rust-lang#9231 suggest TryFrom when truncation possible

Author: navh

clippy_lints/src/casts/cast_possible_truncation.rs

@@ -1,11 +1,11 @@
 use clippy_utils::consts::{constant, Constant};
-use clippy_utils::diagnostics::{span_lint, span_lint_and_then};
+use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg};
 use clippy_utils::expr_or_init;
-use clippy_utils::source::snippet;
+use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::ty::{get_discriminant_value, is_isize_or_usize};
 use rustc_ast::ast;
 use rustc_attr::IntType;
-use rustc_errors::{Applicability, SuggestionStyle};
+use rustc_errors::Applicability;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::{BinOpKind, Expr, ExprKind};
 use rustc_lint::LateContext;
@@ -161,19 +161,18 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>,
         _ => return,
     };
 
-    let snippet = snippet(cx, expr.span, "x");
+    let mut applicability = Applicability::MachineApplicable;
+
+    let snippet = snippet_with_applicability(cx, expr.span, "x", &mut applicability);
     let name_of_cast_from = snippet.split(" as").next().unwrap_or("x");
-    let suggestion = format!("{cast_to}::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,
-        );
-    });
+
+    span_lint_and_sugg(
+        cx,
+        CAST_POSSIBLE_TRUNCATION,
+        expr.span,
+        &msg,
+        "avoid silent truncation by using",
+        format!("{cast_to}::try_from({name_of_cast_from}).unwrap()"),
+        applicability,
+    );
 }

src/docs/cast_possible_truncation.txt

@@ -13,4 +13,14 @@ checks could be beneficial, and suggests implementing TryFrom trait.
 fn as_u8(x: u64) -> u8 {
     x as u8
 }
-```
+```
+
+will silently truncate requiring detection and handling after the fact.
+
+```
+fn as_u8(x: u64) -> u8 {
+    u8::try_from(x).unwrap()
+}
+```
+
+does not, but can now panic. 

tests/ui/cast.stderr

@@ -40,26 +40,15 @@ error: casting `f32` to `i32` may truncate the value
   --> $DIR/cast.rs:24:5
    |
 LL |     1f32 as i32;
-   |     ^^^^^^^^^^^
+   |     ^^^^^^^^^^^ help: avoid silent truncation by using: `i32::try_from(1f32).unwrap()`
    |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
    = note: `-D clippy::cast-possible-truncation` implied by `-D warnings`
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     i32::try_from(1f32);
-   |     ~~~~~~~~~~~~~~~~~~~
 
 error: casting `f32` to `u32` may truncate the value
   --> $DIR/cast.rs:25:5
    |
 LL |     1f32 as u32;
-   |     ^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     u32::try_from(1f32);
-   |     ~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^ help: avoid silent truncation by using: `u32::try_from(1f32).unwrap()`
 
 error: casting `f32` to `u32` may lose the sign of the value
   --> $DIR/cast.rs:25:5
@@ -73,61 +62,31 @@ error: casting `f64` to `f32` may truncate the value
   --> $DIR/cast.rs:26:5
    |
 LL |     1f64 as f32;
-   |     ^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     f32::try_from(1f64);
-   |     ~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^ help: avoid silent truncation by using: `f32::try_from(1f64).unwrap()`
 
 error: casting `i32` to `i8` may truncate the value
   --> $DIR/cast.rs:27:5
    |
 LL |     1i32 as i8;
-   |     ^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     i8::try_from(1i32);
-   |     ~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^ help: avoid silent truncation by using: `i8::try_from(1i32).unwrap()`
 
 error: casting `i32` to `u8` may truncate the value
   --> $DIR/cast.rs:28:5
    |
 LL |     1i32 as u8;
-   |     ^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     u8::try_from(1i32);
-   |     ~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^ help: avoid silent truncation by using: `u8::try_from(1i32).unwrap()`
 
 error: casting `f64` to `isize` may truncate the value
   --> $DIR/cast.rs:29:5
    |
 LL |     1f64 as isize;
-   |     ^^^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     isize::try_from(1f64);
-   |     ~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `isize::try_from(1f64).unwrap()`
 
 error: casting `f64` to `usize` may truncate the value
   --> $DIR/cast.rs:30:5
    |
 LL |     1f64 as usize;
-   |     ^^^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     usize::try_from(1f64);
-   |     ~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `usize::try_from(1f64).unwrap()`
 
