new lint: mem-replace-with-uninit

8 files changed, 171 insertions, 33 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index c39b2e69df9..6c9d2020a82 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1050,6 +1050,7 @@ Released 2018-09-13
 [`mem_discriminant_non_enum`]: https://rust-lang.github.io/rust-clippy/master/index.html#mem_discriminant_non_enum
 [`mem_forget`]: https://rust-lang.github.io/rust-clippy/master/index.html#mem_forget
 [`mem_replace_option_with_none`]: https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_option_with_none
+[`mem_replace_with_uninit`]: https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_uninit
 [`min_max`]: https://rust-lang.github.io/rust-clippy/master/index.html#min_max
 [`misaligned_transmute`]: https://rust-lang.github.io/rust-clippy/master/index.html#misaligned_transmute
 [`misrefactored_assign_op`]: https://rust-lang.github.io/rust-clippy/master/index.html#misrefactored_assign_op
diff --git a/README.md b/README.md
index 4541af9c844..a8e6efc4ba4 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 314 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 315 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 24b8b14f717..79cbb916a5d 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -783,6 +783,7 @@ pub fn register_plugins(reg: &mut rustc_driver::plugin::Registry<'_>, conf: &Con
         matches::SINGLE_MATCH,
         mem_discriminant::MEM_DISCRIMINANT_NON_ENUM,
         mem_replace::MEM_REPLACE_OPTION_WITH_NONE,
+        mem_replace::MEM_REPLACE_WITH_UNINIT,
         methods::CHARS_LAST_CMP,
         methods::CHARS_NEXT_CMP,
         methods::CLONE_DOUBLE_REF,
@@ -1117,6 +1118,7 @@ pub fn register_plugins(reg: &mut rustc_driver::plugin::Registry<'_>, conf: &Con
         loops::REVERSE_RANGE_LOOP,
         loops::WHILE_IMMUTABLE_CONDITION,
         mem_discriminant::MEM_DISCRIMINANT_NON_ENUM,
+        mem_replace::MEM_REPLACE_WITH_UNINIT,
         methods::CLONE_DOUBLE_REF,
         methods::INTO_ITER_ON_ARRAY,
         methods::TEMPORARY_CSTRING_AS_PTR,
diff --git a/clippy_lints/src/mem_replace.rs b/clippy_lints/src/mem_replace.rs
index 7e83e836b85..3e1155806b9 100644
--- a/clippy_lints/src/mem_replace.rs
+++ b/clippy_lints/src/mem_replace.rs
@@ -1,4 +1,6 @@
-use crate::utils::{match_def_path, match_qpath, paths, snippet_with_applicability, span_lint_and_sugg};
+use crate::utils::{
+    match_def_path, match_qpath, paths, snippet_with_applicability, span_help_and_lint, span_lint_and_sugg,
+};
 use if_chain::if_chain;
 use rustc::hir::{Expr, ExprKind, MutMutable, QPath};
 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
@@ -32,7 +34,40 @@ declare_clippy_lint! {
     "replacing an `Option` with `None` instead of `take()`"
 }
 
-declare_lint_pass!(MemReplace => [MEM_REPLACE_OPTION_WITH_NONE]);
+declare_clippy_lint! {
+    /// **What it does:** Checks for `mem::replace(&mut _, mem::uninitialized())`
+    /// and `mem::replace(&mut _, mem::zeroed())`.
+    ///
+    /// **Why is this bad?** This will lead to undefined behavior even if the
+    /// value is overwritten later, because the uninitialized value may be
+    /// observed in the case of a panic.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```
+    /// use std::mem;
+    ///# fn may_panic(v: Vec<i32>) -> Vec<i32> { v }
+    ///
+    /// #[allow(deprecated, invalid_value)]
+    /// fn myfunc (v: &mut Vec<i32>) {
+    ///     let taken_v = unsafe { mem::replace(v, mem::uninitialized()) };
+    ///     let new_v = may_panic(taken_v); // undefined behavior on panic
+    ///     mem::forget(mem::replace(v, new_v));
+    /// }
+    /// ```
+    ///
+    /// The [take_mut](https://docs.rs/take_mut) crate offers a sound solution,
+    /// at the cost of either lazily creating a replacement value or aborting
+    /// on panic, to ensure that the uninitialized value cannot be observed.
+    pub MEM_REPLACE_WITH_UNINIT,
+    correctness,
+    "`mem::replace(&mut _, mem::uninitialized())` or `mem::replace(&mut _, mem::zeroed())`"
+}
+
+declare_lint_pass!(MemReplace =>
+    [MEM_REPLACE_OPTION_WITH_NONE, MEM_REPLACE_WITH_UNINIT]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemReplace {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
@@ -45,37 +80,66 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemReplace {
             if match_def_path(cx, def_id, &paths::MEM_REPLACE);
 
