Lint against indexing and slicing

This can be useful to prevent panics in a codebase. ATM it is a pedantic
lint, but in the future it should be added to the restricions group.

src/array_indexing.rs

@@ -3,7 +3,8 @@ use rustc::middle::const_eval::EvalHint::ExprTypeChecked;
 use rustc::middle::const_eval::{eval_const_expr_partial, ConstVal};
 use rustc::middle::ty::TyArray;
 use rustc_front::hir::*;
-use utils::span_lint;
+use syntax::ast::RangeLimits;
+use utils;
 
 /// **What it does:** Check for out of bounds array indexing with a constant index.
 ///
@@ -17,6 +18,7 @@ use utils::span_lint;
 /// let x = [1,2,3,4];
 /// ...
 /// x[9];
+/// &x[2..9];
 /// ```
 declare_lint! {
     pub OUT_OF_BOUNDS_INDEXING,
@@ -24,28 +26,122 @@ declare_lint! {
     "out of bound constant indexing"
 }
 
+/// **What it does:** Check for usage of indexing or slicing.
+///
+/// **Why is this bad?** Usually, this can be safely allowed. However,
+/// in some domains such as kernel development, a panic can cause the
+/// whole operating system to crash.
+///
+/// **Known problems:** Hopefully none.
+///
+/// **Example:**
+///
+/// ```
+/// ...
+/// x[2];
+/// &x[0..2];
+/// ```
+declare_lint! {
+    pub INDEXING_SLICING,
+    Allow,
+    "indexing/slicing usage"
+}
+
 #[derive(Copy,Clone)]
 pub struct ArrayIndexing;
 
 impl LintPass for ArrayIndexing {
     fn get_lints(&self) -> LintArray {
-        lint_array!(OUT_OF_BOUNDS_INDEXING)
+        lint_array!(INDEXING_SLICING, OUT_OF_BOUNDS_INDEXING)
     }
 }
 
 impl LateLintPass for ArrayIndexing {
     fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
         if let ExprIndex(ref array, ref index) = e.node {
+            // Array with known size can be checked statically
             let ty = cx.tcx.expr_ty(array);
-
             if let TyArray(_, size) = ty.sty {
-                let index = eval_const_expr_partial(cx.tcx, &index, ExprTypeChecked, None);
-                if let Ok(ConstVal::Uint(index)) = index {
-                    if size as u64 <= index {
-                        span_lint(cx, OUT_OF_BOUNDS_INDEXING, e.span, "const index-expr is out of bounds");
+                let size = size as u64;
+
+                // Index is a constant uint
+                let const_index = eval_const_expr_partial(cx.tcx, &index, ExprTypeChecked, None);
+                if let Ok(ConstVal::Uint(const_index)) = const_index {
+                    if size <= const_index {
+                        utils::span_lint(cx, OUT_OF_BOUNDS_INDEXING, e.span, "const index is out of bounds");
+                        utils::span_lint(cx, INDEXING_SLICING, e.span, "indexing may panic");
+                    } else {
+                        // Index is within bounds
+                        return;
                     }
                 }
+
+                // Index is a constant range
+                if let Some(range) = utils::unsugar_range(index) {
+                    let start = range.start.map(|start|
+                        eval_const_expr_partial(cx.tcx, start, ExprTypeChecked, None));
+                    let end = range.end.map(|end|
+                        eval_const_expr_partial(cx.tcx, end, ExprTypeChecked, None));
+
+                    if let Some((start, end)) = to_const_range(start, end, range.limits, size) {
+                        if start >= size && end >= size {
+                            utils::span_lint(cx,
+                                             OUT_OF_BOUNDS_INDEXING,
+                                             e.span,
+                                             "range is out of bounds");
+                            utils::span_lint(cx,
+                                             INDEXING_SLICING,
+                                             e.span,
+                                             "slicing may panic");
+                        } else {
+                            // Range is within bounds
+                            return;
+                        }
+                    }
+                }
+            }
+
+            if let Some(range) = utils::unsugar_range(index) {
+                // Full ranges are always valid
+                if range.start.is_none() && range.end.is_none() {
+                    return;
+                }
+
+                // Impossible to know if indexing or slicing is correct
+                utils::span_lint(cx, INDEXING_SLICING, e.span, "slicing may panic");
+            } else {
+                utils::span_lint(cx, INDEXING_SLICING, e.span, "indexing may panic");
             }
         }
     }
 }
+
+/// Returns an option containing a tuple with the start and end (exclusive) of the range
+///
+/// Note: we assume the start and the end of the range are unsigned, since array slicing
+/// works only on usize
+fn to_const_range<T>(start: Option<Result<ConstVal, T>>,
+                    end: Option<Result<ConstVal, T>>,
+                    limits: RangeLimits,
+                    array_size: u64)
+                    -> Option<(u64, u64)> {
+    let start = match start {
+        Some(Ok(ConstVal::Uint(x))) => x,
+        Some(_) => return None,
+        None => 0,
+    };
+
+    let end = match end {
+        Some(Ok(ConstVal::Uint(x))) => {
+            if limits == RangeLimits::Closed {
+                x
+            } else {
+                x - 1
+            }
+        }
+        Some(_) => return None,
+        None => array_size - 1,
+    };
+
+    Some((start, end))
+}