uninhabited_reference: new lint

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

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,

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`
 }
 

clippy_lints/src/uninhabited_reference.rs

@@ -0,0 +1,65 @@
+use clippy_utils::diagnostics::span_lint;
+use rustc_hir::intravisit::FnKind;
+use rustc_hir::{Body, FnDecl, FnRetTy, TyKind};
+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 `!`.
+    ///
+    /// ### Why is this bad?
+    /// Instances of uninhabited types cannot be created. Creating
+    /// a reference on such a type (which can be done in `unsafe` code)
+    /// would create a risk of dereferencing it, which would be
+    /// undefined behavior.
+    ///
+    /// ### Example
+    /// The following function could dereference its parameter `x` without
+    /// needing `unsafe`. This would be undefined behavior as no instance of
+    /// type `std::convert::Infallible` can ever exist.
+    /// ```no_run
+    /// fn f(x: &std::convert::Infallible) { todo!() }
+    /// ```
+    #[clippy::version = "1.76.0"]
+    pub UNINHABITED_REFERENCE,
+    suspicious,
+    "reference on uninhabited type"
+}
+
+declare_lint_pass!(UninhabitedReference => [UNINHABITED_REFERENCE]);
+
+// This lint checks function parameters and return: this is where references to uninhabited types
+// are most susceptible to be exchanged between safe and unsafe parts of the program.
+impl LateLintPass<'_> for UninhabitedReference {
+    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;
+        }
+        let ret_ty = match fndecl.output {
+            FnRetTy::Return(ty) => Some(ty),
+            FnRetTy::DefaultReturn(_) => None,
+        };
+        for hir_ty in fndecl.inputs.iter().chain(ret_ty) {
+            if 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",
+                );
+            }
+        }
+    }
+}

tests/ui/error_impl_error.rs

@@ -1,4 +1,4 @@
-#![allow(unused)]
+#![allow(unused, clippy::uninhabited_reference)]
 #![warn(clippy::error_impl_error)]
 #![no_main]
 

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),

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),

tests/ui/uninhabited_reference.rs

@@ -0,0 +1,31 @@
+#![warn(clippy::uninhabited_reference)]
+#![allow(unused, clippy::diverging_sub_expression, clippy::trivially_copy_pass_by_ref)]
+#![feature(never_type)]
+
+fn f1(x: &!) {
+    todo!()
+}
+
+fn f2(x: &mut !) {
+    todo!()
+}
+
+fn g(x: &std::convert::Infallible) {
+    todo!()
+}
+
+struct S {
+    a: std::convert::Infallible,
+    b: u32,
+}
+
+fn h(x: &S) {
+    todo!()
+}
+
+fn i() -> &'static ! {
+    let x: &! = panic!();
+    x
+}
+
+fn main() {}

tests/ui/uninhabited_reference.stderr

@@ -0,0 +1,35 @@
+error: dereferencing a reference to an uninhabited type would be undefined behavior
+  --> $DIR/uninhabited_reference.rs:5:10
+   |
+LL | fn f1(x: &!) {
+   |          ^^
+   |
+   = 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 would be undefined behavior
+  --> $DIR/uninhabited_reference.rs:9:10
+   |
+LL | fn f2(x: &mut !) {
+   |          ^^^^^^
+
+error: dereferencing a reference to an uninhabited type would be undefined behavior
+  --> $DIR/uninhabited_reference.rs:13:9
+   |
+LL | fn g(x: &std::convert::Infallible) {
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: dereferencing a reference to an uninhabited type would be undefined behavior
+  --> $DIR/uninhabited_reference.rs:22:9
+   |
+LL | fn h(x: &S) {
+   |         ^^
+
+error: dereferencing a reference to an uninhabited type would be undefined behavior
+  --> $DIR/uninhabited_reference.rs:26:11
+   |
+LL | fn i() -> &'static ! {
+   |           ^^^^^^^^^^
+
+error: aborting due to 5 previous errors
+