Add needless_type_cast lint

CHANGELOG.md

@@ -6688,6 +6688,7 @@ Released 2018-09-13
 [`needless_return`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_return
 [`needless_return_with_question_mark`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_return_with_question_mark
 [`needless_splitn`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_splitn
+[`needless_type_cast`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_type_cast
 [`needless_update`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_update
 [`neg_cmp_op_on_partial_ord`]: https://rust-lang.github.io/rust-clippy/master/index.html#neg_cmp_op_on_partial_ord
 [`neg_multiply`]: https://rust-lang.github.io/rust-clippy/master/index.html#neg_multiply

README.md

@@ -4,7 +4,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are over 750 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are over 800 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 Lints are divided into categories, each with a default [lint level](https://doc.rust-lang.org/rustc/lints/levels.html).
 You can choose how much Clippy is supposed to ~~annoy~~ help you by changing the lint level by category.

book/src/README.md

@@ -5,7 +5,7 @@
 A collection of lints to catch common mistakes and improve your
 [Rust](https://github.com/rust-lang/rust) code.
 
-[There are over 750 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are over 800 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 Lints are divided into categories, each with a default [lint
 level](https://doc.rust-lang.org/rustc/lints/levels.html). You can choose how

clippy_lints/src/casts/mod.rs

@@ -19,6 +19,7 @@ mod fn_to_numeric_cast;
 mod fn_to_numeric_cast_any;
 mod fn_to_numeric_cast_with_truncation;
 mod manual_dangling_ptr;
+mod needless_type_cast;
 mod ptr_as_ptr;
 mod ptr_cast_constness;
 mod ref_as_ptr;
@@ -813,6 +814,32 @@ declare_clippy_lint! {
     "casting a primitive method pointer to any integer type"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for bindings (constants, statics, or let bindings) that are defined
+    /// with one numeric type but are consistently cast to a different type in all usages.
+    ///
+    /// ### Why is this bad?
+    /// If a binding is always cast to a different type when used, it would be clearer
+    /// and more efficient to define it with the target type from the start.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// const SIZE: u16 = 15;
+    /// let arr: [u8; SIZE as usize] = [0; SIZE as usize];
+    /// ```
+    ///
+    /// Use instead:
+    /// ```no_run
+    /// const SIZE: usize = 15;
+    /// let arr: [u8; SIZE] = [0; SIZE];
+    /// ```
+    #[clippy::version = "1.93.0"]
+    pub NEEDLESS_TYPE_CAST,
+    pedantic,
+    "binding defined with one type but always cast to another"
+}
+
 pub struct Casts {
     msrv: Msrv,
 }
@@ -851,6 +878,7 @@ impl_lint_pass!(Casts => [
     AS_POINTER_UNDERSCORE,
     MANUAL_DANGLING_PTR,
     CONFUSING_METHOD_TO_NUMERIC_CAST,
+    NEEDLESS_TYPE_CAST,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Casts {
@@ -920,4 +948,8 @@ impl<'tcx> LateLintPass<'tcx> for Casts {
         cast_slice_different_sizes::check(cx, expr, self.msrv);
         ptr_cast_constness::check_null_ptr_cast_method(cx, expr);
     }
+
+    fn check_body(&mut self, cx: &LateContext<'tcx>, body: &rustc_hir::Body<'tcx>) {
+        needless_type_cast::check(cx, body);
+    }
 }

