rust-lang#9231 suggest TryFrom when truncation possible

Author: navh

clippy_lints/src/casts/cast_possible_truncation.rs

@@ -1,9 +1,11 @@
 use clippy_utils::consts::{constant, Constant};
-use clippy_utils::diagnostics::span_lint;
+use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg};
 use clippy_utils::expr_or_init;
+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;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::{BinOpKind, Expr, ExprKind};
 use rustc_lint::LateContext;
@@ -145,7 +147,7 @@ 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) => {
@@ -159,5 +161,18 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>,
         _ => return,
     };
 
-    span_lint(cx, CAST_POSSIBLE_TRUNCATION, expr.span, &msg);
+    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");
+
+    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

@@ -6,11 +6,21 @@ default.
 ### Why is this bad?
 In some problem domains, it is good practice to avoid
 truncation. This lint can be activated to help assess where additional
-checks could be beneficial.
+checks could be beneficial, and suggests implementing TryFrom trait.
 
 ### Example
 ```
 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

@@ -40,15 +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()`
    |
    = note: `-D clippy::cast-possible-truncation` implied by `-D warnings`
 
 error: casting `f32` to `u32` may truncate the value
   --> $DIR/cast.rs:25:5
    |
 LL |     1f32 as u32;
-   |     ^^^^^^^^^^^
+   |     ^^^^^^^^^^^ 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
@@ -62,31 +62,31 @@ error: casting `f64` to `f32` may truncate the value
   --> $DIR/cast.rs:26:5
    |
 LL |     1f64 as f32;
-   |     ^^^^^^^^^^^
+   |     ^^^^^^^^^^^ 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: 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: 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: 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: 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
@@ -142,19 +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: 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: 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: 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
@@ -168,7 +168,7 @@ error: casting `main::E5` to `i8` may truncate the value
   --> $DIR/cast.rs:178:21
    |
 LL |             let _ = self as i8;
-   |                     ^^^^^^^^^^
+   |                     ^^^^^^^^^^ 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
@@ -180,31 +180,31 @@ error: casting `main::E6` to `i16` may truncate the value
   --> $DIR/cast.rs:193:21
    |
 LL |             let _ = self as i16;
-   |                     ^^^^^^^^^^^
+   |                     ^^^^^^^^^^^ 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: 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: 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: 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: avoid silent truncation by using: `u8::try_from((q / 1000)).unwrap()`
 
 error: aborting due to 33 previous errors
 

tests/ui/cast_size.stderr

@@ -2,7 +2,7 @@ error: casting `isize` to `i8` may truncate the value
   --> $DIR/cast_size.rs:12:5
    |
 LL |     1isize as i8;
-   |     ^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^ help: avoid silent truncation by using: `i8::try_from(1isize).unwrap()`
    |
    = note: `-D clippy::cast-possible-truncation` implied by `-D warnings`
 
@@ -36,25 +36,25 @@ error: casting `isize` to `i32` may truncate the value on targets with 64-bit wi
   --> $DIR/cast_size.rs:19:5
    |
 LL |     1isize as i32;
-   |     ^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `i32::try_from(1isize).unwrap()`
 
 error: casting `isize` to `u32` may truncate the value on targets with 64-bit wide pointers
   --> $DIR/cast_size.rs:20:5
    |
 LL |     1isize as u32;
-   |     ^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `u32::try_from(1isize).unwrap()`
 
 error: casting `usize` to `u32` may truncate the value on targets with 64-bit wide pointers
   --> $DIR/cast_size.rs:21:5
    |
 LL |     1usize as u32;
-   |     ^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `u32::try_from(1usize).unwrap()`
 
 error: casting `usize` to `i32` may truncate the value on targets with 64-bit wide pointers
   --> $DIR/cast_size.rs:22:5
    |
 LL |     1usize as i32;
-   |     ^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `i32::try_from(1usize).unwrap()`
 
 error: casting `usize` to `i32` may wrap around the value on targets with 32-bit wide pointers
   --> $DIR/cast_size.rs:22:5
@@ -68,19 +68,19 @@ error: casting `i64` to `isize` may truncate the value on targets with 32-bit wi
   --> $DIR/cast_size.rs:24:5
    |
 LL |     1i64 as isize;
-   |     ^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `isize::try_from(1i64).unwrap()`
 
 error: casting `i64` to `usize` may truncate the value on targets with 32-bit wide pointers
   --> $DIR/cast_size.rs:25:5
    |
 LL |     1i64 as usize;
-   |     ^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `usize::try_from(1i64).unwrap()`
 
 error: casting `u64` to `isize` may truncate the value on targets with 32-bit wide pointers
   --> $DIR/cast_size.rs:26:5
    |
 LL |     1u64 as isize;
-   |     ^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `isize::try_from(1u64).unwrap()`
 
 error: casting `u64` to `isize` may wrap around the value on targets with 64-bit wide pointers
   --> $DIR/cast_size.rs:26:5
@@ -92,7 +92,7 @@ error: casting `u64` to `usize` may truncate the value on targets with 32-bit wi
   --> $DIR/cast_size.rs:27:5
    |
 LL |     1u64 as usize;
-   |     ^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^ help: avoid silent truncation by using: `usize::try_from(1u64).unwrap()`
 
 error: casting `u32` to `isize` may wrap around the value on targets with 32-bit wide pointers
   --> $DIR/cast_size.rs:28:5