Add missing_transmute_annotation lint

CHANGELOG.md

@@ -5426,6 +5426,7 @@ Released 2018-09-13
 [`missing_safety_doc`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc
 [`missing_spin_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_spin_loop
 [`missing_trait_methods`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_trait_methods
+[`missing_transmute_annotations`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_transmute_annotations
 [`mistyped_literal_suffixes`]: https://rust-lang.github.io/rust-clippy/master/index.html#mistyped_literal_suffixes
 [`mixed_attributes_style`]: https://rust-lang.github.io/rust-clippy/master/index.html#mixed_attributes_style
 [`mixed_case_hex_literals`]: https://rust-lang.github.io/rust-clippy/master/index.html#mixed_case_hex_literals

clippy_lints/src/declared_lints.rs

@@ -677,6 +677,7 @@
     crate::trait_bounds::TYPE_REPETITION_IN_BOUNDS_INFO,
     crate::transmute::CROSSPOINTER_TRANSMUTE_INFO,
     crate::transmute::EAGER_TRANSMUTE_INFO,
+    crate::transmute::MISSING_TRANSMUTE_ANNOTATIONS_INFO,
     crate::transmute::TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS_INFO,
     crate::transmute::TRANSMUTE_BYTES_TO_STR_INFO,
     crate::transmute::TRANSMUTE_FLOAT_TO_INT_INFO,

clippy_lints/src/transmute/missing_transmute_annotations.rs

@@ -0,0 +1,53 @@
+use rustc_errors::Applicability;
+use rustc_hir::{GenericArg, HirId, Node, Path, TyKind};
+use rustc_lint::LateContext;
+use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty::Ty;
+
+use clippy_utils::diagnostics::span_lint_and_sugg;
+
+use crate::transmute::MISSING_TRANSMUTE_ANNOTATIONS;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    path: &Path<'tcx>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+    expr_hir_id: HirId,
+) -> bool {
+    let last = path.segments.last().unwrap();
+    if in_external_macro(cx.tcx.sess, last.ident.span) {
+        // If it comes from a non-local macro, we ignore it.
+        return false;
+    }
+    let args = last.args;
+    let missing_generic = match args {
+        Some(args) if !args.args.is_empty() => args.args.iter().any(|arg| match arg {
+            GenericArg::Infer(_) => true,
+            GenericArg::Type(ty) => matches!(ty.kind, TyKind::Infer),
+            _ => false,
+        }),
+        _ => true,
+    };
+    if !missing_generic {
+        return false;
+    }
+    // If it's being set as a local variable value...
+    if let Some((_, node)) = cx.tcx.hir().parent_iter(expr_hir_id).next()
+        && let Node::Local(local) = node
+        // ... which does have type annotations.
+        && local.ty.is_some()
+    {
+        return false;
+    }
+    span_lint_and_sugg(
+        cx,
+        MISSING_TRANSMUTE_ANNOTATIONS,
+        last.ident.span.with_hi(path.span.hi()),
+        "transmute used without annotations",
+        "consider adding missing annotations",
+        format!("{}::<{from_ty}, {to_ty}>", last.ident.as_str()),
+        Applicability::MaybeIncorrect,
+    );
+    true
+}

clippy_lints/src/transmute/mod.rs

@@ -1,5 +1,6 @@
 mod crosspointer_transmute;
 mod eager_transmute;
+mod missing_transmute_annotations;
 mod transmute_float_to_int;
 mod transmute_int_to_bool;
 mod transmute_int_to_char;
@@ -520,6 +521,37 @@
     "eager evaluation of `transmute`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks if transmute calls have all generics specified.
+    ///
+    /// ### Why is this bad?
+    /// If not set, some unexpected output type could be retrieved instead of the expected one,
+    /// potentially leading to invalid code.
+    ///
+    /// This is particularly dangerous in case a seemingly innocent/unrelated change can cause type
+    /// inference to start inferring a different type. E.g. the transmute is the tail expression of
+    /// an `if` branch, and a different branches type changes, causing the transmute to silently
+    /// have a different type, instead of a proper error.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// # unsafe {
+    /// let x: i32 = std::mem::transmute([1u16, 2u16]);
+    /// # }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// # unsafe {
+    /// let x = std::mem::transmute::<[u16; 2], i32>([1u16, 2u16]);
+    /// # }
+    /// ```
+    #[clippy::version = "1.77.0"]
+    pub MISSING_TRANSMUTE_ANNOTATIONS,
+    suspicious,
+    "warns if a transmute call doesn't have all generics specified"
+}
+
 pub struct Transmute {
     msrv: Msrv,
 }
@@ -542,6 +574,7 @@ pub struct Transmute {
     TRANSMUTING_NULL,
     TRANSMUTE_NULL_TO_FN,
     EAGER_TRANSMUTE,
+    MISSING_TRANSMUTE_ANNOTATIONS,
 ]);
 impl Transmute {
     #[must_use]
@@ -579,6 +612,7 @@ fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
                 | transmuting_null::check(cx, e, arg, to_ty)
                 | transmute_null_to_fn::check(cx, e, arg, to_ty)
                 | transmute_ptr_to_ref::check(cx, e, from_ty, to_ty, arg, path, &self.msrv)
+                | missing_transmute_annotations::check(cx, path, from_ty, to_ty, e.hir_id)
                 | transmute_int_to_char::check(cx, e, from_ty, to_ty, arg, const_context)
                 | transmute_ref_to_ref::check(cx, e, from_ty, to_ty, arg, const_context)
                 | transmute_ptr_to_ptr::check(cx, e, from_ty, to_ty, arg)