Implement syntax tree support for syntax inside string

This allows us to select a string or portions of it and try parsing it as rust
syntax. This is mostly helpful when developing tests where the test
itself contains some rust syntax as a string.

crates/ra_ide_api/src/lib.rs

@@ -32,13 +32,14 @@ mod references;
 mod impls;
 mod assists;
 mod diagnostics;
+mod syntax_tree;
 
 #[cfg(test)]
 mod marks;
 
 use std::sync::Arc;
 
-use ra_syntax::{SourceFile, TreeArc, TextRange, TextUnit, AstNode, algo};
+use ra_syntax::{SourceFile, TreeArc, TextRange, TextUnit};
 use ra_text_edit::TextEdit;
 use ra_db::{
     SourceDatabase, CheckCanceled,
@@ -246,13 +247,7 @@ impl Analysis {
     /// Returns a syntax tree represented as `String`, for debug purposes.
     // FIXME: use a better name here.
     pub fn syntax_tree(&self, file_id: FileId, text_range: Option<TextRange>) -> String {
-        if let Some(text_range) = text_range {
-            let file = self.db.parse(file_id);
-            let node = algo::find_covering_node(file.syntax(), text_range);
-            node.debug_dump()
-        } else {
-            self.db.parse(file_id).syntax().debug_dump()
-        }
+        syntax_tree::syntax_tree(&self.db, file_id, text_range)
     }
 
     /// Returns an edit to remove all newlines in the range, cleaning up minor