clippy_lints/src/casts/needless_type_cast.rs

@@ -0,0 +1,289 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::visitors::{Descend, for_each_expr, for_each_expr_without_closures};
+use core::ops::ControlFlow;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::Applicability;
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::{BlockCheckMode, Body, Expr, ExprKind, HirId, LetStmt, PatKind, StmtKind, UnsafeSource};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{Ty, TypeVisitableExt};
+use rustc_span::Span;
+
+use super::NEEDLESS_TYPE_CAST;
+
+struct BindingInfo<'a> {
+    source_ty: Ty<'a>,
+    ty_span: Span,
+}
+
+struct UsageInfo<'a> {
+    cast_to: Option<Ty<'a>>,
+    in_generic_context: bool,
+}
+
+pub(super) fn check<'a>(cx: &LateContext<'a>, body: &Body<'a>) {
+    let mut bindings: FxHashMap<HirId, BindingInfo<'a>> = FxHashMap::default();
+
+    for_each_expr_without_closures(body.value, |expr| {
+        match expr.kind {
+            ExprKind::Block(block, _) => {
+                for stmt in block.stmts {
+                    if let StmtKind::Let(let_stmt) = stmt.kind {
+                        collect_binding_from_local(cx, let_stmt, &mut bindings);
+                    }
+                }
+            },
+            ExprKind::Let(let_expr) => {
+                collect_binding_from_let(cx, let_expr, &mut bindings);
+            },
+            _ => {},
+        }
+        ControlFlow::<()>::Continue(())
+    });
+
+    #[allow(rustc::potential_query_instability)]
+    let mut binding_vec: Vec<_> = bindings.into_iter().collect();
+    binding_vec.sort_by_key(|(_, info)| info.ty_span.lo());
+
+    for (hir_id, binding_info) in binding_vec {
+        check_binding_usages(cx, body, hir_id, &binding_info);
+    }
+}
+
+fn collect_binding_from_let<'a>(
+    cx: &LateContext<'a>,
+    let_expr: &rustc_hir::LetExpr<'a>,
+    bindings: &mut FxHashMap<HirId, BindingInfo<'a>>,
+) {
+    if let_expr.ty.is_none()
+        || let_expr.span.from_expansion()
+        || has_generic_return_type(cx, let_expr.init)
+        || contains_unsafe(let_expr.init)
+    {
+        return;
+    }
+
+    if let PatKind::Binding(_, hir_id, _, _) = let_expr.pat.kind
+        && let Some(ty_hir) = let_expr.ty
+    {
+        let ty = cx.typeck_results().pat_ty(let_expr.pat);
+        if ty.is_numeric() {
+            bindings.insert(
+                hir_id,
+                BindingInfo {
+                    source_ty: ty,
+                    ty_span: ty_hir.span,
+                },
+            );
+        }
+    }
+}
+
+fn collect_binding_from_local<'a>(
+    cx: &LateContext<'a>,
+    let_stmt: &LetStmt<'a>,
+    bindings: &mut FxHashMap<HirId, BindingInfo<'a>>,
+) {
+    if let_stmt.ty.is_none()
+        || let_stmt.span.from_expansion()
+        || let_stmt
+            .init
+            .is_some_and(|init| has_generic_return_type(cx, init) || contains_unsafe(init))
+    {
+        return;
+    }
+
+    if let PatKind::Binding(_, hir_id, _, _) = let_stmt.pat.kind
+        && let Some(ty_hir) = let_stmt.ty
+    {
+        let ty = cx.typeck_results().pat_ty(let_stmt.pat);
+        if ty.is_numeric() {
+            bindings.insert(
+                hir_id,
+                BindingInfo {
+                    source_ty: ty,
+                    ty_span: ty_hir.span,
+                },
+            );
+        }
+    }
+}
+
+fn contains_unsafe(expr: &Expr<'_>) -> bool {
+    for_each_expr_without_closures(expr, |e| {
+        if let ExprKind::Block(block, _) = e.kind
+            && let BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) = block.rules
+        {
+            return ControlFlow::Break(());
+        }
+        ControlFlow::Continue(())
+    })
+    .is_some()
+}
+
+fn has_generic_return_type(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    match &expr.kind {
+        ExprKind::Block(block, _) => {
+            if let Some(tail_expr) = block.expr {
+                return has_generic_return_type(cx, tail_expr);
+            }
+            false
+        },
+        ExprKind::If(_, then_block, else_expr) => {
+            has_generic_return_type(cx, then_block) || else_expr.is_some_and(|e| has_generic_return_type(cx, e))
+        },
+        ExprKind::Match(_, arms, _) => arms.iter().any(|arm| has_generic_return_type(cx, arm.body)),
+        ExprKind::Loop(block, label, ..) => for_each_expr_without_closures(*block, |e| {
+            match e.kind {
+                ExprKind::Loop(..) => {
+                    // Unlabeled breaks inside nested loops target the inner loop, not ours
+                    return ControlFlow::Continue(Descend::No);
+                },
+                ExprKind::Break(dest, Some(break_expr)) => {
+                    let targets_this_loop =
+                        dest.label.is_none() || dest.label.map(|l| l.ident) == label.map(|l| l.ident);
+                    if targets_this_loop && has_generic_return_type(cx, break_expr) {
+                        return ControlFlow::Break(());
+                    }
+                },
+                _ => {},
+            }
+            ControlFlow::Continue(Descend::Yes)
+        })
+        .is_some(),
+        ExprKind::MethodCall(..) => {
