Uplift the new trait solver

compiler/rustc_next_trait_solver/src/lib.rs

@@ -4,6 +4,12 @@
 //! but were uplifted in the process of making the new trait solver generic.
 //! So if you got to this crate from the old solver, it's totally normal.
 
+#![feature(let_chains)]
+
+// TODO: remove this, use explicit imports.
+#[macro_use]
+extern crate tracing;
+
 pub mod canonicalizer;
 pub mod infcx;
 pub mod resolve;