Fixup path fragments upon MBE transcription

crates/hir-def/src/macro_expansion_tests/mbe.rs

@@ -848,6 +848,37 @@ fn foo() {
     );
 }
 
+#[test]
+fn test_type_path_is_transcribed_as_expr_path() {
+    check(
+        r#"
+macro_rules! m {
+    ($p:path) => { let $p; }
+}
+fn test() {
+    m!(S)
+    m!(S<i32>)
+    m!(S<S<i32>>)
+    m!(S<{ module::CONST < 42 }>)
+}
+"#,
+        expect![[r#"
+macro_rules! m {
+    ($p:path) => { let $p; }
+}
+fn test() {
+    let S;
+    let S:: <i32> ;
+    let S:: <S:: <i32>> ;
+    let S:: < {
+        module::CONST<42
+    }
+    > ;
+}
+"#]],
+    );
+}
+
 #[test]
 fn test_expr() {
     check(

crates/mbe/src/expander.rs

@@ -123,4 +123,14 @@ enum Fragment {
     /// proc-macro delimiter=none. As we later discovered, "none" delimiters are
     /// tricky to handle in the parser, and rustc doesn't handle those either.
     Expr(tt::TokenTree),
+    /// There are roughly two types of paths: paths in expression context, where a
+    /// separator `::` between an identifier and its following generic argument list
+    /// is mandatory, and paths in type context, where `::` can be omitted.
+    ///
+    /// Unlike rustc, we need to transform the parsed fragments back into tokens
+    /// during transcription. When the matched path fragment is a type-context path
+    /// and is trasncribed as an expression-context path, verbatim transcription
+    /// would cause a syntax error. We need to fix it up just before transcribing;
+    /// see `transcriber::fix_up_and_push_path_tt()`.
+    Path(tt::TokenTree),
 }

crates/mbe/src/expander/matcher.rs

@@ -742,7 +742,11 @@ fn match_meta_var(
     is_2021: bool,
 ) -> ExpandResult<Option<Fragment>> {
     let fragment = match kind {
-        MetaVarKind::Path => parser::PrefixEntryPoint::Path,
+        MetaVarKind::Path => {
+            return input
+                .expect_fragment(parser::PrefixEntryPoint::Path)
+                .map(|it| it.map(Fragment::Path));
+        }
         MetaVarKind::Ty => parser::PrefixEntryPoint::Ty,
         MetaVarKind::Pat if is_2021 => parser::PrefixEntryPoint::PatTop,
         MetaVarKind::Pat => parser::PrefixEntryPoint::Pat,
@@ -771,7 +775,7 @@ fn match_meta_var(
                 .expect_fragment(parser::PrefixEntryPoint::Expr)
                 .map(|tt| tt.map(Fragment::Expr));
         }
-        _ => {
+        MetaVarKind::Ident | MetaVarKind::Tt | MetaVarKind::Lifetime | MetaVarKind::Literal => {
             let tt_result = match kind {
                 MetaVarKind::Ident => input
                     .expect_ident()
@@ -799,7 +803,7 @@ fn match_meta_var(
                         })
                         .map_err(|()| ExpandError::binding_error("expected literal"))
                 }
-                _ => Err(ExpandError::UnexpectedToken),
+                _ => unreachable!(),
             };
             return tt_result.map(|it| Some(Fragment::Tokens(it))).into();
         }

crates/mbe/src/expander/transcriber.rs

@@ -400,7 +400,8 @@ fn push_fragment(buf: &mut Vec<tt::TokenTree>, fragment: Fragment) {
             }
             buf.push(tt.into())
         }
-        Fragment::Tokens(tt) | Fragment::Expr(tt) => buf.push(tt),
+        Fragment::Path(tt::TokenTree::Subtree(tt)) => fix_up_and_push_path_tt(buf, tt),
+        Fragment::Tokens(tt) | Fragment::Expr(tt) | Fragment::Path(tt) => buf.push(tt),
     }
 }
 
@@ -411,6 +412,45 @@ fn push_subtree(buf: &mut Vec<tt::TokenTree>, tt: tt::Subtree) {
     }
 }
 
+/// Inserts the path separator `::` between an identifier and its following generic
+/// argument list, and then pushes into the buffer. See [`Fragment::Path`] for why
+/// we need this fixup.
+fn fix_up_and_push_path_tt(buf: &mut Vec<tt::TokenTree>, subtree: tt::Subtree) {
+    stdx::always!(matches!(subtree.delimiter.kind, tt::DelimiterKind::Invisible));
+    let mut prev_was_ident = false;
+    // Note that we only need to fix up the top-level `TokenTree`s because the
+    // context of the paths in the descendant `Subtree`s won't be changed by the
+    // mbe transcription.
+    for tt in subtree.token_trees {
+        if prev_was_ident {
+            // Pedantically, `(T) -> U` in `FnOnce(T) -> U` is treated as a generic
+            // argument list and thus needs `::` between it and `FnOnce`. However in
+            // today's Rust this type of path *semantically* cannot appear as a
+            // top-level expression-context path, so we can safely ignore it.
+            if let tt::TokenTree::Leaf(tt::Leaf::Punct(tt::Punct { char: '<', .. })) = tt {
+                buf.push(
+                    tt::Leaf::Punct(tt::Punct {
+                        char: ':',
+                        spacing: tt::Spacing::Joint,
+                        span: tt::Span::unspecified(),
+                    })
+                    .into(),
+                );
+                buf.push(
+                    tt::Leaf::Punct(tt::Punct {
+                        char: ':',
+                        spacing: tt::Spacing::Alone,
+                        span: tt::Span::unspecified(),
+                    })
+                    .into(),
+                );
+            }
+        }
+        prev_was_ident = matches!(tt, tt::TokenTree::Leaf(tt::Leaf::Ident(_)));
+        buf.push(tt);
+    }
+}
+
 /// Handles `${count(t, depth)}`. `our_depth` is the recursion depth and `count_depth` is the depth
 /// defined by the metavar expression.
 fn count(

crates/tt/src/lib.rs

@@ -122,7 +122,6 @@ impl_from!(Literal<Span>, Punct<Span>, Ident<Span> for Leaf);
 
 #[derive(Clone, PartialEq, Eq, Hash)]
 pub struct Subtree<Span> {
-    // FIXME, this should not be Option
     pub delimiter: Delimiter<Span>,
     pub token_trees: Vec<TokenTree<Span>>,
 }