Auto merge of #10038 - xFrednet:9231-sugg-try-from, r=Jarcho

`cast_possible_truncation` Suggest TryFrom when truncation possible

This fixes the last issues from #9664 as the author seems to be inactive. The PR author was sadly kept during the rebase, due to the conflict resolution.

IDK if it's worth it do to a full review, I only added the last commit, everything else remained the same, besides a rebase.

---

changelog: Sugg: [`cast_possible_truncation`]: Now suggests using `try_from` or allowing the lint
[#10038](#10038)
<!-- changelog_checked -->

closes: #9231

clippy_lints/src/casts/cast_possible_truncation.rs

@@ -1,11 +1,14 @@
 use clippy_utils::consts::{constant, Constant};
-use clippy_utils::diagnostics::span_lint;
+use clippy_utils::diagnostics::{span_lint, span_lint_and_then};
 use clippy_utils::expr_or_init;
+use clippy_utils::source::snippet;
 use clippy_utils::ty::{get_discriminant_value, is_isize_or_usize};
+use rustc_errors::{Applicability, SuggestionStyle};
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::{BinOpKind, Expr, ExprKind};
 use rustc_lint::LateContext;
 use rustc_middle::ty::{self, FloatTy, Ty};
+use rustc_span::Span;
 use rustc_target::abi::IntegerType;
 
 use super::{utils, CAST_ENUM_TRUNCATION, CAST_POSSIBLE_TRUNCATION};
@@ -74,7 +77,14 @@ fn apply_reductions(cx: &LateContext<'_>, nbits: u64, expr: &Expr<'_>, signed: b
     }
 }
 
-pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+pub(super) fn check(
+    cx: &LateContext<'_>,
+    expr: &Expr<'_>,
+    cast_expr: &Expr<'_>,
+    cast_from: Ty<'_>,
+    cast_to: Ty<'_>,
+    cast_to_span: Span,
+) {
     let msg = match (cast_from.kind(), cast_to.is_integral()) {
         (ty::Int(_) | ty::Uint(_), true) => {
             let from_nbits = apply_reductions(
@@ -139,7 +149,7 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>,
                 );
                 return;
             }
-            format!("casting `{cast_from}` to `{cast_to}` may truncate the value{suffix}",)
+            format!("casting `{cast_from}` to `{cast_to}` may truncate the value{suffix}")
         },
 
         (ty::Float(_), true) => {
@@ -153,5 +163,19 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>,
         _ => return,
     };
 
-    span_lint(cx, CAST_POSSIBLE_TRUNCATION, expr.span, &msg);
+    let name_of_cast_from = snippet(cx, cast_expr.span, "..");
+    let cast_to_snip = snippet(cx, cast_to_span, "..");
+    let suggestion = format!("{cast_to_snip}::try_from({name_of_cast_from})");
+
+    span_lint_and_then(cx, CAST_POSSIBLE_TRUNCATION, expr.span, &msg, |diag| {
+        diag.help("if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...");
+        diag.span_suggestion_with_style(
+            expr.span,
+            "... or use `try_from` and handle the error accordingly",
+            suggestion,
+            Applicability::Unspecified,
+            // always show the suggestion in a separate line
+            SuggestionStyle::ShowAlways,
+        );
+    });
 }

clippy_lints/src/casts/mod.rs

@@ -80,7 +80,8 @@ declare_clippy_lint! {
     /// ### What it does
     /// Checks for casts between numerical types that may
     /// truncate large values. This is expected behavior, so the cast is `Allow` by
-    /// default.
+    /// default. It suggests user either explicitly ignore the lint,
+    /// or use `try_from()` and handle the truncation, default, or panic explicitly.
     ///
     /// ### Why is this bad?
     /// In some problem domains, it is good practice to avoid
@@ -93,6 +94,21 @@ declare_clippy_lint! {
     ///     x as u8
     /// }
     /// ```
+    /// Use instead:
+    /// ```
+    /// fn as_u8(x: u64) -> u8 {
+    ///     if let Ok(x) = u8::try_from(x) {
+    ///         x
+    ///     } else {
+    ///         todo!();
+    ///     }
+    /// }
+    /// // Or
+    /// #[allow(clippy::cast_possible_truncation)]
+    /// fn as_u16(x: u64) -> u16 {
+    ///     x as u16
+    /// }
+    /// ```
     #[clippy::version = "pre 1.29.0"]
     pub CAST_POSSIBLE_TRUNCATION,
     pedantic,
@@ -712,7 +728,7 @@ impl<'tcx> LateLintPass<'tcx> for Casts {
             fn_to_numeric_cast_with_truncation::check(cx, expr, cast_expr, cast_from, cast_to);
 
             if cast_to.is_numeric() && !in_external_macro(cx.sess(), expr.span) {
-                cast_possible_truncation::check(cx, expr, cast_expr, cast_from, cast_to);
+                cast_possible_truncation::check(cx, expr, cast_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_precision_loss::check(cx, expr, cast_from, cast_to);

tests/ui/cast.rs

@@ -28,6 +28,7 @@ fn main() {
     1i32 as u8;
     1f64 as isize;
     1f64 as usize;
+    1f32 as u32 as u16;
     // Test clippy::cast_possible_wrap
     1u8 as i8;
     1u16 as i16;