Rollup merge of rust-lang#51417 - pietroalbini:revert-49719, r=nikoma…

…tsakis

Revert rust-lang#49719

This also needs to be backported into beta.

Fixes rust-lang#51416.
r? @nikomatsakis

src/libsyntax/ext/tt/quoted.rs

@@ -386,26 +386,72 @@ where
 {
     // We basically look at two token trees here, denoted as #1 and #2 below
     let span = match parse_kleene_op(input, span) {
-        // #1 is any KleeneOp (`?`)
-        Ok(Ok(op)) if op == KleeneOp::ZeroOrOne => {
-            if !features.macro_at_most_once_rep
-                && !attr::contains_name(attrs, "allow_internal_unstable")
-            {
-                let explain = feature_gate::EXPLAIN_MACRO_AT_MOST_ONCE_REP;
-                emit_feature_err(
-                    sess,
-                    "macro_at_most_once_rep",
-                    span,
-                    GateIssue::Language,
-                    explain,
-                );
+        // #1 is a `+` or `*` KleeneOp
+        //
+        // `?` is ambiguous: it could be a separator or a Kleene::ZeroOrOne, so we need to look
+        // ahead one more token to be sure.
+        Ok(Ok(op)) if op != KleeneOp::ZeroOrOne => return (None, op),
+
+        // #1 is `?` token, but it could be a Kleene::ZeroOrOne without a separator or it could
+        // be a `?` separator followed by any Kleene operator. We need to look ahead 1 token to
+        // find out which.
+        Ok(Ok(op)) => {
+            assert_eq!(op, KleeneOp::ZeroOrOne);
+
+            // Lookahead at #2. If it is a KleenOp, then #1 is a separator.
+            let is_1_sep = if let Some(&tokenstream::TokenTree::Token(_, ref tok2)) = input.peek() {
+                kleene_op(tok2).is_some()
+            } else {
+                false
+            };
+
+            if is_1_sep {
+                // #1 is a separator and #2 should be a KleepeOp::*
+                // (N.B. We need to advance the input iterator.)
+                match parse_kleene_op(input, span) {
+                    // #2 is a KleeneOp (this is the only valid option) :)
+                    Ok(Ok(op)) if op == KleeneOp::ZeroOrOne => {
+                        if !features.macro_at_most_once_rep
+                            && !attr::contains_name(attrs, "allow_internal_unstable")
+                        {
+                            let explain = feature_gate::EXPLAIN_MACRO_AT_MOST_ONCE_REP;
+                            emit_feature_err(
+                                sess,
+                                "macro_at_most_once_rep",
+                                span,
+                                GateIssue::Language,
+                                explain,
+                            );
+                        }
+                        return (Some(token::Question), op);
+                    }
+                    Ok(Ok(op)) => return (Some(token::Question), op),
+
+                    // #2 is a random token (this is an error) :(
+                    Ok(Err((_, span))) => span,
+
+                    // #2 is not even a token at all :(
+                    Err(span) => span,
+                }
+            } else {
+                if !features.macro_at_most_once_rep
+                    && !attr::contains_name(attrs, "allow_internal_unstable")
+                {
+                    let explain = feature_gate::EXPLAIN_MACRO_AT_MOST_ONCE_REP;
+                    emit_feature_err(
+                        sess,
+                        "macro_at_most_once_rep",
+                        span,
+                        GateIssue::Language,
+                        explain,
+                    );
+                }
+
+                // #2 is a random tree and #1 is KleeneOp::ZeroOrOne
+                return (None, op);
             }
-            return (None, op);
         }
 
-        // #1 is any KleeneOp (`+`, `*`)
-        Ok(Ok(op)) => return (None, op),
-
         // #1 is a separator followed by #2, a KleeneOp
         Ok(Err((tok, span))) => match parse_kleene_op(input, span) {
             // #2 is a KleeneOp :D
@@ -421,11 +467,8 @@ where
                         GateIssue::Language,
                         explain,
                     );
-                } else {
-                    sess.span_diagnostic
-                        .span_err(span, "`?` macro repetition does not allow a separator");
                 }
-                return (None, op);
+                return (Some(tok), op);
             }
             Ok(Ok(op)) => return (Some(tok), op),
 
@@ -440,7 +483,9 @@ where
         Err(span) => span,
     };
 
