Simplify lifetime-differences-only detection

Now instead of reinventing the wheel with differ_only_in_lifetimes(), we
use TyCtxt's erase_regions()

Author: devonhollowood

clippy_lints/src/transmute.rs

@@ -1,8 +1,7 @@
 use rustc::lint::*;
-use rustc::ty::{self, Ty, walk::TypeWalker};
+use rustc::ty::{self, Ty};
 use rustc::hir::*;
 use std::borrow::Cow;
-use std::mem;
 use syntax::ast;
 use utils::{last_path_segment, match_def_path, paths, snippet, span_lint, span_lint_and_then};
 use utils::{opt_def_id, sugg};
@@ -364,7 +363,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Transmute {
                                             }
                                         )
                                     } else {
-                                        if !differ_only_in_lifetime_params(from_ty, to_ty) {
+                                        if cx.tcx.erase_regions(&from_ty) != cx.tcx.erase_regions(&to_ty) {
                                             span_lint_and_then(
                                                 cx,
                                                 TRANSMUTE_PTR_TO_PTR,
@@ -448,48 +447,6 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Transmute {
     }
 }
 
-/// Returns true if `type1` and `type2` are the same type except for their lifetime parameters
-fn differ_only_in_lifetime_params(type1: Ty, type2: Ty) -> bool {
-    use rustc::ty::TypeVariants::*;
-    if TypeWalker::new(type1).count() != TypeWalker::new(type2).count() {
-        return false;
-    }
-    TypeWalker::new(type1)
-        .zip(TypeWalker::new(type2))
-        .all(|(t1, t2)| {
-            match (&t1.sty, &t2.sty) {
-                // types with generic parameters which can contain lifetimes
-                (TyAdt(_, sub1), TyAdt(_, sub2))
-                | (TyFnDef(_, sub1), TyFnDef(_, sub2))
-                | (TyAnon(_, sub1), TyAnon(_, sub2))
-                => {
-                    // Iterate over generic parameters, which are either Lifetimes or Types.
-                    // Here we only need to check that they are the same type of thing, because
-                    // if they are both Lifetimes then we don't care about their equality, and if
-                    // they are both Types, we will check their equality later in the type walk.
-                    sub1.iter().count() == sub2.iter().count()
-                    && sub1.iter().zip(sub2.iter()).all(|(k1, k2)| {
-                        mem::discriminant(&k1.unpack()) == mem::discriminant(&k2.unpack())
-                    })
-                }
-                // types without subtypes: check that the types are equal
-                (TyBool, TyBool)
-                | (TyChar, TyChar)
-                | (TyInt(_), TyInt(_))
-                | (TyUint(_), TyUint(_))
-                | (TyFloat(_), TyFloat(_))
-                | (TyForeign(_), TyForeign(_))
-                | (TyStr, TyStr)
-                | (TyNever, TyNever)
-                | (TyInfer(_), TyInfer(_))
-                => t1.sty == t2.sty,
-                // types with subtypes: return true for now if they are the same sort of type.
-                // we will check their subtypes later
-                (sty1, sty2) => mem::discriminant(sty1) == mem::discriminant(sty2)
-            }
-        })
-}
-
 /// Get the snippet of `Bar` in `…::transmute<Foo, &Bar>`. If that snippet is
 /// not available , use
 /// the type's `ToString` implementation. In weird cases it could lead to types