Implement slow-path for FirstSets::first

When 2 or more sequences share the same span, we can't use the precomputed map
for their first set. So we compute it recursively.

Fixes #62831.

src/libsyntax/ext/tt/macro_rules.rs

@@ -625,38 +625,37 @@ impl FirstSets {
                     return first;
                 }
                 TokenTree::Sequence(sp, ref seq_rep) => {
-                    match self.first.get(&sp.entire()) {
-                        Some(&Some(ref subfirst)) => {
-                            // If the sequence contents can be empty, then the first
-                            // token could be the separator token itself.
-
-                            if let (Some(sep), true) = (&seq_rep.separator, subfirst.maybe_empty) {
-                                first.add_one_maybe(TokenTree::Token(sep.clone()));
-                            }
-
-                            assert!(first.maybe_empty);
-                            first.add_all(subfirst);
-                            if subfirst.maybe_empty
-                                || seq_rep.kleene.op == quoted::KleeneOp::ZeroOrMore
-                                || seq_rep.kleene.op == quoted::KleeneOp::ZeroOrOne
-                            {
-                                // continue scanning for more first
-                                // tokens, but also make sure we
-                                // restore empty-tracking state
-                                first.maybe_empty = true;
-                                continue;
-                            } else {
-                                return first;
-                            }
-                        }
-
+                    let subfirst_owned;
+                    let subfirst = match self.first.get(&sp.entire()) {
+                        Some(&Some(ref subfirst)) => subfirst,
                         Some(&None) => {
-                            panic!("assume all sequences have (unique) spans for now");
+                            subfirst_owned = self.first(&seq_rep.tts[..]);
+                            &subfirst_owned
                         }
-
                         None => {
                             panic!("We missed a sequence during FirstSets construction");
                         }
+                    };
+
+                    // If the sequence contents can be empty, then the first
+                    // token could be the separator token itself.
+                    if let (Some(sep), true) = (&seq_rep.separator, subfirst.maybe_empty) {
+                        first.add_one_maybe(TokenTree::Token(sep.clone()));
+                    }
+
+                    assert!(first.maybe_empty);
+                    first.add_all(subfirst);
+                    if subfirst.maybe_empty
+                        || seq_rep.kleene.op == quoted::KleeneOp::ZeroOrMore
+                        || seq_rep.kleene.op == quoted::KleeneOp::ZeroOrOne
+                    {
+                        // Continue scanning for more first
+                        // tokens, but also make sure we
+                        // restore empty-tracking state.
+                        first.maybe_empty = true;
+                        continue;
+                    } else {
+                        return first;
                     }
                 }
             }

src/test/ui/macros/auxiliary/proc_macro_sequence.rs

@@ -0,0 +1,36 @@
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+#![feature(proc_macro_span, proc_macro_hygiene, proc_macro_quote)]
+
+extern crate proc_macro;
+
+use proc_macro::{quote, Span, TokenStream};
+
+fn assert_same_span(a: Span, b: Span) {
+    assert_eq!(a.start(), b.start());
+    assert_eq!(a.end(), b.end());
+}
+
+// This macro generates a macro with the same macro definition as `manual_foo` in
+// `same-sequence-span.rs` but with the same span for all sequences.
+#[proc_macro]
+pub fn make_foo(_: TokenStream) -> TokenStream {
+    let result = quote! {
+        macro_rules! generated_foo {
+            (1 $$x:expr $$($$y:tt,)* $$(= $$z:tt)*) => {};
+        }
+    };
+
+    // Check that all spans are equal.
+    let mut span = None;
+    for tt in result.clone() {
+        match span {
+            None => span = Some(tt.span()),
+            Some(span) => assert_same_span(tt.span(), span),
+        }
+    }
+
+    result
+}

src/test/ui/macros/same-sequence-span.rs

@@ -0,0 +1,23 @@
+// aux-build:proc_macro_sequence.rs
+
+// Regression test for issue #62831: Check that multiple sequences with the same span in the
+// left-hand side of a macro definition behave as if they had unique spans, and in particular that
+// they don't crash the compiler.
+
+#![feature(proc_macro_hygiene)]
+#![allow(unused_macros)]
+
+extern crate proc_macro_sequence;
+
+// When ignoring spans, this macro has the same macro definition as `generated_foo` in
+// `proc_macro_sequence.rs`.
+macro_rules! manual_foo {
+    (1 $x:expr $($y:tt,)*   //~ERROR `$x:expr` may be followed by `$y:tt`
+               $(= $z:tt)*  //~ERROR `$x:expr` may be followed by `=`
+    ) => {};
+}
+
+proc_macro_sequence::make_foo!(); //~ERROR `$x:expr` may be followed by `$y:tt`
+                                  //~^ERROR `$x:expr` may be followed by `=`
+
+fn main() {}

src/test/ui/macros/same-sequence-span.stderr

@@ -0,0 +1,34 @@
+error: `$x:expr` may be followed by `$y:tt`, which is not allowed for `expr` fragments
+  --> $DIR/same-sequence-span.rs:15:18
+   |
+LL |     (1 $x:expr $($y:tt,)*
+   |                  ^^^^^ not allowed after `expr` fragments
+   |
+   = note: allowed there are: `=>`, `,` or `;`
+
+error: `$x:expr` may be followed by `=`, which is not allowed for `expr` fragments
+  --> $DIR/same-sequence-span.rs:16:18
+   |
+LL |                $(= $z:tt)*
+   |                  ^ not allowed after `expr` fragments
+   |
+   = note: allowed there are: `=>`, `,` or `;`
+
+error: `$x:expr` may be followed by `$y:tt`, which is not allowed for `expr` fragments
+  --> $DIR/same-sequence-span.rs:20:1
+   |
+LL | proc_macro_sequence::make_foo!();
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not allowed after `expr` fragments
+   |
+   = note: allowed there are: `=>`, `,` or `;`
+
+error: `$x:expr` may be followed by `=`, which is not allowed for `expr` fragments
+  --> $DIR/same-sequence-span.rs:20:1
+   |
+LL | proc_macro_sequence::make_foo!();
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not allowed after `expr` fragments
+   |
+   = note: allowed there are: `=>`, `,` or `;`
+
+error: aborting due to 4 previous errors
+