, so we can't easily wrap it.
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 pub use imp::FileName;
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 #[derive(Clone, PartialEq, Eq)]
 pub struct SourceFile(imp::SourceFile);
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 impl SourceFile {
     /// Get the path to this source file as a string.
     pub fn path(&self) -> &FileName {
@@ -123,21 +123,21 @@
     }
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 impl AsRef<FileName> for SourceFile {
     fn as_ref(&self) -> &FileName {
         self.0.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,
@@ -162,30 +162,30 @@
         Span(imp::Span::def_site())
     }
 
-    /// This method is only available when the `"unstable"` feature is enabled.
-    #[cfg(feature = "unstable")]
+    /// This method is only available when the `"nightly"` feature is enabled.
+    #[cfg(feature = "nightly")]
     pub fn unstable(self) -> proc_macro::Span {
         self.0.unstable()
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn source_file(&self) -> SourceFile {
         SourceFile(self.0.source_file())
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn start(&self) -> LineColumn {
         let imp::LineColumn{ line, column } = self.0.start();
         LineColumn { line: line, column: column }
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn end(&self) -> LineColumn {
         let imp::LineColumn{ line, column } = self.0.end();
         LineColumn { line: line, column: column }
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn join(&self, other: Span) -> Option<Span> {
         self.0.join(other.0).map(Span)
     }
diff --git a/src/stable.rs b/src/stable.rs
index d892ae5..984a146 100644
--- a/src/stable.rs
+++ b/src/stable.rs
@@ -1,7 +1,7 @@
 use std::ascii;
 use std::borrow::Borrow;
 use std::cell::RefCell;
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 use std::cmp;
 use std::collections::HashMap;
 use std::fmt;
@@ -36,7 +36,7 @@
     }
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 fn get_cursor(src: &str) -> Cursor {
     // Create a dummy file & add it to the codemap
     CODEMAP.with(|cm| {
@@ -50,7 +50,7 @@
     })
 }
 
-#[cfg(not(procmacro2_unstable))]
+#[cfg(not(procmacro2_semver_exempt))]
 fn get_cursor(src: &str) -> Cursor {
     Cursor {
         rest: src,
@@ -163,24 +163,24 @@
     }
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 #[derive(Clone, PartialEq, Eq, Debug)]
 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)
     }
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 #[derive(Clone, PartialEq, Eq)]
 pub struct SourceFile {
     name: FileName,
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 impl SourceFile {
     /// Get the path to this source file as a string.
     pub fn path(&self) -> &FileName {
@@ -193,14 +193,14 @@
     }
 }
 
-#[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 {
         f.debug_struct("SourceFile")
@@ -210,14 +210,14 @@
     }
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct LineColumn {
     pub line: usize,
     pub column: usize,
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 thread_local! {
     static CODEMAP: RefCell<Codemap> = RefCell::new(Codemap {
         // NOTE: We start with a single dummy file which all call_site() and
@@ -230,14 +230,14 @@
     });
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 struct FileInfo {
     name: String,
     span: Span,
     lines: Vec<usize>,
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 impl FileInfo {
     fn offset_line_column(&self, offset: usize) -> LineColumn {
         assert!(self.span_within(Span { lo: offset as u32, hi: offset as u32 }));
@@ -260,7 +260,7 @@
 }
 
 /// Computes the offsets of each line in the given source string.
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 fn lines_offsets(s: &str) -> Vec<usize> {
     let mut lines = vec![0];
     let mut prev = 0;
@@ -271,12 +271,12 @@
     lines
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 struct Codemap {
     files: Vec<FileInfo>,
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 impl Codemap {
     fn next_start_pos(&self) -> u32 {
         // Add 1 so there's always space between files.
@@ -313,19 +313,19 @@
 
 #[derive(Clone, Copy, Debug)]
 pub struct Span {
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     lo: u32,
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     hi: u32,
 }
 
 impl Span {
-    #[cfg(not(procmacro2_unstable))]
+    #[cfg(not(procmacro2_semver_exempt))]
     pub fn call_site() -> Span {
         Span {}
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn call_site() -> Span {
         Span { lo: 0, hi: 0 }
     }
@@ -334,7 +334,7 @@
         Span::call_site()
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn source_file(&self) -> SourceFile {
         CODEMAP.with(|cm| {
             let cm = cm.borrow();
@@ -345,7 +345,7 @@
         })
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn start(&self) -> LineColumn {
         CODEMAP.with(|cm| {
             let cm = cm.borrow();
@@ -354,7 +354,7 @@
         })
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn end(&self) -> LineColumn {
         CODEMAP.with(|cm| {
             let cm = cm.borrow();
@@ -363,7 +363,7 @@
         })
     }
 
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn join(&self, other: Span) -> Option<Span> {
         CODEMAP.with(|cm| {
             let cm = cm.borrow();
@@ -578,7 +578,7 @@
     |trees| ::TokenStream(TokenStream { inner: trees })
 ));
 
-#[cfg(not(procmacro2_unstable))]
+#[cfg(not(procmacro2_semver_exempt))]
 fn token_tree(input: Cursor) -> PResult<TokenTree> {
     let (input, kind) = token_kind(input)?;
     Ok((input, TokenTree {
@@ -587,7 +587,7 @@
     }))
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 fn token_tree(input: Cursor) -> PResult<TokenTree> {
     let input = skip_whitespace(input);
     let lo = input.off;
diff --git a/src/strnom.rs b/src/strnom.rs
index 2f9e73f..35acff7 100644
--- a/src/strnom.rs
+++ b/src/strnom.rs
@@ -9,18 +9,18 @@
 #[derive(Copy, Clone, Eq, PartialEq)]
 pub struct Cursor<'a> {
     pub rest: &'a str,
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub off: u32,
 }
 
 impl<'a> Cursor<'a> {
-    #[cfg(not(procmacro2_unstable))]
+    #[cfg(not(procmacro2_semver_exempt))]
     pub fn advance(&self, amt: usize) -> Cursor<'a> {
         Cursor {
             rest: &self.rest[amt..],
         }
     }
-    #[cfg(procmacro2_unstable)]
+    #[cfg(procmacro2_semver_exempt)]
     pub fn advance(&self, amt: usize) -> Cursor<'a> {
         Cursor {
             rest: &self.rest[amt..],
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)
     }
diff --git a/tests/test.rs b/tests/test.rs
index 6442ba6..89f7b10 100644
--- a/tests/test.rs
+++ b/tests/test.rs
@@ -2,11 +2,11 @@
 
 use proc_macro2::{Term, Literal, TokenStream};
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 use proc_macro2::TokenNode;
 
-#[cfg(procmacro2_unstable)]
-#[cfg(not(feature = "unstable"))]
+#[cfg(procmacro2_semver_exempt)]
+#[cfg(not(feature = "nightly"))]
 use proc_macro2::Span;
 
 #[test]
@@ -72,7 +72,7 @@
     fail("'mut");
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 #[test]
 fn span_test() {
     fn check_spans(p: &str, mut lines: &[(usize, usize, usize, usize)]) {
@@ -120,8 +120,8 @@
 ]);
 }
 
-#[cfg(procmacro2_unstable)]
-#[cfg(not(feature = "unstable"))]
+#[cfg(procmacro2_semver_exempt)]
+#[cfg(not(feature = "nightly"))]
 #[test]
 fn default_span() {
     let start = Span::call_site().start();
@@ -135,7 +135,7 @@
     assert!(!source_file.is_real());
 }
 
-#[cfg(procmacro2_unstable)]
+#[cfg(procmacro2_semver_exempt)]
 #[test]
 fn span_join() {
     let source1 =