2 files changed, 42 insertions, 7 deletions
diff --git a/library/proc_macro/src/bridge/mod.rs b/library/proc_macro/src/bridge/mod.rs
index bb05506defd..2df287f7d93 100644
--- a/library/proc_macro/src/bridge/mod.rs
+++ b/library/proc_macro/src/bridge/mod.rs
@@ -62,6 +62,7 @@ macro_rules! with_api {
                 fn clone($self: &$S::TokenStream) -> $S::TokenStream;
                 fn new() -> $S::TokenStream;
                 fn is_empty($self: &$S::TokenStream) -> bool;
+                fn expand_expr($self: &$S::TokenStream) -> Result<$S::TokenStream, ()>;
                 fn from_str(src: &str) -> $S::TokenStream;
                 fn to_string($self: &$S::TokenStream) -> String;
                 fn from_token_tree(
diff --git a/library/proc_macro/src/lib.rs b/library/proc_macro/src/lib.rs
index 9d673d69687..ef96d72a38b 100644
--- a/library/proc_macro/src/lib.rs
+++ b/library/proc_macro/src/lib.rs
@@ -88,12 +88,6 @@ impl !Sync for TokenStream {}
 #[derive(Debug)]
 pub struct LexError;
 
-impl LexError {
-    fn new() -> Self {
-        LexError
-    }
-}
-
 #[stable(feature = "proc_macro_lexerror_impls", since = "1.44.0")]
 impl fmt::Display for LexError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -109,6 +103,28 @@ impl !Send for LexError {}
 #[stable(feature = "proc_macro_lib", since = "1.15.0")]
 impl !Sync for LexError {}
 
+/// Error returned from `TokenStream::expand_expr`.
+#[unstable(feature = "proc_macro_expand", issue = "90765")]
+#[non_exhaustive]
+#[derive(Debug)]
+pub struct ExpandError;
+
+#[unstable(feature = "proc_macro_expand", issue = "90765")]
+impl fmt::Display for ExpandError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("macro expansion failed")
+    }
+}
+
+#[unstable(feature = "proc_macro_expand", issue = "90765")]
+impl error::Error for ExpandError {}
+
+#[unstable(feature = "proc_macro_expand", issue = "90765")]
+impl !Send for ExpandError {}
+
+#[unstable(feature = "proc_macro_expand", issue = "90765")]
+impl !Sync for ExpandError {}
+
 impl TokenStream {
     /// Returns an empty `TokenStream` containing no token trees.
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
@@ -121,6 +137,24 @@ impl TokenStream {
     pub fn is_empty(&self) -> bool {
         self.0.is_empty()
     }
+
+    /// Parses this `TokenStream` as an expression and attempts to expand any
+    /// macros within it. Returns the expanded `TokenStream`.
+    ///
+    /// Currently only expressions expanding to literals will succeed, although
+    /// this may be relaxed in the future.
+    ///
+    /// NOTE: In error conditions, `expand_expr` may leave macros unexpanded,
+    /// report an error, failing compilation, and/or return an `Err(..)`. The
+    /// specific behavior for any error condition, and what conditions are
+    /// considered errors, is unspecified and may change in the future.
+    #[unstable(feature = "proc_macro_expand", issue = "90765")]
+    pub fn expand_expr(&self) -> Result<TokenStream, ExpandError> {
+        match bridge::client::TokenStream::expand_expr(&self.0) {
+            Ok(stream) => Ok(TokenStream(stream)),
+            Err(_) => Err(ExpandError),
+        }
+    }
 }
 
 /// Attempts to break the string into tokens and parse those tokens into a token stream.
@@ -1211,7 +1245,7 @@ impl FromStr for Literal {
     fn from_str(src: &str) -> Result<Self, LexError> {
         match bridge::client::Literal::from_str(src) {
             Ok(literal) => Ok(Literal(literal)),
-            Err(()) => Err(LexError::new()),
+            Err(()) => Err(LexError),
         }
     }
 }