1 files changed, 143 insertions, 0 deletions

diff --git a/crates/hir_expand/src/proc_macro.rs b/crates/hir_expand/src/proc_macro.rs
new file mode 100644
index 000000000..80255ea32
--- /dev/null
+++ b/crates/hir_expand/src/proc_macro.rs
@@ -0,0 +1,143 @@
+//! Proc Macro Expander stub
+
+use crate::{db::AstDatabase, LazyMacroId};
+use base_db::{CrateId, ProcMacroId};
+use tt::buffer::{Cursor, TokenBuffer};
+
+#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
+pub struct ProcMacroExpander {
+    krate: CrateId,
+    proc_macro_id: ProcMacroId,
+}
+
+macro_rules! err {
+    ($fmt:literal, $($tt:tt),*) => {
+        mbe::ExpandError::ProcMacroError(tt::ExpansionError::Unknown(format!($fmt, $($tt),*)))
+    };
+    ($fmt:literal) => {
+        mbe::ExpandError::ProcMacroError(tt::ExpansionError::Unknown($fmt.to_string()))
+    }
+}
+
+impl ProcMacroExpander {
+    pub fn new(krate: CrateId, proc_macro_id: ProcMacroId) -> ProcMacroExpander {
+        ProcMacroExpander { krate, proc_macro_id }
+    }
+
+    pub fn expand(
+        self,
+        db: &dyn AstDatabase,
+        _id: LazyMacroId,
+        tt: &tt::Subtree,
+    ) -> Result<tt::Subtree, mbe::ExpandError> {
+        let krate_graph = db.crate_graph();
+        let proc_macro = krate_graph[self.krate]
+            .proc_macro
+            .get(self.proc_macro_id.0 as usize)
+            .clone()
+            .ok_or_else(|| err!("No derive macro found."))?;
+
+        let tt = remove_derive_attrs(tt)
+            .ok_or_else(|| err!("Fail to remove derive for custom derive"))?;
+
+        proc_macro.expander.expand(&tt, None).map_err(mbe::ExpandError::from)
+    }
+}
+
+fn eat_punct(cursor: &mut Cursor, c: char) -> bool {
+    if let Some(tt::TokenTree::Leaf(tt::Leaf::Punct(punct))) = cursor.token_tree() {
+        if punct.char == c {
+            *cursor = cursor.bump();
+            return true;
+        }
+    }
+    false
+}
+
+fn eat_subtree(cursor: &mut Cursor, kind: tt::DelimiterKind) -> bool {
+    if let Some(tt::TokenTree::Subtree(subtree)) = cursor.token_tree() {
+        if Some(kind) == subtree.delimiter_kind() {
+            *cursor = cursor.bump_subtree();
+            return true;
+        }
+    }
+    false
+}
+
+fn eat_ident(cursor: &mut Cursor, t: &str) -> bool {
+    if let Some(tt::TokenTree::Leaf(tt::Leaf::Ident(ident))) = cursor.token_tree() {
+        if t == ident.text.as_str() {
+            *cursor = cursor.bump();
+            return true;
+        }
+    }
+    false
+}
+
+fn remove_derive_attrs(tt: &tt::Subtree) -> Option<tt::Subtree> {
+    let buffer = TokenBuffer::new(&tt.token_trees);
+    let mut p = buffer.begin();
+    let mut result = tt::Subtree::default();
+
+    while !p.eof() {
+        let curr = p;
+
+        if eat_punct(&mut p, '#') {
+            eat_punct(&mut p, '!');
+            let parent = p;
+            if eat_subtree(&mut p, tt::DelimiterKind::Bracket) {
+                if eat_ident(&mut p, "derive") {
+                    p = parent.bump();
+                    continue;
+                }
+            }
+        }
+
+        result.token_trees.push(curr.token_tree()?.clone());
+        p = curr.bump();
+    }
+
+    Some(result)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use test_utils::assert_eq_text;
+
+    #[test]
+    fn test_remove_derive_attrs() {
+        let tt = mbe::parse_to_token_tree(
+            r#"
+    #[allow(unused)]
+    #[derive(Copy)]
+    #[derive(Hello)]
+    struct A {
+        bar: u32
+    }
+"#,
+        )
+        .unwrap()
+        .0;
+        let result = format!("{:#?}", remove_derive_attrs(&tt).unwrap());
+
+        assert_eq_text!(
+            &result,
+            r#"
+SUBTREE $
+  PUNCH   # [alone] 0
+  SUBTREE [] 1
+    IDENT   allow 2
+    SUBTREE () 3
+      IDENT   unused 4
+  IDENT   struct 15
+  IDENT   A 16
+  SUBTREE {} 17
+    IDENT   bar 18
+    PUNCH   : [alone] 19
+    IDENT   u32 20
+"#
+            .trim()
+        );
+    }
+}