             // Check that second argument is `Option::None`
-            if let ExprKind::Path(ref replacement_qpath) = func_args[1].node;
-            if match_qpath(replacement_qpath, &paths::OPTION_NONE);
-
             then {
-                // Since this is a late pass (already type-checked),
-                // and we already know that the second argument is an
-                // `Option`, we do not need to check the first
-                // argument's type. All that's left is to get
-                // replacee's path.
-                let replaced_path = match func_args[0].node {
-                    ExprKind::AddrOf(MutMutable, ref replaced) => {
-                        if let ExprKind::Path(QPath::Resolved(None, ref replaced_path)) = replaced.node {
-                            replaced_path
-                        } else {
-                            return
-                        }
-                    },
-                    ExprKind::Path(QPath::Resolved(None, ref replaced_path)) => replaced_path,
-                    _ => return,
-                };
+                if let ExprKind::Path(ref replacement_qpath) = func_args[1].node {
+                    if match_qpath(replacement_qpath, &paths::OPTION_NONE) {
 
-                let mut applicability = Applicability::MachineApplicable;
-                span_lint_and_sugg(
-                    cx,
-                    MEM_REPLACE_OPTION_WITH_NONE,
-                    expr.span,
-                    "replacing an `Option` with `None`",
-                    "consider `Option::take()` instead",
-                    format!("{}.take()", snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)),
-                    applicability,
-                );
+                        // Since this is a late pass (already type-checked),
+                        // and we already know that the second argument is an
+                        // `Option`, we do not need to check the first
+                        // argument's type. All that's left is to get
+                        // replacee's path.
+                        let replaced_path = match func_args[0].node {
+                            ExprKind::AddrOf(MutMutable, ref replaced) => {
+                                if let ExprKind::Path(QPath::Resolved(None, ref replaced_path)) = replaced.node {
+                                    replaced_path
+                                } else {
+                                    return
+                                }
+                            },
+                            ExprKind::Path(QPath::Resolved(None, ref replaced_path)) => replaced_path,
+                            _ => return,
+                        };
+
+                        let mut applicability = Applicability::MachineApplicable;
+                        span_lint_and_sugg(
+                            cx,
+                            MEM_REPLACE_OPTION_WITH_NONE,
+                            expr.span,
+                            "replacing an `Option` with `None`",
+                            "consider `Option::take()` instead",
+                            format!("{}.take()", snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)),
+                            applicability,
+                        );
+                    }
+                }
+                if let ExprKind::Call(ref repl_func, ref repl_args) = func_args[1].node {
+                    if_chain! {
+                        if repl_args.is_empty();
+                        if let ExprKind::Path(ref repl_func_qpath) = repl_func.node;
+                        if let Some(repl_def_id) = cx.tables.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
+                        then {
+                            if match_def_path(cx, repl_def_id, &paths::MEM_UNINITIALIZED) {
+                                span_help_and_lint(
+                                    cx,
+                                    MEM_REPLACE_WITH_UNINIT,
+                                    expr.span,
+                                    "replacing with `mem::uninitialized()`",
+                                    "consider using the `take_mut` crate instead",
+                                );
+                            } else if match_def_path(cx, repl_def_id, &paths::MEM_ZEROED) &&
+                                    !cx.tables.expr_ty(&func_args[1]).is_primitive() {
+                                span_help_and_lint(
+                                    cx,
+                                    MEM_REPLACE_WITH_UNINIT,
+                                    expr.span,
+                                    "replacing with `mem::zeroed()`",
+                                    "consider using a default value or the `take_mut` crate instead",
+                                );
+                            }
+                        }
+                    }
+                }
             }
         }
     }
diff --git a/clippy_lints/src/utils/paths.rs b/clippy_lints/src/utils/paths.rs
index 9b88a0d3089..cda3d2024d2 100644
--- a/clippy_lints/src/utils/paths.rs
+++ b/clippy_lints/src/utils/paths.rs
@@ -52,6 +52,8 @@ pub const MEM_FORGET: [&str; 3] = ["core", "mem", "forget"];
 pub const MEM_MAYBEUNINIT: [&str; 4] = ["core", "mem", "maybe_uninit", "MaybeUninit"];
 pub const MEM_MAYBEUNINIT_UNINIT: [&str; 5] = ["core", "mem", "maybe_uninit", "MaybeUninit", "uninit"];
 pub const MEM_REPLACE: [&str; 3] = ["core", "mem", "replace"];
+pub const MEM_UNINITIALIZED: [&str; 3] = ["core", "mem", "uninitialized"];
+pub const MEM_ZEROED: [&str; 3] = ["core", "mem", "zeroed"];
 pub const MUTEX: [&str; 4] = ["std", "sync", "mutex", "Mutex"];
 pub const OPEN_OPTIONS: [&str; 3] = ["std", "fs", "OpenOptions"];
 pub const OPS_MODULE: [&str; 2] = ["core", "ops"];
diff --git a/src/lintlist/mod.rs b/src/lintlist/mod.rs
index f430e46ae67..49b7731865c 100644
--- a/src/lintlist/mod.rs
+++ b/src/lintlist/mod.rs
@@ -6,7 +6,7 @@ pub use lint::Lint;
 pub use lint::LINT_LEVELS;
 
