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/syntax_tree.rs

@@ -0,0 +1,85 @@
+use ra_db::SourceDatabase;
+use crate::db::RootDatabase;
+use ra_syntax::{
+    SourceFile, SyntaxNode, TextRange, AstNode,
+    algo::{self, visit::{visitor, Visitor}}, ast::{self, AstToken}
+};
+
+pub use ra_db::FileId;
+
+pub(crate) fn syntax_tree(
+    db: &RootDatabase,
+    file_id: FileId,
+    text_range: Option<TextRange>,
+) -> String {
+    if let Some(text_range) = text_range {
+        let file = db.parse(file_id);
+        let node = algo::find_covering_node(file.syntax(), text_range);
+
+        if let Some(tree) = syntax_tree_for_string(node, text_range) {
+            return tree;
+        }
+
+        node.debug_dump()
+    } else {
+        db.parse(file_id).syntax().debug_dump()
+    }
+}
+
+/// Attempts parsing the selected contents of a string literal
+/// as rust syntax and returns its syntax tree
+fn syntax_tree_for_string(node: &SyntaxNode, text_range: TextRange) -> Option<String> {
+    // When the range is inside a string
+    // we'll attempt parsing it as rust syntax
+    // to provide the syntax tree of the contents of the string
+    visitor()
+        .visit(|node: &ast::String| syntax_tree_for_token(node, text_range))
+        .visit(|node: &ast::RawString| syntax_tree_for_token(node, text_range))
+        .accept(node)?
+}
+
+fn syntax_tree_for_token<T: AstToken>(node: &T, text_range: TextRange) -> Option<String> {
+    // Range of the full node
+    let node_range = node.syntax().range();
+    let text = node.text().to_string();
+
+    // We start at some point inside the node
+    // Either we have selected the whole string
+    // or our selection is inside it
+    let start = text_range.start() - node_range.start();
+
+    // how many characters we have selected
+    let len = text_range.len().to_usize();
+
+    let node_len = node_range.len().to_usize();
+
+    let start = start.to_usize();
+
+    // We want to cap our length
+    let len = len.min(node_len);
+
+    // Ensure our slice is inside the actual string
+    let end = if start + len < text.len() { start + len } else { text.len() - start };
+
+    let text = &text[start..end];
+
+    // Remove possible extra string quotes from the start
+    // and the end of the string
+    let text = text
+        .trim_start_matches('r')
+        .trim_start_matches('#')
+        .trim_start_matches('"')
+        .trim_end_matches('#')
+        .trim_end_matches('"')
+        .trim();
+
+    let parsed = SourceFile::parse(&text);
+
+    // If the "file" parsed without errors,
+    // return its syntax
+    if parsed.errors().is_empty() {
+        return Some(parsed.syntax().debug_dump());
+    }
+
+    None
+}