uninhabited_reference: new lint

Author: samueltardieu

Diff for: CHANGELOG.md

@@ -5581,6 +5581,7 @@ Released 2018-09-13
 [`undropped_manually_drops`]: https://rust-lang.github.io/rust-clippy/master/index.html#undropped_manually_drops
 [`unicode_not_nfc`]: https://rust-lang.github.io/rust-clippy/master/index.html#unicode_not_nfc
 [`unimplemented`]: https://rust-lang.github.io/rust-clippy/master/index.html#unimplemented
+[`uninhabited_reference`]: https://rust-lang.github.io/rust-clippy/master/index.html#uninhabited_reference
 [`uninit_assumed_init`]: https://rust-lang.github.io/rust-clippy/master/index.html#uninit_assumed_init
 [`uninit_vec`]: https://rust-lang.github.io/rust-clippy/master/index.html#uninit_vec
 [`uninlined_format_args`]: https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args

Diff for: clippy_lints/src/declared_lints.rs

@@ -677,6 +677,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::unicode::INVISIBLE_CHARACTERS_INFO,
     crate::unicode::NON_ASCII_LITERAL_INFO,
     crate::unicode::UNICODE_NOT_NFC_INFO,
+    crate::uninhabited_reference::UNINHABITED_REFERENCE_INFO,
     crate::uninit_vec::UNINIT_VEC_INFO,
     crate::unit_return_expecting_ord::UNIT_RETURN_EXPECTING_ORD_INFO,
     crate::unit_types::LET_UNIT_VALUE_INFO,

Diff for: clippy_lints/src/lib.rs

@@ -325,6 +325,7 @@ mod tuple_array_conversions;
 mod types;
 mod undocumented_unsafe_blocks;
 mod unicode;
+mod uninhabited_reference;
 mod uninit_vec;
 mod unit_return_expecting_ord;
 mod unit_types;
@@ -1069,6 +1070,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(iter_without_into_iter::IterWithoutIntoIter));
     store.register_late_pass(|_| Box::new(iter_over_hash_type::IterOverHashType));
     store.register_late_pass(|_| Box::new(impl_hash_with_borrow_str_and_bytes::ImplHashWithBorrowStrBytes));
+    store.register_late_pass(|_| Box::new(uninhabited_reference::UninhabitedReference));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

Diff for: clippy_lints/src/uninhabited_reference.rs

@@ -0,0 +1,81 @@
+use clippy_utils::diagnostics::span_lint;
+use rustc_hir::intravisit::FnKind;
+use rustc_hir::{Body, ExprKind, FnDecl, FnRetTy, TyKind, UnOp};
+use rustc_hir_analysis::hir_ty_to_ty;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// It detects references to uninhabited types, such as `!` and
+    /// warns when those are either dereferenced or returned from a function.
+    ///
+    /// ### Why is this bad?
+    /// Dereferencing a reference to an uninhabited type would create
+    /// an instance of such a type, which cannot exist. This constitutes
+    /// undefined behaviour. Such a reference could have been created
+    /// by `unsafe` code.
+    ///
+    /// ### Example
+    /// The following function can return a reference to an uninhabited type
+    /// (`Infallible`) because it uses `unsafe` code to create it. However,
+    /// the user of such a function could dereference the return value and
+    /// trigger an undefined behaviour from safe code.
+    ///
+    /// ```no_run
+    /// fn create_ref() -> std::convert::Infallible {
+    ///     unsafe { std::mem::transmute(&()) }
+    /// }
+    /// ```
+    #[clippy::version = "1.76.0"]
+    pub UNINHABITED_REFERENCE,
+    suspicious,
+    "reference to uninhabited type"
+}
+
+declare_lint_pass!(UninhabitedReference => [UNINHABITED_REFERENCE]);
+
+impl LateLintPass<'_> for UninhabitedReference {
+    fn check_expr_post(&mut self, cx: &LateContext<'_>, expr: &'_ rustc_hir::Expr<'_>) {
+        if expr.span.from_expansion() {
+            return;
+        }
+
+        if let ExprKind::Unary(UnOp::Deref, _) = expr.kind {
+            let ty = cx.typeck_results().expr_ty_adjusted(expr);
+            if ty.is_privately_uninhabited(cx.tcx, cx.param_env) {
+                span_lint(
+                    cx,
+                    UNINHABITED_REFERENCE,
+                    expr.span,
+                    "dereferencing a reference to an uninhabited type is undefined behavior",
+                );
+            }
+        }
+    }
+
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'_>,
+        kind: FnKind<'_>,
+        fndecl: &'_ FnDecl<'_>,
+        _: &'_ Body<'_>,
+        span: rustc_span::Span,
+        _: rustc_span::def_id::LocalDefId,
+    ) {
+        if span.from_expansion() || matches!(kind, FnKind::Closure) {
+            return;
+        }
+        if let FnRetTy::Return(hir_ty) = fndecl.output
+            && let TyKind::Ref(_, mut_ty) = hir_ty.kind
+            && hir_ty_to_ty(cx.tcx, mut_ty.ty).is_privately_uninhabited(cx.tcx, cx.param_env)
+        {
+            span_lint(
+                cx,
+                UNINHABITED_REFERENCE,
+                hir_ty.span,
+                "dereferencing a reference to an uninhabited type would be undefined behavior",
+            );
+        }
+    }
+}

Diff for: tests/ui/infallible_destructuring_match.fixed

@@ -1,6 +1,6 @@
 #![feature(exhaustive_patterns, never_type)]
 #![allow(dead_code, unreachable_code, unused_variables)]
-#![allow(clippy::let_and_return)]
+#![allow(clippy::let_and_return, clippy::uninhabited_reference)]
 
 enum SingleVariantEnum {
     Variant(i32),

Diff for: tests/ui/infallible_destructuring_match.rs

@@ -1,6 +1,6 @@
 #![feature(exhaustive_patterns, never_type)]
 #![allow(dead_code, unreachable_code, unused_variables)]
-#![allow(clippy::let_and_return)]
+#![allow(clippy::let_and_return, clippy::uninhabited_reference)]
 
 enum SingleVariantEnum {
     Variant(i32),

Diff for: tests/ui/uninhabited_reference.rs

@@ -0,0 +1,11 @@
+#![warn(clippy::uninhabited_reference)]
+#![feature(never_type)]
+
+fn ret_uninh_ref() -> &'static std::convert::Infallible {
+    unsafe { std::mem::transmute(&()) }
+}
+
+fn main() {
+    let x = ret_uninh_ref();
+    let _ = *x;
+}

Diff for: tests/ui/uninhabited_reference.stderr

@@ -0,0 +1,17 @@
+error: dereferencing a reference to an uninhabited type would be undefined behavior
+  --> $DIR/uninhabited_reference.rs:4:23
+   |
+LL | fn ret_uninh_ref() -> &'static std::convert::Infallible {
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::uninhabited-reference` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::uninhabited_reference)]`
+
+error: dereferencing a reference to an uninhabited type is undefined behavior
+  --> $DIR/uninhabited_reference.rs:10:13
+   |
+LL |     let _ = *x;
+   |             ^^
+
+error: aborting due to 2 previous errors
+