rust-lang#9231 suggest TryFrom when truncation possible

Author: navh

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_size.stderr

@@ -2,14 +2,9 @@ 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()`
    |
-   = 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 |     i8::try_from(1isize);
-   |     ~~~~~~~~~~~~~~~~~~~~
 
 error: casting `isize` to `f64` causes a loss of precision on targets with 64-bit wide pointers (`isize` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
   --> $DIR/cast_size.rs:15:5
@@ -41,49 +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: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     i32::try_from(1isize);
-   |     ~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^ 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: 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(1isize);
-   |     ~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^ 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: 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(1usize);
-   |     ~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^ 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: if this is intentional allow the lint with `#[allow(clippy::cast_precision_loss)]` ...
-help: ... or use `try_from` and handle the error accordingly
-   |
-LL |     i32::try_from(1usize);
-   |     ~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^ 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
@@ -97,37 +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: 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(1i64);
-   |     ~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^ 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: 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(1i64);
-   |     ~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^ 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: 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(1u64);
-   |     ~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^ 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
@@ -139,13 +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: 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(1u64);
-   |     ~~~~~~~~~~~~~~~~~~~~~
+   |     ^^^^^^^^^^^^^ 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