1 files changed, 126 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/casts/unnecessary_cast.rs b/src/tools/clippy/clippy_lints/src/casts/unnecessary_cast.rs
new file mode 100644
index 000000000..fff7da8e3
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/casts/unnecessary_cast.rs
@@ -0,0 +1,126 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::numeric_literal::NumericLiteral;
+use clippy_utils::source::snippet_opt;
+use if_chain::if_chain;
+use rustc_ast::{LitFloatType, LitIntType, LitKind};
+use rustc_errors::Applicability;
+use rustc_hir::def::Res;
+use rustc_hir::{Expr, ExprKind, Lit, QPath, TyKind, UnOp};
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty::{self, FloatTy, InferTy, Ty};
+
+use super::UNNECESSARY_CAST;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &Expr<'tcx>,
+    cast_expr: &Expr<'tcx>,
+    cast_from: Ty<'tcx>,
+    cast_to: Ty<'tcx>,
+) -> bool {
+    // skip non-primitive type cast
+    if_chain! {
+        if let ExprKind::Cast(_, cast_to) = expr.kind;
+        if let TyKind::Path(QPath::Resolved(_, path)) = &cast_to.kind;
+        if let Res::PrimTy(_) = path.res;
+        then {}
+        else {
+            return false
+        }
+    }
+
+    if let Some(lit) = get_numeric_literal(cast_expr) {
+        let literal_str = snippet_opt(cx, cast_expr.span).unwrap_or_default();
+
+        if_chain! {
+            if let LitKind::Int(n, _) = lit.node;
+            if let Some(src) = snippet_opt(cx, cast_expr.span);
+            if cast_to.is_floating_point();
+            if let Some(num_lit) = NumericLiteral::from_lit_kind(&src, &lit.node);
+            let from_nbits = 128 - n.leading_zeros();
+            let to_nbits = fp_ty_mantissa_nbits(cast_to);
+            if from_nbits != 0 && to_nbits != 0 && from_nbits <= to_nbits && num_lit.is_decimal();
+            then {
+                lint_unnecessary_cast(cx, expr, num_lit.integer, cast_from, cast_to);
+                return true
+            }
+        }
+
+        match lit.node {
+            LitKind::Int(_, LitIntType::Unsuffixed) if cast_to.is_integral() => {
+                lint_unnecessary_cast(cx, expr, &literal_str, cast_from, cast_to);
+            },
+            LitKind::Float(_, LitFloatType::Unsuffixed) if cast_to.is_floating_point() => {
+                lint_unnecessary_cast(cx, expr, &literal_str, cast_from, cast_to);
+            },
+            LitKind::Int(_, LitIntType::Unsuffixed) | LitKind::Float(_, LitFloatType::Unsuffixed) => {},
+            LitKind::Int(_, LitIntType::Signed(_) | LitIntType::Unsigned(_))
+            | LitKind::Float(_, LitFloatType::Suffixed(_))
+                if cast_from.kind() == cast_to.kind() =>
+            {
+                if let Some(src) = snippet_opt(cx, cast_expr.span) {
+                    if let Some(num_lit) = NumericLiteral::from_lit_kind(&src, &lit.node) {
+                        lint_unnecessary_cast(cx, expr, num_lit.integer, cast_from, cast_to);
+                    }
+                }
+            },
+            _ => {
+                if cast_from.kind() == cast_to.kind() && !in_external_macro(cx.sess(), expr.span) {
+                    span_lint_and_sugg(
+                        cx,
+                        UNNECESSARY_CAST,
+                        expr.span,
+                        &format!(
+                            "casting to the same type is unnecessary (`{}` -> `{}`)",
+                            cast_from, cast_to
+                        ),
+                        "try",
+                        literal_str,
+                        Applicability::MachineApplicable,
+                    );
+                    return true;
+                }
+            },
+        }
+    }
+
+    false
+}
+
+fn lint_unnecessary_cast(cx: &LateContext<'_>, expr: &Expr<'_>, literal_str: &str, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    let literal_kind_name = if cast_from.is_integral() { "integer" } else { "float" };
+    span_lint_and_sugg(
+        cx,
+        UNNECESSARY_CAST,
+        expr.span,
+        &format!("casting {} literal to `{}` is unnecessary", literal_kind_name, cast_to),
+        "try",
+        format!("{}_{}", literal_str.trim_end_matches('.'), cast_to),
+        Applicability::MachineApplicable,
+    );
+}
+
+fn get_numeric_literal<'e>(expr: &'e Expr<'e>) -> Option<&'e Lit> {
+    match expr.kind {
+        ExprKind::Lit(ref lit) => Some(lit),
+        ExprKind::Unary(UnOp::Neg, e) => {
+            if let ExprKind::Lit(ref lit) = e.kind {
+                Some(lit)
+            } else {
+                None
+            }
+        },
+        _ => None,
+    }
+}
+
+/// Returns the mantissa bits wide of a fp type.
+/// Will return 0 if the type is not a fp
+fn fp_ty_mantissa_nbits(typ: Ty<'_>) -> u32 {
+    match typ.kind() {
+        ty::Float(FloatTy::F32) => 23,
+        ty::Float(FloatTy::F64) | ty::Infer(InferTy::FloatVar(_)) => 52,
+        _ => 0,
+    }
+}