 error: casting `f64` to `usize` may lose the sign of the value
   --> $DIR/cast.rs:30:5
@@ -183,37 +142,19 @@ error: casting `i64` to `i8` may truncate the value
   --> $DIR/cast.rs:108:5
    |
 LL |     (-99999999999i64).min(1) as i8; // should be linted because signed
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     i8::try_from((-99999999999i64).min(1)); // should be linted because signed
-   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: avoid silent truncation by using: `i8::try_from((-99999999999i64).min(1)).unwrap()`
 
 error: casting `u64` to `u8` may truncate the value
   --> $DIR/cast.rs:120:5
    |
 LL |     999999u64.clamp(0, 256) as u8; // should still be linted
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     u8::try_from(999999u64.clamp(0, 256)); // should still be linted
-   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: avoid silent truncation by using: `u8::try_from(999999u64.clamp(0, 256)).unwrap()`
 
 error: casting `main::E2` to `u8` may truncate the value
   --> $DIR/cast.rs:141:21
    |
 LL |             let _ = self as u8;
-   |                     ^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |             let _ = u8::try_from(self);
-   |                     ~~~~~~~~~~~~~~~~~~
+   |                     ^^^^^^^^^^ help: avoid silent truncation by using: `u8::try_from(self).unwrap()`
 
 error: casting `main::E2::B` to `u8` will truncate the value
   --> $DIR/cast.rs:142:21
@@ -227,13 +168,7 @@ error: casting `main::E5` to `i8` may truncate the value
   --> $DIR/cast.rs:178:21
    |
 LL |             let _ = self as i8;
-   |                     ^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |             let _ = i8::try_from(self);
-   |                     ~~~~~~~~~~~~~~~~~~
+   |                     ^^^^^^^^^^ help: avoid silent truncation by using: `i8::try_from(self).unwrap()`
 
 error: casting `main::E5::A` to `i8` will truncate the value
   --> $DIR/cast.rs:179:21
@@ -245,61 +180,31 @@ error: casting `main::E6` to `i16` may truncate the value
   --> $DIR/cast.rs:193:21
    |
 LL |             let _ = self as i16;
-   |                     ^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |             let _ = i16::try_from(self);
-   |                     ~~~~~~~~~~~~~~~~~~~
+   |                     ^^^^^^^^^^^ help: avoid silent truncation by using: `i16::try_from(self).unwrap()`
 
 error: casting `main::E7` to `usize` may truncate the value on targets with 32-bit wide pointers
   --> $DIR/cast.rs:208:21
    |
 LL |             let _ = self as usize;
-   |                     ^^^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |             let _ = usize::try_from(self);
-   |                     ~~~~~~~~~~~~~~~~~~~~~
+   |                     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `usize::try_from(self).unwrap()`
 
 error: casting `main::E10` to `u16` may truncate the value
   --> $DIR/cast.rs:249:21
    |
 LL |             let _ = self as u16;
-   |                     ^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |             let _ = u16::try_from(self);
-   |                     ~~~~~~~~~~~~~~~~~~~
+   |                     ^^^^^^^^^^^ help: avoid silent truncation by using: `u16::try_from(self).unwrap()`
 
 error: casting `u32` to `u8` may truncate the value
   --> $DIR/cast.rs:257:13
    |
 LL |     let c = (q >> 16) as u8;
-   |             ^^^^^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     let c = u8::try_from((q >> 16));
-   |             ~~~~~~~~~~~~~~~~~~~~~~~
+   |             ^^^^^^^^^^^^^^^ help: avoid silent truncation by using: `u8::try_from((q >> 16)).unwrap()`
 
 error: casting `u32` to `u8` may truncate the value
   --> $DIR/cast.rs:260:13
    |
 LL |     let c = (q / 1000) as u8;
-   |             ^^^^^^^^^^^^^^^^
-   |
-   = help: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     let c = u8::try_from((q / 1000));
-   |             ~~~~~~~~~~~~~~~~~~~~~~~~
+   |             ^^^^^^^^^^^^^^^^ help: avoid silent truncation by using: `u8::try_from((q / 1000)).unwrap()`
 
 error: aborting due to 33 previous errors