Add suggestion to cast_sign_loss and cast_possible_wrap using the cast_{un,}signed() methods (#15384)

changelog: [`cast_sign_loss`, `cast_possible_wrap`]: add suggestion
using `cast_{un,}signed()` methods

Fixes #14150

This PR adds a suggestion to `cast_sign_loss` and `cast_possible_wrap`,
which fixes reported warnings using the `cast_{un,}signed()` methods, as
proposed in #14150. Suggestions are only made if
MSRV is >=1.87.

Also, I'm not 100% sure about the suggestion message yet. I'm open for
suggestions.

Author: Jarcho

clippy_lints/src/casts/cast_possible_wrap.rs

@@ -1,4 +1,7 @@
 use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::msrvs::{self, Msrv};
+use clippy_utils::sugg::Sugg;
+use rustc_errors::Applicability;
 use rustc_hir::Expr;
 use rustc_lint::LateContext;
 use rustc_middle::ty::Ty;
@@ -16,7 +19,14 @@ enum EmitState {
     LintOnPtrSize(u64),
 }
 
-pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+pub(super) fn check(
+    cx: &LateContext<'_>,
+    expr: &Expr<'_>,
+    cast_op: &Expr<'_>,
+    cast_from: Ty<'_>,
+    cast_to: Ty<'_>,
+    msrv: Msrv,
+) {
     let (Some(from_nbits), Some(to_nbits)) = (
         utils::int_ty_to_nbits(cx.tcx, cast_from),
         utils::int_ty_to_nbits(cx.tcx, cast_to),
@@ -85,5 +95,23 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, ca
                 .note("`usize` and `isize` may be as small as 16 bits on some platforms")
                 .note("for more information see https://doc.rust-lang.org/reference/types/numeric.html#machine-dependent-integer-types");
         }
+
+        if msrv.meets(cx, msrvs::INTEGER_SIGN_CAST)
+            && let Some(cast) = utils::is_signedness_cast(cast_from, cast_to)
+        {
+            let method = match cast {
+                utils::CastTo::Signed => "cast_signed()",
+                utils::CastTo::Unsigned => "cast_unsigned()",
+            };
+            let mut app = Applicability::MaybeIncorrect;
+            let sugg = Sugg::hir_with_context(cx, cast_op, expr.span.ctxt(), "..", &mut app);
+
+            diag.span_suggestion(
+                expr.span,
+                format!("if this is intentional, use `{method}` instead"),
+                format!("{}.{method}", sugg.maybe_paren()),
+                app,
+            );
+        }
     });
 }

clippy_lints/src/casts/cast_sign_loss.rs

@@ -2,15 +2,18 @@ use std::convert::Infallible;
 use std::ops::ControlFlow;
 
 use clippy_utils::consts::{ConstEvalCtxt, Constant};
-use clippy_utils::diagnostics::span_lint;
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::msrvs::{self, Msrv};
+use clippy_utils::sugg::Sugg;
 use clippy_utils::visitors::{Descend, for_each_expr_without_closures};
 use clippy_utils::{method_chain_args, sext, sym};
+use rustc_errors::Applicability;
 use rustc_hir::{BinOpKind, Expr, ExprKind};
 use rustc_lint::LateContext;
 use rustc_middle::ty::{self, Ty};
 use rustc_span::Symbol;
 
-use super::CAST_SIGN_LOSS;
+use super::{CAST_SIGN_LOSS, utils};
 
 /// A list of methods that can never return a negative value.
 /// Includes methods that panic rather than returning a negative value.
@@ -42,13 +45,33 @@ pub(super) fn check<'cx>(
     cast_op: &Expr<'_>,
     cast_from: Ty<'cx>,
     cast_to: Ty<'_>,
+    msrv: Msrv,
 ) {
     if should_lint(cx, cast_op, cast_from, cast_to) {
-        span_lint(
+        span_lint_and_then(
             cx,
             CAST_SIGN_LOSS,
             expr.span,
             format!("casting `{cast_from}` to `{cast_to}` may lose the sign of the value"),
+            |diag| {
+                if msrv.meets(cx, msrvs::INTEGER_SIGN_CAST)
+                    && let Some(cast) = utils::is_signedness_cast(cast_from, cast_to)
+                {
+                    let method = match cast {
+                        utils::CastTo::Signed => "cast_signed()",
+                        utils::CastTo::Unsigned => "cast_unsigned()",
+                    };
+                    let mut app = Applicability::MaybeIncorrect;
+                    let sugg = Sugg::hir_with_context(cx, cast_op, expr.span.ctxt(), "..", &mut app);
+
+                    diag.span_suggestion(
+                        expr.span,
+                        format!("if this is intentional, use `{method}` instead"),
+                        format!("{}.{method}", sugg.maybe_paren()),
+                        app,
+                    );
+                }
+            },
         );
     }
 }

