restore precise parser error message

Author: cyrgani

compiler/rustc_errors/src/diagnostic.rs

@@ -1419,6 +1419,13 @@ impl<'a, G: EmissionGuarantee> Diag<'a, G> {
         drop(self);
     }
 
+    /// Cancels this diagnostic and returns its first message, if it exists.
+    pub fn cancel_into_message(self) -> Option<String> {
+        let s = self.diag.as_ref()?.messages.get(0)?.0.as_str().map(ToString::to_string);
+        self.cancel();
+        s
+    }
+
     /// See `DiagCtxt::stash_diagnostic` for details.
     pub fn stash(mut self, span: Span, key: StashKey) -> Option<ErrorGuaranteed> {
         let diag = self.take_diag();

compiler/rustc_expand/src/proc_macro_server.rs

@@ -563,15 +563,22 @@ impl server::TokenStream for Rustc<'_, '_> {
     }
 
     fn from_str(&mut self, src: &str) -> Result<Self::TokenStream, String> {
+        const ERROR_MSG: &str = "cannot parse string into token stream";
+
         source_str_to_stream(
             self.psess(),
             FileName::proc_macro_source_code(src),
             src.to_string(),
             Some(self.call_site),
         )
         .map_err(|diags| {
-            diags.into_iter().for_each(Diag::cancel);
-            "cannot parse string into token stream".to_string()
+            let mut messages = diags.into_iter().map(Diag::cancel_into_message).flatten();
+            if let Some(msg) = messages.next() {
+                messages.for_each(drop);
+                format!("{ERROR_MSG}: {msg}")
+            } else {
+                ERROR_MSG.to_string()
+            }
         })
     }
 

tests/ui/proc-macro/auxiliary/invalid-punct-ident.rs

@@ -20,6 +20,9 @@ pub fn invalid_raw_ident(_: TokenStream) -> TokenStream {
 
 #[proc_macro]
 pub fn lexer_failure(_: TokenStream) -> TokenStream {
-    assert!("a b ) c".parse::<TokenStream>().is_err());
+    assert_eq!(
+        "a b ) c".parse::<TokenStream>().unwrap_err().to_string(),
+        "cannot parse string into token stream: unexpected closing delimiter: `)`"
+    );
     TokenStream::new()
 }