1 files changed, 34 insertions, 9 deletions

diff --git a/crates/ra_hir_expand/src/db.rs b/crates/ra_hir_expand/src/db.rs
index f5c8bc22f..29dde3d80 100644
--- a/crates/ra_hir_expand/src/db.rs
+++ b/crates/ra_hir_expand/src/db.rs
@@ -72,6 +72,30 @@ pub trait AstDatabase: SourceDatabase {
     fn intern_eager_expansion(&self, eager: EagerCallLoc) -> EagerMacroId;
 }
 
+/// This expands the given macro call, but with different arguments. This is
+/// used for completion, where we want to see what 'would happen' if we insert a
+/// token. The `token_to_map` mapped down into the expansion, with the mapped
+/// token returned.
+pub fn expand_hypothetical(
+    db: &impl AstDatabase,
+    actual_macro_call: MacroCallId,
+    hypothetical_args: &ra_syntax::ast::TokenTree,
+    token_to_map: ra_syntax::SyntaxToken,
+) -> Option<(SyntaxNode, ra_syntax::SyntaxToken)> {
+    let macro_file = MacroFile { macro_call_id: actual_macro_call };
+    let (tt, tmap_1) = mbe::syntax_node_to_token_tree(hypothetical_args.syntax()).unwrap();
+    let range =
+        token_to_map.text_range().checked_sub(hypothetical_args.syntax().text_range().start())?;
+    let token_id = tmap_1.token_by_range(range)?;
+    let macro_def = expander(db, actual_macro_call)?;
+    let (node, tmap_2) =
+        parse_macro_with_arg(db, macro_file, Some(std::sync::Arc::new((tt, tmap_1))))?;
+    let token_id = macro_def.0.map_id_down(token_id);
+    let range = tmap_2.range_by_token(token_id)?.by_kind(token_to_map.kind())?;
+    let token = ra_syntax::algo::find_covering_element(&node.syntax_node(), range).into_token()?;
+    Some((node.syntax_node(), token))
+}
+
 pub(crate) fn ast_id_map(db: &dyn AstDatabase, file_id: HirFileId) -> Arc<AstIdMap> {
     let map =
         db.parse_or_expand(file_id).map_or_else(AstIdMap::default, |it| AstIdMap::from_source(&it));
@@ -133,10 +157,7 @@ pub(crate) fn macro_expand(
     macro_expand_with_arg(db, id, None)
 }
 
-pub fn expander(
-    db: &dyn AstDatabase,
-    id: MacroCallId,
-) -> Option<Arc<(TokenExpander, mbe::TokenMap)>> {
+fn expander(db: &dyn AstDatabase, id: MacroCallId) -> Option<Arc<(TokenExpander, mbe::TokenMap)>> {
     let lazy_id = match id {
         MacroCallId::LazyMacro(id) => id,
         MacroCallId::EagerMacro(_id) => {
@@ -149,7 +170,7 @@ pub fn expander(
     Some(macro_rules)
 }
 
-pub(crate) fn macro_expand_with_arg(
+fn macro_expand_with_arg(
     db: &dyn AstDatabase,
     id: MacroCallId,
     arg: Option<Arc<(tt::Subtree, mbe::TokenMap)>>,
@@ -158,7 +179,9 @@ pub(crate) fn macro_expand_with_arg(
         MacroCallId::LazyMacro(id) => id,
         MacroCallId::EagerMacro(id) => {
             if arg.is_some() {
-                return Err("hypothetical macro expansion not implemented for eager macro".to_owned());
+                return Err(
+                    "hypothetical macro expansion not implemented for eager macro".to_owned()
+                );
             } else {
                 return Ok(db.lookup_intern_eager_expansion(id).subtree);
             }
@@ -225,13 +248,15 @@ pub fn parse_macro_with_arg(
                     .collect::<Vec<_>>()
                     .join("\n");
 
-                    eprintln!(
+                    log::warn!(
                         "fail on macro_parse: (reason: {} macro_call: {:#}) parents: {}",
-                        err, node.value, parents
+                        err,
+                        node.value,
+                        parents
                     );
                 }
                 _ => {
-                    eprintln!("fail on macro_parse: (reason: {})", err);
+                    log::warn!("fail on macro_parse: (reason: {})", err);
                 }
             }
         })