clippy_lints/src/casts/mod.rs

@@ -890,9 +890,9 @@ impl<'tcx> LateLintPass<'tcx> for Casts {
             if cast_to.is_numeric() {
                 cast_possible_truncation::check(cx, expr, cast_from_expr, cast_from, cast_to, cast_to_hir.span);
                 if cast_from.is_numeric() {
-                    cast_possible_wrap::check(cx, expr, cast_from, cast_to);
+                    cast_possible_wrap::check(cx, expr, cast_from_expr, cast_from, cast_to, self.msrv);
                     cast_precision_loss::check(cx, expr, cast_from, cast_to);
-                    cast_sign_loss::check(cx, expr, cast_from_expr, cast_from, cast_to);
+                    cast_sign_loss::check(cx, expr, cast_from_expr, cast_from, cast_to, self.msrv);
                     cast_abs_to_unsigned::check(cx, expr, cast_from_expr, cast_from, cast_to, self.msrv);
                     cast_nan_to_int::check(cx, expr, cast_from_expr, cast_from, cast_to);
                 }

clippy_lints/src/casts/utils.rs

@@ -60,3 +60,18 @@ pub(super) fn enum_ty_to_nbits(adt: AdtDef<'_>, tcx: TyCtxt<'_>) -> u64 {
         neg_bits.max(pos_bits).into()
     }
 }
+
+pub(super) enum CastTo {
+    Signed,
+    Unsigned,
+}
+/// Returns `Some` if the type cast is between 2 integral types that differ
+/// only in signedness, otherwise `None`. The value of `Some` is which
+/// signedness is casted to.
+pub(super) fn is_signedness_cast(cast_from: Ty<'_>, cast_to: Ty<'_>) -> Option<CastTo> {
+    match (cast_from.kind(), cast_to.kind()) {
+        (ty::Int(from), ty::Uint(to)) if from.to_unsigned() == *to => Some(CastTo::Unsigned),
+        (ty::Uint(from), ty::Int(to)) if *from == to.to_unsigned() => Some(CastTo::Signed),
+        _ => None,
+    }
+}

clippy_utils/src/msrvs.rs

@@ -24,7 +24,7 @@ macro_rules! msrv_aliases {
 // names may refer to stabilized feature flags or library items
 msrv_aliases! {
     1,88,0 { LET_CHAINS }
-    1,87,0 { OS_STR_DISPLAY, INT_MIDPOINT, CONST_CHAR_IS_DIGIT, UNSIGNED_IS_MULTIPLE_OF }
+    1,87,0 { OS_STR_DISPLAY, INT_MIDPOINT, CONST_CHAR_IS_DIGIT, UNSIGNED_IS_MULTIPLE_OF, INTEGER_SIGN_CAST }
     1,85,0 { UINT_FLOAT_MIDPOINT, CONST_SIZE_OF_VAL }
     1,84,0 { CONST_OPTION_AS_SLICE, MANUAL_DANGLING_PTR }
     1,83,0 { CONST_EXTERN_FN, CONST_FLOAT_BITS_CONV, CONST_FLOAT_CLASSIFY, CONST_MUT_REFS, CONST_UNWRAP }

tests/ui/cast.rs

@@ -569,3 +569,16 @@ fn issue12721() {
     (255 % 999999u64) as u8;
     //~^ cast_possible_truncation
 }
+
+mod issue14150 {
+    #[clippy::msrv = "1.87"]
+    fn msrv_supports_cast_signed() {
+        _ = 1u8 as i8;
+        //~^ cast_possible_wrap
+    }
+    #[clippy::msrv = "1.86"]
+    fn msrv_doesnt_supports_cast_signed() {
+        _ = 1u8 as i8;
+        //~^ cast_possible_wrap
+    }
+}