5 files changed, 57 insertions, 41 deletions

diff --git a/library/proc_macro/src/bridge/client.rs b/library/proc_macro/src/bridge/client.rs
index 1516f084a..4461b2180 100644
--- a/library/proc_macro/src/bridge/client.rs
+++ b/library/proc_macro/src/bridge/client.rs
@@ -176,8 +176,6 @@ define_handles! {
     FreeFunctions,
     TokenStream,
     SourceFile,
-    MultiSpan,
-    Diagnostic,
 
     'interned:
     Span,
diff --git a/library/proc_macro/src/bridge/mod.rs b/library/proc_macro/src/bridge/mod.rs
index 5cde966bf..4c1e196b5 100644
--- a/library/proc_macro/src/bridge/mod.rs
+++ b/library/proc_macro/src/bridge/mod.rs
@@ -57,6 +57,7 @@ macro_rules! with_api {
                 fn track_env_var(var: &str, value: Option<&str>);
                 fn track_path(path: &str);
                 fn literal_from_str(s: &str) -> Result<Literal<$S::Span, $S::Symbol>, ()>;
+                fn emit_diagnostic(diagnostic: Diagnostic<$S::Span>);
             },
             TokenStream {
                 fn drop($self: $S::TokenStream);
@@ -87,22 +88,6 @@ macro_rules! with_api {
                 fn path($self: &$S::SourceFile) -> String;
                 fn is_real($self: &$S::SourceFile) -> bool;
             },
-            MultiSpan {
-                fn drop($self: $S::MultiSpan);
-                fn new() -> $S::MultiSpan;
-                fn push($self: &mut $S::MultiSpan, span: $S::Span);
-            },
-            Diagnostic {
-                fn drop($self: $S::Diagnostic);
-                fn new(level: Level, msg: &str, span: $S::MultiSpan) -> $S::Diagnostic;
-                fn sub(
-                    $self: &mut $S::Diagnostic,
-                    level: Level,
-                    msg: &str,
-                    span: $S::MultiSpan,
-                );
-                fn emit($self: $S::Diagnostic);
-            },
             Span {
                 fn debug($self: $S::Span) -> String;
                 fn source_file($self: $S::Span) -> $S::SourceFile;
@@ -510,6 +495,18 @@ compound_traits!(
     }
 );
 
+#[derive(Clone, Debug)]
+pub struct Diagnostic<Span> {
+    pub level: Level,
+    pub message: String,
+    pub spans: Vec<Span>,
+    pub children: Vec<Diagnostic<Span>>,
+}
+
+compound_traits!(
+    struct Diagnostic<Span> { level, message, spans, children }
+);
+
 /// Globals provided alongside the initial inputs for a macro expansion.
 /// Provides values such as spans which are used frequently to avoid RPC.
 #[derive(Clone)]
diff --git a/library/proc_macro/src/bridge/server.rs b/library/proc_macro/src/bridge/server.rs
index e068ec60b..8202c40d6 100644
--- a/library/proc_macro/src/bridge/server.rs
+++ b/library/proc_macro/src/bridge/server.rs
@@ -2,6 +2,7 @@
 
 use super::*;
 
+use std::cell::Cell;
 use std::marker::PhantomData;
 
 // FIXME(eddyb) generate the definition of `HandleStore` in `server.rs`.
@@ -11,8 +12,6 @@ pub trait Types {
     type FreeFunctions: 'static;
     type TokenStream: 'static + Clone;
     type SourceFile: 'static + Clone;
-    type MultiSpan: 'static;
-    type Diagnostic: 'static;
     type Span: 'static + Copy + Eq + Hash;
     type Symbol: 'static;
 }
@@ -145,6 +144,38 @@ pub trait ExecutionStrategy {
     ) -> Buffer;
 }
 
