Auto merge of #11878 - samueltardieu:uninhabited_reference, r=flip1995

uninhabited_reference: new lint

Close #11851

The lint is implemented on function parameters and return types, as this is the place where the risk of exchanging references to uninhabited types is the highest. Other constructs, such as in a local variable,
would require the use of `unsafe` and will clearly be done on purpose.

changelog: [`uninhabited_reference`]: new lint

CHANGELOG.md

@@ -5584,6 +5584,7 @@ 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_references`]: https://rust-lang.github.io/rust-clippy/master/index.html#uninhabited_references
 [`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

@@ -679,6 +679,7 @@
     crate::unicode::INVISIBLE_CHARACTERS_INFO,
     crate::unicode::NON_ASCII_LITERAL_INFO,
     crate::unicode::UNICODE_NOT_NFC_INFO,
+    crate::uninhabited_references::UNINHABITED_REFERENCES_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

@@ -326,6 +326,7 @@
 mod types;
 mod undocumented_unsafe_blocks;
 mod unicode;
+mod uninhabited_references;
 mod uninit_vec;
 mod unit_return_expecting_ord;
 mod unit_types;
@@ -1071,6 +1072,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     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(repeat_vec_with_capacity::RepeatVecWithCapacity));
+    store.register_late_pass(|_| Box::new(uninhabited_references::UninhabitedReferences));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/uninhabited_references.rs

@@ -0,0 +1,84 @@
+use clippy_utils::diagnostics::span_lint;
+use rustc_hir::intravisit::FnKind;
+use rustc_hir::{Body, Expr, ExprKind, FnDecl, FnRetTy, TyKind, UnOp};
+use rustc_hir_analysis::hir_ty_to_ty;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::lint::in_external_macro;
+use rustc_session::declare_lint_pass;
+use rustc_span::def_id::LocalDefId;
+use rustc_span::Span;
+
+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 behavior from safe code.
+    ///
+    /// ```no_run
+    /// fn create_ref() -> &'static std::convert::Infallible {
+    ///     unsafe { std::mem::transmute(&()) }
+    /// }
+    /// ```
+    #[clippy::version = "1.76.0"]
+    pub UNINHABITED_REFERENCES,
+    suspicious,
+    "reference to uninhabited type"
+}
+
+declare_lint_pass!(UninhabitedReferences => [UNINHABITED_REFERENCES]);
+
+impl LateLintPass<'_> for UninhabitedReferences {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &'_ Expr<'_>) {
+        if in_external_macro(cx.tcx.sess, expr.span) {
+            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_REFERENCES,
+                    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: Span,
+        _: LocalDefId,
+    ) {
+        if in_external_macro(cx.tcx.sess, span) || 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_REFERENCES,
+                hir_ty.span,
+                "dereferencing a reference to an uninhabited type would be undefined behavior",
+            );
+        }
+    }
+}

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_references)]
 
 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_references)]
 
 enum SingleVariantEnum {
     Variant(i32),

tests/ui/uninhabited_references.rs

@@ -0,0 +1,22 @@
+#![warn(clippy::uninhabited_references)]
+#![feature(never_type)]
+
+fn ret_uninh_ref() -> &'static std::convert::Infallible {
+    unsafe { std::mem::transmute(&()) }
+}
+
+macro_rules! ret_something {
+    ($name:ident, $ty:ty) => {
+        fn $name(x: &$ty) -> &$ty {
+            &*x
+        }
+    };
+}
+
+ret_something!(id_u32, u32);
+ret_something!(id_never, !);
+
+fn main() {
+    let x = ret_uninh_ref();
+    let _ = *x;
+}

tests/ui/uninhabited_references.stderr

@@ -0,0 +1,39 @@
+error: dereferencing a reference to an uninhabited type would be undefined behavior
+  --> $DIR/uninhabited_references.rs:4:23
+   |
+LL | fn ret_uninh_ref() -> &'static std::convert::Infallible {
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::uninhabited-references` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::uninhabited_references)]`
+
+error: dereferencing a reference to an uninhabited type would be undefined behavior
+  --> $DIR/uninhabited_references.rs:10:30
+   |
+LL |         fn $name(x: &$ty) -> &$ty {
+   |                              ^^^^
+...
+LL | ret_something!(id_never, !);
+   | --------------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `ret_something` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: dereferencing a reference to an uninhabited type is undefined behavior
+  --> $DIR/uninhabited_references.rs:11:14
+   |
+LL |             &*x
+   |              ^^
+...
+LL | ret_something!(id_never, !);
+   | --------------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `ret_something` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: dereferencing a reference to an uninhabited type is undefined behavior
+  --> $DIR/uninhabited_references.rs:21:13
+   |
+LL |     let _ = *x;
+   |             ^^
+
+error: aborting due to 4 previous errors
+