Add misaligned_transmute lint

clippy_lints/src/lib.rs

@@ -603,6 +603,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         transmute::TRANSMUTE_PTR_TO_REF,
         transmute::USELESS_TRANSMUTE,
         transmute::WRONG_TRANSMUTE,
+        transmute::MISALIGNED_TRANSMUTE,
         types::ABSURD_EXTREME_COMPARISONS,
         types::BORROWED_BOX,
         types::BOX_VEC,

clippy_lints/src/transmute.rs

@@ -3,7 +3,8 @@ use rustc::ty::{self, Ty};
 use rustc::hir::*;
 use std::borrow::Cow;
 use syntax::ast;
-use utils::{last_path_segment, match_def_path, paths, snippet, span_lint, span_lint_and_then};
+use utils::{last_path_segment, match_def_path, paths, snippet, span_lint, span_lint_and_then,
+            alignment};
 use utils::{opt_def_id, sugg};
 
 /// **What it does:** Checks for transmutes that can't ever be correct on any
@@ -168,6 +169,23 @@ declare_lint! {
     "transmutes from an integer to a float"
 }
 
+/// **What it does:** Checks for transmutes to a potentially less-aligned type.
+///
+/// **Why is this bad?** This might result in undefined behavior.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// // u32 is 32-bit aligned; u8 is 8-bit aligned
+/// let _: u32 = unsafe { std::mem::transmute([0u8; 4]) };
+/// ```
+declare_lint! {
+    pub MISALIGNED_TRANSMUTE,
+    Warn,
+    "transmutes to a potentially less-aligned type"
+}
+
 pub struct Transmute;
 
 impl LintPass for Transmute {
@@ -180,7 +198,8 @@ impl LintPass for Transmute {
             TRANSMUTE_INT_TO_CHAR,
             TRANSMUTE_BYTES_TO_STR,
             TRANSMUTE_INT_TO_BOOL,
-            TRANSMUTE_INT_TO_FLOAT
+            TRANSMUTE_INT_TO_FLOAT,
+            MISALIGNED_TRANSMUTE
         )
     }
 }
@@ -201,6 +220,18 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Transmute {
                                 e.span,
                                 &format!("transmute from a type (`{}`) to itself", from_ty),
                             ),
+                            _ if alignment(cx, from_ty).map(|a| a.abi())
+                                < alignment(cx, to_ty).map(|a| a.abi())
+                                => span_lint(
+                                    cx,
+                                    MISALIGNED_TRANSMUTE,
+                                    e.span,
+                                    &format!(
+                                        "transmute from `{}` to a less-aligned type (`{}`)",
+                                        from_ty,
+                                        to_ty,
+                                    )
+                            ),
                             (&ty::TyRef(_, rty), &ty::TyRawPtr(ptr_ty)) => span_lint_and_then(
                                 cx,
                                 USELESS_TRANSMUTE,

clippy_lints/src/utils/mod.rs

@@ -9,7 +9,7 @@ use rustc::lint::{LateContext, Level, Lint, LintContext};
 use rustc::session::Session;
 use rustc::traits;
 use rustc::ty::{self, Ty, TyCtxt};
-use rustc::ty::layout::LayoutOf;
+use rustc::ty::layout::{LayoutOf, Align};
 use rustc_errors;
 use std::borrow::Cow;
 use std::env;
@@ -1056,3 +1056,8 @@ pub fn get_arg_name(pat: &Pat) -> Option<ast::Name> {
         _ => None,
     }
 }
+
+/// Returns alignment for a type, or None if alignment is undefined
+pub fn alignment<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, ty: Ty<'tcx>) -> Option<Align> {
+    (cx.tcx, cx.param_env).layout_of(ty).ok().map(|layout| layout.align)
+}

tests/ui/transmute.rs

@@ -140,4 +140,11 @@ fn bytes_to_str(b: &[u8], mb: &mut [u8]) {
     let _: &mut str = unsafe { std::mem::transmute(mb) };
 }
 
+#[warn(misaligned_transmute)]
+fn misaligned_transmute() {
+    let _: u32 = unsafe { std::mem::transmute([0u8; 4]) }; // err
+    let _: u32 = unsafe { std::mem::transmute(0f32) }; // ok (alignment-wise)
+    let _: [u8; 4] = unsafe { std::mem::transmute(0u32) }; // ok (alignment-wise)
+}
+
 fn main() { }

tests/ui/transmute.stderr

@@ -204,5 +204,13 @@ error: transmute from a `&mut [u8]` to a `&mut str`
 140 |     let _: &mut str = unsafe { std::mem::transmute(mb) };
     |                                ^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::str::from_utf8_mut(mb).unwrap()`
 
-error: aborting due to 32 previous errors
+error: transmute from `[u8; 4]` to a less-aligned type (`u32`)
+   --> $DIR/transmute.rs:145:27
+    |
+145 |     let _: u32 = unsafe { std::mem::transmute([0u8; 4]) }; // err
+    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    |
+    = note: `-D misaligned-transmute` implied by `-D warnings`
+
+error: aborting due to 33 previous errors