2 files changed, 117 insertions, 0 deletions

diff --git a/crates/ra_ide_api/src/expand_macro.rs b/crates/ra_ide_api/src/expand_macro.rs
new file mode 100644
index 000000000..48dc90932
--- /dev/null
+++ b/crates/ra_ide_api/src/expand_macro.rs
@@ -0,0 +1,112 @@
+//! FIXME: write short doc here
+
+use crate::{db::RootDatabase, FilePosition};
+use ra_db::SourceDatabase;
+use rustc_hash::FxHashMap;
+
+use hir::db::AstDatabase;
+use ra_syntax::{
+    algo::{find_node_at_offset, replace_descendants},
+    ast::{self},
+    AstNode, NodeOrToken, SyntaxKind, SyntaxNode, WalkEvent,
+};
+
+fn insert_whitespaces(syn: SyntaxNode) -> String {
+    let mut res = String::new();
+
+    let mut token_iter = syn
+        .preorder_with_tokens()
+        .filter_map(|event| {
+            if let WalkEvent::Enter(NodeOrToken::Token(token)) = event {
+                Some(token)
+            } else {
+                None
+            }
+        })
+        .peekable();
+
+    while let Some(token) = token_iter.next() {
+        res += &token.text().to_string();
+        if token.kind().is_keyword()
+            || token.kind().is_literal()
+            || token.kind() == SyntaxKind::IDENT
+        {
+            if !token_iter.peek().map(|it| it.kind().is_punct()).unwrap_or(false) {
+                res += " ";
+            }
+        }
+    }
+
+    res
+}
+
+fn expand_macro_recur(
+    db: &RootDatabase,
+    source: hir::Source<&SyntaxNode>,
+    macro_call: &ast::MacroCall,
+) -> Option<SyntaxNode> {
+    let analyzer = hir::SourceAnalyzer::new(db, source, None);
+    let expansion = analyzer.expand(db, &macro_call)?;
+    let expanded: SyntaxNode = db.parse_or_expand(expansion.file_id())?;
+
+    let children = expanded.descendants().filter_map(ast::MacroCall::cast);
+    let mut replaces = FxHashMap::default();
+
+    for child in children.into_iter() {
+        let source = hir::Source::new(expansion.file_id(), source.ast);
+        let new_node = expand_macro_recur(db, source, &child)?;
+
+        replaces.insert(child.syntax().clone().into(), new_node.into());
+    }
+
+    Some(replace_descendants(&expanded, &replaces))
+}
+
+pub(crate) fn expand_macro(db: &RootDatabase, position: FilePosition) -> Option<(String, String)> {
+    let parse = db.parse(position.file_id);
+    let file = parse.tree();
+    let name_ref = find_node_at_offset::<ast::NameRef>(file.syntax(), position.offset)?;
+    let mac = name_ref.syntax().ancestors().find_map(ast::MacroCall::cast)?;
+
+    let source = hir::Source::new(position.file_id.into(), mac.syntax());
+
+    let expanded = expand_macro_recur(db, source, &mac)?;
+
+    // FIXME:
+    // macro expansion may lose all white space information
+    // But we hope someday we can use ra_fmt for that
+    let res = insert_whitespaces(expanded);
+    Some((name_ref.text().to_string(), res))
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::mock_analysis::analysis_and_position;
+
+    fn check_expand_macro(fixture: &str, expected: (&str, &str)) {
+        let (analysis, pos) = analysis_and_position(fixture);
+
+        let result = analysis.expand_macro(pos).unwrap().unwrap();
+        assert_eq!(result, (expected.0.to_string(), expected.1.to_string()));
+    }
+
+    #[test]
+    fn macro_expand_recursive_expansion() {
+        check_expand_macro(
+            r#"
+        //- /lib.rs
+        macro_rules! bar {
+            () => { fn  b() {} }
+        }
+        macro_rules! foo {
+            () => { bar!(); }
+        }
+        macro_rules! baz {
+            () => { foo!(); }
+        }        
+        f<|>oo!();
+        "#,
+            ("foo", "fn b(){}"),
+        );
+    }
+}
diff --git a/crates/ra_ide_api/src/lib.rs b/crates/ra_ide_api/src/lib.rs
index 110ddcd62..d1b73ef6f 100644
--- a/crates/ra_ide_api/src/lib.rs
+++ b/crates/ra_ide_api/src/lib.rs
@@ -42,6 +42,7 @@ mod display;
 mod inlay_hints;
 mod wasm_shims;
 mod expand;
+mod expand_macro;
 
 #[cfg(test)]
 mod marks;
@@ -296,6 +297,10 @@ impl Analysis {
         self.with_db(|db| syntax_tree::syntax_tree(&db, file_id, text_range))
     }
 
+    pub fn expand_macro(&self, position: FilePosition) -> Cancelable<Option<(String, String)>> {
+        self.with_db(|db| expand_macro::expand_macro(db, position))
+    }
+
     /// Returns an edit to remove all newlines in the range, cleaning up minor
     /// stuff like trailing commas.
     pub fn join_lines(&self, frange: FileRange) -> Cancelable<SourceChange> {