move db basics to ra_db

This should allow to move hir to a separate crate

crates/ra_db/src/syntax_ptr.rs

@@ -0,0 +1,48 @@
+use ra_syntax::{SourceFileNode, SyntaxKind, SyntaxNode, SyntaxNodeRef, TextRange};
+
+/// A pionter to a syntax node inside a file.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct LocalSyntaxPtr {
+    range: TextRange,
+    kind: SyntaxKind,
+}
+
+impl LocalSyntaxPtr {
+    pub fn new(node: SyntaxNodeRef) -> LocalSyntaxPtr {
+        LocalSyntaxPtr {
+            range: node.range(),
+            kind: node.kind(),
+        }
+    }
+
+    pub fn resolve(self, file: &SourceFileNode) -> SyntaxNode {
+        let mut curr = file.syntax();
+        loop {
+            if curr.range() == self.range && curr.kind() == self.kind {
+                return curr.owned();
+            }
+            curr = curr
+                .children()
+                .find(|it| self.range.is_subrange(&it.range()))
+                .unwrap_or_else(|| panic!("can't resolve local ptr to SyntaxNode: {:?}", self))
+        }
+    }
+
+    pub fn range(self) -> TextRange {
+        self.range
+    }
+}
+
+#[test]
+fn test_local_syntax_ptr() {
+    use ra_syntax::{ast, AstNode};
+    let file = SourceFileNode::parse("struct Foo { f: u32, }");
+    let field = file
+        .syntax()
+        .descendants()
+        .find_map(ast::NamedFieldDef::cast)
+        .unwrap();
+    let ptr = LocalSyntaxPtr::new(field.syntax());
+    let field_syntax = ptr.resolve(&file);
+    assert_eq!(field.syntax(), field_syntax);
+}