-    if !features.macro_at_most_once_rep && !attr::contains_name(attrs, "allow_internal_unstable") {
+    if !features.macro_at_most_once_rep
+        && !attr::contains_name(attrs, "allow_internal_unstable")
+    {
         sess.span_diagnostic
             .span_err(span, "expected one of: `*`, `+`, or `?`");
     } else {

src/test/run-pass/macro-at-most-once-rep.rs

@@ -32,13 +32,25 @@ macro_rules! foo {
     } }
 }
 
+macro_rules! baz {
+    ($($a:ident),? ; $num:expr) => { { // comma separator is meaningless for `?`
+        let mut x = 0;
+
+        $(
+            x += $a;
+         )?
+
+        assert_eq!(x, $num);
+    } }
+}
+
 macro_rules! barplus {
     ($($a:ident)?+ ; $num:expr) => { {
         let mut x = 0;
 
         $(
             x += $a;
-         )?
+         )+
 
         assert_eq!(x, $num);
     } }
@@ -50,7 +62,7 @@ macro_rules! barstar {
 
         $(
             x += $a;
-         )?
+         )*
 
         assert_eq!(x, $num);
     } }
@@ -62,10 +74,15 @@ pub fn main() {
     // accept 0 or 1 repetitions
     foo!( ; 0);
     foo!(a ; 1);
+    baz!( ; 0);
+    baz!(a ; 1);
 
     // Make sure using ? as a separator works as before
-    barplus!(+ ; 0);
-    barplus!(a + ; 1);
-    barstar!(* ; 0);
-    barstar!(a * ; 1);
+    barplus!(a ; 1);
+    barplus!(a?a ; 2);
+    barplus!(a?a?a ; 3);
+    barstar!( ; 0);
+    barstar!(a ; 1);
+    barstar!(a?a ; 2);
+    barstar!(a?a?a ; 3);
 }

src/test/ui/macros/macro-at-most-once-rep-ambig.rs

@@ -8,26 +8,30 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// Tests the behavior of various Kleene operators in macros with respect to `?` terminals. In
-// particular, `?` in the position of a separator and of a Kleene operator is tested.
+// The logic for parsing Kleene operators in macros has a special case to disambiguate `?`.
+// Specifically, `$(pat)?` is the ZeroOrOne operator whereas `$(pat)?+` or `$(pat)?*` are the
+// ZeroOrMore and OneOrMore operators using `?` as a separator. These tests are intended to
+// exercise that logic in the macro parser.
+//
+// Moreover, we also throw in some tests for using a separator with `?`, which is meaningless but
+// included for consistency with `+` and `*`.
+//
+// This test focuses on error cases.
 
 #![feature(macro_at_most_once_rep)]
 
-// should match `` and `a`
 macro_rules! foo {
     ($(a)?) => {}
 }
 
 macro_rules! baz {
-    ($(a),?) => {} //~ ERROR `?` macro repetition does not allow a separator
+    ($(a),?) => {} // comma separator is meaningless for `?`
 }
 
-// should match `+` and `a+`
 macro_rules! barplus {
     ($(a)?+) => {}
 }
 
-// should match `*` and `a*`
 macro_rules! barstar {
     ($(a)?*) => {}
 }
@@ -36,14 +40,14 @@ pub fn main() {
     foo!(a?a?a); //~ ERROR no rules expected the token `?`
     foo!(a?a); //~ ERROR no rules expected the token `?`
     foo!(a?); //~ ERROR no rules expected the token `?`
+    baz!(a?a?a); //~ ERROR no rules expected the token `?`
+    baz!(a?a); //~ ERROR no rules expected the token `?`
+    baz!(a?); //~ ERROR no rules expected the token `?`
+    baz!(a,); //~ ERROR unexpected end of macro invocation
+    baz!(a?a?a,); //~ ERROR no rules expected the token `?`
+    baz!(a?a,); //~ ERROR no rules expected the token `?`
+    baz!(a?,); //~ ERROR no rules expected the token `?`
     barplus!(); //~ ERROR unexpected end of macro invocation
-    barstar!(); //~ ERROR unexpected end of macro invocation
-    barplus!(a?); //~ ERROR no rules expected the token `?`
-    barplus!(a); //~ ERROR unexpected end of macro invocation
-    barstar!(a?); //~ ERROR no rules expected the token `?`
-    barstar!(a); //~ ERROR unexpected end of macro invocation
-    barplus!(+); // ok
-    barstar!(*); // ok
-    barplus!(a+); // ok
-    barstar!(a*); // ok
+    barplus!(a?); //~ ERROR unexpected end of macro invocation
+    barstar!(a?); //~ ERROR unexpected end of macro invocation
 }

