Add ra_ide_api::expand

This module should handle all tricky bits with mapping macro-expanded
HirFileId to original files the user actually can see in the editor

crates/ra_ide_api/src/expand.rs

@@ -0,0 +1,42 @@
+//! Utilities to work with files, produced by macros.
+use std::iter::successors;
+
+use hir::Source;
+use ra_db::FileId;
+use ra_syntax::{ast, AstNode, SyntaxNode, SyntaxToken};
+
+use crate::{db::RootDatabase, FileRange};
+
+pub(crate) fn original_range(db: &RootDatabase, node: Source<&SyntaxNode>) -> FileRange {
+    let text_range = node.ast.text_range();
+    let (file_id, range) = node
+        .file_id
+        .expansion_info(db)
+        .and_then(|expansion_info| expansion_info.find_range(text_range))
+        .unwrap_or((node.file_id, text_range));
+
+    // FIXME: handle recursive macro generated macro
+    FileRange { file_id: file_id.original_file(db), range }
+}
+
+pub(crate) fn descend_into_macros(
+    db: &RootDatabase,
+    file_id: FileId,
+    token: SyntaxToken,
+) -> Source<SyntaxToken> {
+    let src = Source::new(file_id.into(), token);
+
+    successors(Some(src), |token| {
+        let macro_call = token.ast.ancestors().find_map(ast::MacroCall::cast)?;
+        let tt = macro_call.token_tree()?;
+        if !token.ast.text_range().is_subrange(&tt.syntax().text_range()) {
+            return None;
+        }
+        let source_analyzer =
+            hir::SourceAnalyzer::new(db, token.with_ast(token.ast.parent()).as_ref(), None);
+        let exp = source_analyzer.expand(db, &macro_call)?;
+        exp.map_token_down(db, token.as_ref())
+    })
+    .last()
+    .unwrap()
+}