Implement the unused variable pass on the MIR

Author: ecstatic-morse

Cargo.lock

@@ -4164,6 +4164,7 @@ dependencies = [
  "rustc_infer",
  "rustc_macros",
  "rustc_middle",
+ "rustc_mir",
  "rustc_session",
  "rustc_span",
  "rustc_target",

src/librustc_mir_build/Cargo.toml

@@ -21,6 +21,7 @@ rustc_errors = { path = "../librustc_errors" }
 rustc_hir = { path = "../librustc_hir" }
 rustc_infer = { path = "../librustc_infer" }
 rustc_macros = { path = "../librustc_macros" }
+rustc_mir = { path = "../librustc_mir" } # Just for dataflow
 rustc_serialize = { path = "../libserialize", package = "serialize" }
 rustc_session = { path = "../librustc_session" }
 rustc_span = { path = "../librustc_span" }

src/librustc_mir_build/build/mod.rs

@@ -183,6 +183,7 @@ fn mir_build(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Body<'_> {
         };
 
         lints::unconditional_recursion::check(tcx, &body, def_id);
+        lints::unused_variables::check(tcx, &body, def_id);
 
         // The borrow checker will replace all the regions here with its own
         // inference variables. There's no point having non-erased regions here.

src/librustc_mir_build/lib.rs

@@ -9,6 +9,7 @@
 #![feature(const_panic)]
 #![feature(crate_visibility_modifier)]
 #![feature(bool_to_option)]
+#![feature(in_band_lifetimes)]
 #![feature(or_patterns)]
 #![recursion_limit = "256"]
 

src/librustc_mir_build/lints/mod.rs

@@ -4,3 +4,4 @@
 //! user's code as possible.
 
 pub mod unconditional_recursion;
+pub mod unused_variables;