+            if let Some(def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id) {
+                let sig = cx.tcx.fn_sig(def_id).instantiate_identity();
+                let ret_ty = sig.output().skip_binder();
+                return ret_ty.has_param();
+            }
+            false
+        },
+        ExprKind::Call(callee, _) => {
+            if let ExprKind::Path(qpath) = &callee.kind {
+                let res = cx.qpath_res(qpath, callee.hir_id);
+                if let Res::Def(DefKind::Fn | DefKind::AssocFn, def_id) = res {
+                    let sig = cx.tcx.fn_sig(def_id).instantiate_identity();
+                    let ret_ty = sig.output().skip_binder();
+                    return ret_ty.has_param();
+                }
+            }
+            false
+        },
+        _ => false,
+    }
+}
+
+fn is_generic_res(cx: &LateContext<'_>, res: Res) -> bool {
+    let has_type_params = |def_id| {
+        cx.tcx
+            .generics_of(def_id)
+            .own_params
+            .iter()
+            .any(|p| p.kind.is_ty_or_const())
+    };
+    match res {
+        Res::Def(DefKind::Fn | DefKind::AssocFn, def_id) => has_type_params(def_id),
+        // Ctor → Variant → ADT: constructor's parent is variant, variant's parent is the ADT
+        Res::Def(DefKind::Ctor(..), def_id) => has_type_params(cx.tcx.parent(cx.tcx.parent(def_id))),
+        _ => false,
+    }
+}
+
+fn is_cast_in_generic_context<'a>(cx: &LateContext<'a>, cast_expr: &Expr<'a>) -> bool {
+    let mut current_id = cast_expr.hir_id;
+
+    loop {
+        let parent_id = cx.tcx.parent_hir_id(current_id);
+        if parent_id == current_id {
+            return false;
+        }
+
+        let parent = cx.tcx.hir_node(parent_id);
+
+        match parent {
+            rustc_hir::Node::Expr(parent_expr) => {
+                match &parent_expr.kind {
+                    ExprKind::Closure(_) => return false,
+                    ExprKind::Call(callee, _) => {
+                        if let ExprKind::Path(qpath) = &callee.kind {
+                            let res = cx.qpath_res(qpath, callee.hir_id);
+                            if is_generic_res(cx, res) {
+                                return true;
+                            }
+                        }
+                    },
+                    ExprKind::MethodCall(..) => {
+                        if let Some(def_id) = cx.typeck_results().type_dependent_def_id(parent_expr.hir_id)
+                            && cx
+                                .tcx
+                                .generics_of(def_id)
+                                .own_params
+                                .iter()
+                                .any(|p| p.kind.is_ty_or_const())
+                        {
+                            return true;
+                        }
+                    },
+                    _ => {},
+                }
+                current_id = parent_id;
+            },
+            _ => return false,
+        }
+    }
+}
+
+fn check_binding_usages<'a>(cx: &LateContext<'a>, body: &Body<'a>, hir_id: HirId, binding_info: &BindingInfo<'a>) {
+    let mut usages = Vec::new();
+
+    for_each_expr(cx, body.value, |expr| {
+        if let ExprKind::Path(ref qpath) = expr.kind
+            && !expr.span.from_expansion()
+            && let Res::Local(id) = cx.qpath_res(qpath, expr.hir_id)
+            && id == hir_id
+        {
+            let parent_id = cx.tcx.parent_hir_id(expr.hir_id);
+            let parent = cx.tcx.hir_node(parent_id);
+
+            let usage = if let rustc_hir::Node::Expr(parent_expr) = parent
+                && let ExprKind::Cast(..) = parent_expr.kind
+                && !parent_expr.span.from_expansion()
+            {
+                UsageInfo {
+                    cast_to: Some(cx.typeck_results().expr_ty(parent_expr)),
+                    in_generic_context: is_cast_in_generic_context(cx, parent_expr),
+                }
+            } else {
+                UsageInfo {
+                    cast_to: None,
+                    in_generic_context: false,
+                }
+            };
+            usages.push(usage);
+        }
+        ControlFlow::<()>::Continue(())
+    });
+
+    let Some(first_target) = usages
+        .first()
+        .and_then(|u| u.cast_to)
+        .filter(|&t| t != binding_info.source_ty)
+        .filter(|&t| usages.iter().all(|u| u.cast_to == Some(t) && !u.in_generic_context))
+    else {
+        return;
+    };
+
+    span_lint_and_sugg(
+        cx,
+        NEEDLESS_TYPE_CAST,
+        binding_info.ty_span,
+        format!(
+            "this binding is defined as `{}` but is always cast to `{}`",
+            binding_info.source_ty, first_target
+        ),
+        "consider defining it as",
+        first_target.to_string(),
+        Applicability::MaybeIncorrect,
+    );
+}

clippy_lints/src/declared_lints.rs

@@ -70,6 +70,7 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::casts::FN_TO_NUMERIC_CAST_ANY_INFO,
     crate::casts::FN_TO_NUMERIC_CAST_WITH_TRUNCATION_INFO,
     crate::casts::MANUAL_DANGLING_PTR_INFO,
+    crate::casts::NEEDLESS_TYPE_CAST_INFO,
     crate::casts::PTR_AS_PTR_INFO,
     crate::casts::PTR_CAST_CONSTNESS_INFO,
     crate::casts::REF_AS_PTR_INFO,