 // begin lint list, do not remove this comment, it’s used in `update_lints`
-pub const ALL_LINTS: [Lint; 314] = [
+pub const ALL_LINTS: [Lint; 315] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -1044,6 +1044,13 @@ pub const ALL_LINTS: [Lint; 314] = [
         module: "mem_replace",
     },
     Lint {
+        name: "mem_replace_with_uninit",
+        group: "correctness",
+        desc: "`mem::replace(&mut _, mem::uninitialized())` or `mem::replace(&mut _, mem::zeroed())`",
+        deprecation: None,
+        module: "mem_replace",
+    },
+    Lint {
         name: "min_max",
         group: "correctness",
         desc: "`min(_, max(_, _))` (or vice versa) with bounds clamping the result to a constant",
diff --git a/tests/ui/repl_uninit.rs b/tests/ui/repl_uninit.rs
new file mode 100644
index 00000000000..346972b7bb4
--- /dev/null
+++ b/tests/ui/repl_uninit.rs
@@ -0,0 +1,35 @@
+#![allow(deprecated, invalid_value)]
+#![warn(clippy::all)]
+
+use std::mem;
+
+fn might_panic<X>(x: X) -> X {
+    // in practice this would be a possibly-panicky operation
+    x
+}
+
+fn main() {
+    let mut v = vec![0i32; 4];
+    // the following is UB if `might_panic` panics
+    unsafe {
+        let taken_v = mem::replace(&mut v, mem::uninitialized());
+        let new_v = might_panic(taken_v);
+        std::mem::forget(mem::replace(&mut v, new_v));
+    }
+
+    unsafe {
+        let taken_v = mem::replace(&mut v, mem::zeroed());
+        let new_v = might_panic(taken_v);
+        std::mem::forget(mem::replace(&mut v, new_v));
+    }
+
+    // this is silly but OK, because usize is a primitive type
+    let mut u: usize = 42;
+    let uref = &mut u;
+    let taken_u = unsafe { mem::replace(uref, mem::zeroed()) };
+    *uref = taken_u + 1;
+
+    // this is still not OK, because uninit
+    let taken_u = unsafe { mem::replace(uref, mem::uninitialized()) };
+    *uref = taken_u + 1;
+}
diff --git a/tests/ui/repl_uninit.stderr b/tests/ui/repl_uninit.stderr
new file mode 100644
index 00000000000..c1f55d7601e
--- /dev/null
+++ b/tests/ui/repl_uninit.stderr
@@ -0,0 +1,27 @@
+error: replacing with `mem::uninitialized()`
+  --> $DIR/repl_uninit.rs:15:23
+   |
+LL |         let taken_v = mem::replace(&mut v, mem::uninitialized());
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::mem-replace-with-uninit` implied by `-D warnings`
+   = help: consider using the `take_mut` crate instead
+
+error: replacing with `mem::zeroed()`
+  --> $DIR/repl_uninit.rs:21:23
+   |
+LL |         let taken_v = mem::replace(&mut v, mem::zeroed());
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using a default value or the `take_mut` crate instead
+
+error: replacing with `mem::uninitialized()`
+  --> $DIR/repl_uninit.rs:33:28
+   |
+LL |     let taken_u = unsafe { mem::replace(uref, mem::uninitialized()) };
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using the `take_mut` crate instead
+
+error: aborting due to 3 previous errors
+