New lint: Suggest ptr.add([usize]) over ptr.offset([usize] as isize).

First part of #3047.

clippy_lints/src/lib.rs

@@ -142,6 +142,7 @@ pub mod panic_unimplemented;
 pub mod partialeq_ne_impl;
 pub mod precedence;
 pub mod ptr;
+pub mod ptr_offset_with_cast;
 pub mod question_mark;
 pub mod ranges;
 pub mod redundant_field_names;
@@ -408,6 +409,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
     reg.register_late_lint_pass(box default_trait_access::DefaultTraitAccess);
     reg.register_late_lint_pass(box indexing_slicing::IndexingSlicing);
     reg.register_late_lint_pass(box non_copy_const::NonCopyConst);
+    reg.register_late_lint_pass(box ptr_offset_with_cast::Pass);
 
     reg.register_lint_group("clippy_restriction", vec![
         arithmetic::FLOAT_ARITHMETIC,
@@ -631,6 +633,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         ptr::CMP_NULL,
         ptr::MUT_FROM_REF,
         ptr::PTR_ARG,
+        ptr_offset_with_cast::PTR_OFFSET_WITH_CAST,
         question_mark::QUESTION_MARK,
         ranges::ITERATOR_STEP_BY_ZERO,
         ranges::RANGE_MINUS_ONE,
@@ -755,6 +758,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         panic_unimplemented::PANIC_PARAMS,
         ptr::CMP_NULL,
         ptr::PTR_ARG,
+        ptr_offset_with_cast::PTR_OFFSET_WITH_CAST,
         question_mark::QUESTION_MARK,
         redundant_field_names::REDUNDANT_FIELD_NAMES,
         regex::REGEX_MACRO,

clippy_lints/src/ptr_offset_with_cast.rs

@@ -0,0 +1,130 @@
+use rustc::{declare_lint, hir, lint, lint_array, ty};
+use syntax::ast;
+use crate::utils;
+
+/// **What it does:** Checks for usage of the `offset` pointer method with a `usize` casted to an
+/// `isize`.
+///
+/// **Why is this bad?** If we’re always increasing the pointer address, we can avoid the numeric
+/// cast by using the `add` method instead.
+///
+/// **Known problems:** None
+///
+/// **Example:**
+/// ```rust
+/// let vec = vec![b'a', b'b', b'c'];
+/// let ptr = vec.as_ptr();
+/// let offset = 1_usize;
+///
+/// unsafe { ptr.offset(offset as isize); }
+/// ```
+///
+/// Could be written:
+///
+/// ```rust
+/// let vec = vec![b'a', b'b', b'c'];
+/// let ptr = vec.as_ptr();
+/// let offset = 1_usize;
+///
+/// unsafe { ptr.add(offset); }
+/// ```
+declare_clippy_lint! {
+    pub PTR_OFFSET_WITH_CAST,
+    style,
+    "uneeded pointer offset cast"
+}
+
+#[derive(Copy, Clone, Debug)]
+pub struct Pass;
+
+impl lint::LintPass for Pass {
+    fn get_lints(&self) -> lint::LintArray {
+        lint_array!(PTR_OFFSET_WITH_CAST)
+    }
+}
+
+impl<'a, 'tcx> lint::LateLintPass<'a, 'tcx> for Pass {
+    fn check_expr(&mut self, cx: &lint::LateContext<'a, 'tcx>, expr: &'tcx hir::Expr) {
+        // Check if the expressions is a ptr.offset method call
+        let [receiver_expr, arg_expr] = match expr_as_ptr_offset_call(cx, expr) {
+            Some(call_arg) => call_arg,
+            None => return,
+        };
+
+        // Check if the parameter to ptr.offset is a cast from usize to isize
+        let cast_lhs_expr = match expr_as_cast_from_usize(cx, arg_expr) {
+            Some(cast_lhs_expr) => cast_lhs_expr,
+            None => return,
+        };
+
+        utils::span_lint_and_sugg(
+            cx,
+            PTR_OFFSET_WITH_CAST,
+            expr.span,
+            "use of `offset` with a `usize` casted to an `isize`",
+            "try",
+            build_suggestion(cx, receiver_expr, cast_lhs_expr),
+        );
+    }
+}
+
+// If the given expression is a cast from a usize, return the lhs of the cast
+fn expr_as_cast_from_usize<'a, 'tcx>(
+    cx: &lint::LateContext<'a, 'tcx>,
+    expr: &'tcx hir::Expr,
+) -> Option<&'tcx hir::Expr> {
+    if let hir::ExprKind::Cast(ref cast_lhs_expr, _) = expr.node {
+        if is_expr_ty_usize(cx, &cast_lhs_expr) {
+            return Some(cast_lhs_expr);
+        }
+    }
+    None
+}
+
+// If the given expression is a ptr::offset method call, return the receiver and the arg of the
+// method call.
+fn expr_as_ptr_offset_call<'a, 'tcx>(
+    cx: &lint::LateContext<'a, 'tcx>,
+    expr: &'tcx hir::Expr,
+) -> Option<[&'tcx hir::Expr; 2]> {
+    if let hir::ExprKind::MethodCall(ref path_segment, _, ref args) = expr.node {
+        if path_segment.ident.name == "offset" && is_expr_ty_raw_ptr(cx, &args[0]) {
+            return Some([&args[0], &args[1]]);
+        }
+    }
+    None
+}
+
+// Is the type of the expression a usize?
+fn is_expr_ty_usize<'a, 'tcx>(
+    cx: &lint::LateContext<'a, 'tcx>,
+    expr: &hir::Expr,
+) -> bool {
+    cx.tables.expr_ty(expr).sty == ty::TyKind::Uint(ast::UintTy::Usize)
+}
+
+// Is the type of the expression a raw pointer?
+fn is_expr_ty_raw_ptr<'a, 'tcx>(
+    cx: &lint::LateContext<'a, 'tcx>,
+    expr: &hir::Expr,
+) -> bool {
+    if let ty::RawPtr(..) = cx.tables.expr_ty(expr).sty {
+        true
+    } else {
+        false
+    }
+}
+
+fn build_suggestion<'a, 'tcx>(
+    cx: &lint::LateContext<'a, 'tcx>,
+    receiver_expr: &hir::Expr,
+    cast_lhs_expr: &hir::Expr,
+) -> String {
+    match (
+        utils::snippet_opt(cx, receiver_expr.span),
+        utils::snippet_opt(cx, cast_lhs_expr.span)
+    ) {
+        (Some(receiver), Some(cast_lhs)) => format!("{}.add({})", receiver, cast_lhs),
+        _ => String::new(),
+    }
+}

tests/ui/ptr_offset_with_cast.rs

@@ -0,0 +1,14 @@
+fn main() {
+    let vec = vec![b'a', b'b', b'c'];
+    let ptr = vec.as_ptr();
+
+    let offset_u8 = 1_u8;
+    let offset_usize = 1_usize;
+    let offset_isize = 1_isize;
+
+    unsafe {
+        ptr.offset(offset_usize as isize);
+        ptr.offset(offset_isize as isize);
+        ptr.offset(offset_u8 as isize);
+    }
+}

tests/ui/ptr_offset_with_cast.stderr

@@ -0,0 +1,10 @@
+error: use of `offset` with a `usize` casted to an `isize`
+  --> $DIR/ptr_offset_with_cast.rs:10:9
+   |
+10 |         ptr.offset(offset_usize as isize);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `ptr.add(offset_usize)`
+   |
+   = note: `-D ptr-offset-with-cast` implied by `-D warnings`
+
+error: aborting due to previous error
+