Fix some warnings in no_std builds.

The dbg! macro expands to nothing in no_std mode, so variables that are
only used for debugging prompt unused variable warnings.

Also, allow unstable_features in no_std builds, since they use
feature(alloc), which is an unstable feature.

lib/codegen/src/lib.rs

@@ -1,7 +1,8 @@
 //! Cretonne code generation library.
 
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
-#![warn(unused_import_braces, unstable_features)]
+#![warn(unused_import_braces)]
+#![cfg_attr(feature = "std", warn(unstable_features))]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature="cargo-clippy", allow(
 // Rustfmt 0.9.0 is at odds with this lint:

lib/codegen/src/regalloc/coalescing.rs

@@ -6,6 +6,7 @@
 //! parameter will belong to the same virtual register as the EBB parameter value itself.
 
 use cursor::{Cursor, EncCursor};
+#[cfg(feature = "std")]
 use dbg::DisplayList;
 use dominator_tree::{DominatorTree, DominatorTreePreorder};
 use flowgraph::ControlFlowGraph;
@@ -546,8 +547,8 @@ impl<'a> Context<'a> {
             return false;
         }
 
-        let vreg = self.virtregs.unify(self.values);
-        dbg!("-> merged into {} = {}", vreg, DisplayList(self.values));
+        let _vreg = self.virtregs.unify(self.values);
+        dbg!("-> merged into {} = {}", _vreg, DisplayList(self.values));
         true
     }
 

lib/codegen/src/regalloc/coloring.rs

@@ -857,8 +857,11 @@ impl<'a> Context<'a> {
                     let added = self.try_add_var(rc, throughs);
                     debug_assert!(added, "Ran out of registers in {}", rc);
                 }
-                Err(SolverError::Global(value)) => {
-                    dbg!("Not enough global registers for {}, trying as local", value);
+                Err(SolverError::Global(_value)) => {
+                    dbg!(
+                        "Not enough global registers for {}, trying as local",
+                        _value
+                    );
                     // We'll clear the `is_global` flag on all solver variables and instead make a
                     // note to replace all global defines with local defines followed by a copy.
                     *replace_global_defines = true;