src/test/ui/macros/macro-at-most-once-rep-ambig.stderr

@@ -1,62 +1,80 @@
-error: `?` macro repetition does not allow a separator
-  --> $DIR/macro-at-most-once-rep-ambig.rs:22:10
-   |
-LL |     ($(a),?) => {} //~ ERROR `?` macro repetition does not allow a separator
-   |          ^
-
 error: no rules expected the token `?`
-  --> $DIR/macro-at-most-once-rep-ambig.rs:36:11
+  --> $DIR/macro-at-most-once-rep-ambig.rs:40:11
    |
 LL |     foo!(a?a?a); //~ ERROR no rules expected the token `?`
    |           ^
 
 error: no rules expected the token `?`
-  --> $DIR/macro-at-most-once-rep-ambig.rs:37:11
+  --> $DIR/macro-at-most-once-rep-ambig.rs:41:11
    |
 LL |     foo!(a?a); //~ ERROR no rules expected the token `?`
    |           ^
 
 error: no rules expected the token `?`
-  --> $DIR/macro-at-most-once-rep-ambig.rs:38:11
+  --> $DIR/macro-at-most-once-rep-ambig.rs:42:11
    |
 LL |     foo!(a?); //~ ERROR no rules expected the token `?`
    |           ^
 
-error: unexpected end of macro invocation
-  --> $DIR/macro-at-most-once-rep-ambig.rs:39:5
+error: no rules expected the token `?`
+  --> $DIR/macro-at-most-once-rep-ambig.rs:43:11
    |
-LL |     barplus!(); //~ ERROR unexpected end of macro invocation
-   |     ^^^^^^^^^^^
+LL |     baz!(a?a?a); //~ ERROR no rules expected the token `?`
+   |           ^
 
-error: unexpected end of macro invocation
-  --> $DIR/macro-at-most-once-rep-ambig.rs:40:5
+error: no rules expected the token `?`
+  --> $DIR/macro-at-most-once-rep-ambig.rs:44:11
    |
-LL |     barstar!(); //~ ERROR unexpected end of macro invocation
-   |     ^^^^^^^^^^^
+LL |     baz!(a?a); //~ ERROR no rules expected the token `?`
+   |           ^
 
 error: no rules expected the token `?`
-  --> $DIR/macro-at-most-once-rep-ambig.rs:41:15
+  --> $DIR/macro-at-most-once-rep-ambig.rs:45:11
    |
-LL |     barplus!(a?); //~ ERROR no rules expected the token `?`
-   |               ^
+LL |     baz!(a?); //~ ERROR no rules expected the token `?`
+   |           ^
 
 error: unexpected end of macro invocation
-  --> $DIR/macro-at-most-once-rep-ambig.rs:42:14
+  --> $DIR/macro-at-most-once-rep-ambig.rs:46:11
+   |
+LL |     baz!(a,); //~ ERROR unexpected end of macro invocation
+   |           ^
+
+error: no rules expected the token `?`
+  --> $DIR/macro-at-most-once-rep-ambig.rs:47:11
+   |
+LL |     baz!(a?a?a,); //~ ERROR no rules expected the token `?`
+   |           ^
+
+error: no rules expected the token `?`
+  --> $DIR/macro-at-most-once-rep-ambig.rs:48:11
    |
-LL |     barplus!(a); //~ ERROR unexpected end of macro invocation
-   |              ^
+LL |     baz!(a?a,); //~ ERROR no rules expected the token `?`
+   |           ^
 
 error: no rules expected the token `?`
-  --> $DIR/macro-at-most-once-rep-ambig.rs:43:15
+  --> $DIR/macro-at-most-once-rep-ambig.rs:49:11
+   |
+LL |     baz!(a?,); //~ ERROR no rules expected the token `?`
+   |           ^
+
+error: unexpected end of macro invocation
+  --> $DIR/macro-at-most-once-rep-ambig.rs:50:5
+   |
+LL |     barplus!(); //~ ERROR unexpected end of macro invocation
+   |     ^^^^^^^^^^^
+
+error: unexpected end of macro invocation
+  --> $DIR/macro-at-most-once-rep-ambig.rs:51:15
    |
-LL |     barstar!(a?); //~ ERROR no rules expected the token `?`
+LL |     barplus!(a?); //~ ERROR unexpected end of macro invocation
    |               ^
 
 error: unexpected end of macro invocation
-  --> $DIR/macro-at-most-once-rep-ambig.rs:44:14
+  --> $DIR/macro-at-most-once-rep-ambig.rs:52:15
    |
-LL |     barstar!(a); //~ ERROR unexpected end of macro invocation
-   |              ^
+LL |     barstar!(a?); //~ ERROR unexpected end of macro invocation
+   |               ^
 
-error: aborting due to 10 previous errors
+error: aborting due to 13 previous errors