+thread_local! {
+    /// While running a proc-macro with the same-thread executor, this flag will
+    /// be set, forcing nested proc-macro invocations (e.g. due to
+    /// `TokenStream::expand_expr`) to be run using a cross-thread executor.
+    ///
+    /// This is required as the thread-local state in the proc_macro client does
+    /// not handle being re-entered, and will invalidate all `Symbol`s when
+    /// entering a nested macro.
+    static ALREADY_RUNNING_SAME_THREAD: Cell<bool> = Cell::new(false);
+}
+
+/// Keep `ALREADY_RUNNING_SAME_THREAD` (see also its documentation)
+/// set to `true`, preventing same-thread reentrance.
+struct RunningSameThreadGuard(());
+
+impl RunningSameThreadGuard {
+    fn new() -> Self {
+        let already_running = ALREADY_RUNNING_SAME_THREAD.replace(true);
+        assert!(
+            !already_running,
+            "same-thread nesting (\"reentrance\") of proc macro executions is not supported"
+        );
+        RunningSameThreadGuard(())
+    }
+}
+
+impl Drop for RunningSameThreadGuard {
+    fn drop(&mut self) {
+        ALREADY_RUNNING_SAME_THREAD.set(false);
+    }
+}
+
 pub struct MaybeCrossThread<P> {
     cross_thread: bool,
     marker: PhantomData<P>,
@@ -167,7 +198,7 @@ where
         run_client: extern "C" fn(BridgeConfig<'_>) -> Buffer,
         force_show_panics: bool,
     ) -> Buffer {
-        if self.cross_thread {
+        if self.cross_thread || ALREADY_RUNNING_SAME_THREAD.get() {
             <CrossThread<P>>::new().run_bridge_and_client(
                 dispatcher,
                 input,
@@ -190,6 +221,8 @@ impl ExecutionStrategy for SameThread {
         run_client: extern "C" fn(BridgeConfig<'_>) -> Buffer,
         force_show_panics: bool,
     ) -> Buffer {
+        let _guard = RunningSameThreadGuard::new();
+
         let mut dispatch = |buf| dispatcher.dispatch(buf);
 
         run_client(BridgeConfig {
diff --git a/library/proc_macro/src/diagnostic.rs b/library/proc_macro/src/diagnostic.rs
index 6e46dc036..5a209f7c7 100644
--- a/library/proc_macro/src/diagnostic.rs
+++ b/library/proc_macro/src/diagnostic.rs
@@ -161,22 +161,15 @@ impl Diagnostic {
     /// Emit the diagnostic.
     #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
     pub fn emit(self) {
-        fn to_internal(spans: Vec<Span>) -> crate::bridge::client::MultiSpan {
-            let mut multi_span = crate::bridge::client::MultiSpan::new();
-            for span in spans {
-                multi_span.push(span.0);
+        fn to_internal(diag: Diagnostic) -> crate::bridge::Diagnostic<crate::bridge::client::Span> {
+            crate::bridge::Diagnostic {
+                level: diag.level,
+                message: diag.message,
+                spans: diag.spans.into_iter().map(|s| s.0).collect(),
+                children: diag.children.into_iter().map(to_internal).collect(),
             }
-            multi_span
         }
 
-        let mut diag = crate::bridge::client::Diagnostic::new(
-            self.level,
-            &self.message[..],
-            to_internal(self.spans),
-        );
-        for c in self.children {
-            diag.sub(c.level, &c.message[..], to_internal(c.spans));
-        }
-        diag.emit();
+        crate::bridge::client::FreeFunctions::emit_diagnostic(to_internal(self));
     }
 }
diff --git a/library/proc_macro/src/lib.rs b/library/proc_macro/src/lib.rs
index 8e478cd7b..495c1c5ae 100644
--- a/library/proc_macro/src/lib.rs
+++ b/library/proc_macro/src/lib.rs
@@ -1353,12 +1353,7 @@ impl Literal {
     /// Byte string literal.
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     pub fn byte_string(bytes: &[u8]) -> Literal {
-        let string = bytes
-            .iter()
-            .cloned()
-            .flat_map(std::ascii::escape_default)
-            .map(Into::<char>::into)
-            .collect::<String>();
+        let string = bytes.escape_ascii().to_string();
         Literal::new(bridge::LitKind::ByteStr, &string, None)
     }