Add lint for useless transmutes

Closes #441.

src/lib.rs

@@ -55,6 +55,7 @@ pub mod needless_features;
 pub mod needless_update;
 pub mod no_effect;
 pub mod temporary_assignment;
+pub mod transmute;
 
 mod reexport {
     pub use syntax::ast::{Name, Ident, NodeId};
@@ -104,6 +105,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_late_lint_pass(box no_effect::NoEffectPass);
     reg.register_late_lint_pass(box map_clone::MapClonePass);
     reg.register_late_lint_pass(box temporary_assignment::TemporaryAssignmentPass);
+    reg.register_late_lint_pass(box transmute::UselessTransmute);
 
     reg.register_lint_group("clippy_pedantic", vec![
         methods::OPTION_UNWRAP_USED,
@@ -175,6 +177,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         returns::LET_AND_RETURN,
         returns::NEEDLESS_RETURN,
         temporary_assignment::TEMPORARY_ASSIGNMENT,
+        transmute::USELESS_TRANSMUTE,
         types::BOX_VEC,
         types::LET_UNIT_VALUE,
         types::LINKEDLIST,

src/transmute.rs

@@ -0,0 +1,37 @@
+use rustc::lint::*;
+use rustc_front::hir::*;
+use utils;
+
+declare_lint! {
+    pub USELESS_TRANSMUTE,
+    Warn,
+    "transmutes that have the same to and from types"
+}
+
+pub struct UselessTransmute;
+
+impl LintPass for UselessTransmute {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(USELESS_TRANSMUTE)
+    }
+}
+
+impl LateLintPass for UselessTransmute {
+    fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
+        if let ExprCall(ref path_expr, ref args) = e.node {
+            if let ExprPath(None, _) = path_expr.node {
+                let def_id = cx.tcx.def_map.borrow()[&path_expr.id].def_id();
+
+                if utils::match_def_path(cx, def_id, &["core", "intrinsics", "transmute"]) {
+                    let from_ty = cx.tcx.expr_ty(&args[0]);
+                    let to_ty = cx.tcx.expr_ty(e);
+
+                    if from_ty == to_ty {
+                        cx.span_lint(USELESS_TRANSMUTE, e.span,
+                                     &format!("transmute from a type (`{}`) to itself", from_ty));
+                    }
+                }
+            }
+        }
+    }
+}