Auto merge of rust-lang#6716 - magurotuna:refactor-transmute-mod, r=f…

…lip1995

Refactor: arrange transmute lints

This PR arranges `transmute` lints so that they can be accessed more easily.
Basically, I followed the instruction described in rust-lang#6680 as to how to do the refactoring.

- `declare_clippy_lint!` and `impl LintPass` is placed in `transmute/mod.rs`
- Uitlity functions is placed in `transmute/utils.rs`
- Each lint function about `transmute` is moved into its own module, like `transmute/useless_transmute.rs`

For ease of review, I refactored step by step, keeping each commit small. For instance, all I did in
2451781 was to move  `useless_transmute` into its own module.

---

changelog: Refactor `transmute.rs` file into `transmute` module.

clippy_lints/src/transmute/crosspointer_transmute.rs

@@ -0,0 +1,37 @@
+use super::CROSSPOINTER_TRANSMUTE;
+use crate::utils::span_lint;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+/// Checks for `crosspointer_transmute` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::RawPtr(from_ptr), _) if from_ptr.ty == to_ty => {
+            span_lint(
+                cx,
+                CROSSPOINTER_TRANSMUTE,
+                e.span,
+                &format!(
+                    "transmute from a type (`{}`) to the type that it points to (`{}`)",
+                    from_ty, to_ty
+                ),
+            );
+            true
+        },
+        (_, ty::RawPtr(to_ptr)) if to_ptr.ty == from_ty => {
+            span_lint(
+                cx,
+                CROSSPOINTER_TRANSMUTE,
+                e.span,
+                &format!(
+                    "transmute from a type (`{}`) to a pointer to that type (`{}`)",
+                    from_ty, to_ty
+                ),
+            );
+            true
+        },
+        _ => false,
+    }
+}