Show the unexpected token when parsing derive macro output.

Author: m-ou-se

compiler/rustc_expand/src/proc_macro.rs

@@ -133,11 +133,21 @@ impl MultiItemModifier for ProcMacroDerive {
             }
         }
 
-        // fail if there have been errors emitted,
-        // or if there's any unparsed tokens left.
-        if parser.token != token::Eof
-            || ecx.sess.parse_sess.span_diagnostic.err_count() > error_count_before
-        {
+        // check if the entire stream was parsed
+        if parser.token != token::Eof {
+            ecx.struct_span_err(
+                parser.token.span,
+                &format!(
+                    "expected item, found {}",
+                    rustc_parse::parser::token_descr(&parser.token)
+                ),
+            )
+            .span_label(parser.token.span, "expected item")
+            .emit();
+        }
+
+        // fail if there have been errors emitted
+        if ecx.sess.parse_sess.span_diagnostic.err_count() > error_count_before {
             ecx.struct_span_err(span, "proc-macro derive produced unparseable tokens").emit();
         }
 

compiler/rustc_parse/src/parser/mod.rs

@@ -394,7 +394,7 @@ fn token_descr_opt(token: &Token) -> Option<&'static str> {
     })
 }
 
-pub(super) fn token_descr(token: &Token) -> String {
+pub fn token_descr(token: &Token) -> String {
     let token_str = pprust::token_to_string(token);
     match token_descr_opt(token) {
         Some(prefix) => format!("{} `{}`", prefix, token_str),

src/test/ui/proc-macro/derive-bad.rs

@@ -10,6 +10,7 @@ struct A; //~ ERROR the name `A` is defined multiple times
 
 #[derive(B)]
 //~^ ERROR proc-macro derive produced unparseable tokens
+//~| ERROR expected item, found `{`
 struct B;
 
 fn main() {}

src/test/ui/proc-macro/derive-bad.stderr

@@ -23,12 +23,20 @@ LL | struct A;
    |
    = note: `A` must be defined only once in the type namespace of this module
 
+error: expected item, found `{`
+  --> $DIR/derive-bad.rs:11:10
+   |
+LL | #[derive(B)]
+   |          ^ expected item
+   |
+   = note: this error originates in the derive macro `B` (in Nightly builds, run with -Z macro-backtrace for more info)
+
 error: proc-macro derive produced unparseable tokens
   --> $DIR/derive-bad.rs:11:10
    |
 LL | #[derive(B)]
    |          ^
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0428`.