Fix an incorrect error message regarding the size of usize and isize in cast_precision_loss. (#14966)

When trying to cast a `usize` or `isize` using `as` to a type of
potentially smaller size the `cast_precision_loss` would claim that the
size of `usize` is "32 or 64 bits". It is now corrected to "16, 32, or
64 bits".

changelog: [`cast_precision_loss`]: fix the error messages claim
regarding the size of `usize` and `isize`.

Author: llogiq

clippy_lints/src/casts/cast_precision_loss.rs

@@ -23,29 +23,21 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, ca
 
     let cast_to_f64 = to_nbits == 64;
     let mantissa_nbits = if cast_to_f64 { 52 } else { 23 };
-    let arch_dependent = is_isize_or_usize(cast_from) && cast_to_f64;
-    let arch_dependent_str = "on targets with 64-bit wide pointers ";
-    let from_nbits_str = if arch_dependent {
-        "64".to_owned()
-    } else if is_isize_or_usize(cast_from) {
-        // FIXME: handle 16 bits `usize` type
-        "32 or 64".to_owned()
+
+    let has_width = if is_isize_or_usize(cast_from) {
+        "can be up to 64 bits wide depending on the target architecture".to_owned()
     } else {
-        from_nbits.to_string()
+        format!("is {from_nbits} bits wide")
     };
 
     span_lint(
         cx,
         CAST_PRECISION_LOSS,
         expr.span,
         format!(
-            "casting `{0}` to `{1}` causes a loss of precision {2}(`{0}` is {3} bits wide, \
-             but `{1}`'s mantissa is only {4} bits wide)",
-            cast_from,
-            if cast_to_f64 { "f64" } else { "f32" },
-            if arch_dependent { arch_dependent_str } else { "" },
-            from_nbits_str,
-            mantissa_nbits
+            "casting `{cast_from}` to `{cast_to}` may cause a loss of precision \
+            (`{cast_from}` {has_width}, \
+             but `{cast_to}`'s mantissa is only {mantissa_nbits} bits wide)",
         ),
     );
 }

tests/ui/cast.stderr

@@ -1,4 +1,4 @@
-error: casting `i32` to `f32` causes a loss of precision (`i32` is 32 bits wide, but `f32`'s mantissa is only 23 bits wide)
+error: casting `i32` to `f32` may cause a loss of precision (`i32` is 32 bits wide, but `f32`'s mantissa is only 23 bits wide)
   --> tests/ui/cast.rs:23:5
    |
 LL |     x0 as f32;
@@ -7,31 +7,31 @@ LL |     x0 as f32;
    = note: `-D clippy::cast-precision-loss` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::cast_precision_loss)]`
 
-error: casting `i64` to `f32` causes a loss of precision (`i64` is 64 bits wide, but `f32`'s mantissa is only 23 bits wide)
+error: casting `i64` to `f32` may cause a loss of precision (`i64` is 64 bits wide, but `f32`'s mantissa is only 23 bits wide)
   --> tests/ui/cast.rs:27:5
    |
 LL |     x1 as f32;
    |     ^^^^^^^^^
 
-error: casting `i64` to `f64` causes a loss of precision (`i64` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
+error: casting `i64` to `f64` may cause a loss of precision (`i64` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
   --> tests/ui/cast.rs:30:5
    |
 LL |     x1 as f64;
    |     ^^^^^^^^^
 
-error: casting `u32` to `f32` causes a loss of precision (`u32` is 32 bits wide, but `f32`'s mantissa is only 23 bits wide)
+error: casting `u32` to `f32` may cause a loss of precision (`u32` is 32 bits wide, but `f32`'s mantissa is only 23 bits wide)
   --> tests/ui/cast.rs:34:5
    |
 LL |     x2 as f32;
    |     ^^^^^^^^^
 
-error: casting `u64` to `f32` causes a loss of precision (`u64` is 64 bits wide, but `f32`'s mantissa is only 23 bits wide)
+error: casting `u64` to `f32` may cause a loss of precision (`u64` is 64 bits wide, but `f32`'s mantissa is only 23 bits wide)
   --> tests/ui/cast.rs:38:5
    |
 LL |     x3 as f32;
    |     ^^^^^^^^^
 
-error: casting `u64` to `f64` causes a loss of precision (`u64` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
+error: casting `u64` to `f64` may cause a loss of precision (`u64` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
   --> tests/ui/cast.rs:41:5
    |
 LL |     x3 as f64;

tests/ui/cast_size.r32bit.stderr

@@ -13,7 +13,7 @@ LL -     1isize as i8;
 LL +     i8::try_from(1isize);
    |
 
-error: casting `isize` to `f32` causes a loss of precision (`isize` is 32 or 64 bits wide, but `f32`'s mantissa is only 23 bits wide)
+error: casting `isize` to `f32` may cause a loss of precision (`isize` can be up to 64 bits wide depending on the target architecture, but `f32`'s mantissa is only 23 bits wide)
   --> tests/ui/cast_size.rs:24:5
    |
 LL |     x0 as f32;
@@ -22,19 +22,19 @@ LL |     x0 as f32;
    = note: `-D clippy::cast-precision-loss` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::cast_precision_loss)]`
 
-error: casting `usize` to `f32` causes a loss of precision (`usize` is 32 or 64 bits wide, but `f32`'s mantissa is only 23 bits wide)
+error: casting `usize` to `f32` may cause a loss of precision (`usize` can be up to 64 bits wide depending on the target architecture, but `f32`'s mantissa is only 23 bits wide)
   --> tests/ui/cast_size.rs:26:5
    |
 LL |     x1 as f32;
    |     ^^^^^^^^^
 
-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)
+error: casting `isize` to `f64` may cause 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)
   --> tests/ui/cast_size.rs:28:5
    |
 LL |     x0 as f64;
    |     ^^^^^^^^^
 
-error: casting `usize` to `f64` causes a loss of precision on targets with 64-bit wide pointers (`usize` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
+error: casting `usize` to `f64` may cause a loss of precision on targets with 64-bit wide pointers (`usize` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
   --> tests/ui/cast_size.rs:30:5
    |
 LL |     x1 as f64;
@@ -165,13 +165,13 @@ error: casting `u32` to `isize` may wrap around the value on targets with 32-bit
 LL |     1u32 as isize;
    |     ^^^^^^^^^^^^^
 
-error: casting `i32` to `f32` causes a loss of precision (`i32` is 32 bits wide, but `f32`'s mantissa is only 23 bits wide)
+error: casting `i32` to `f32` may cause a loss of precision (`i32` is 32 bits wide, but `f32`'s mantissa is only 23 bits wide)
   --> tests/ui/cast_size.rs:61:5
    |
 LL |     999_999_999 as f32;
    |     ^^^^^^^^^^^^^^^^^^
 
-error: casting `usize` to `f64` causes a loss of precision on targets with 64-bit wide pointers (`usize` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
+error: casting `usize` to `f64` may cause a loss of precision on targets with 64-bit wide pointers (`usize` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
   --> tests/ui/cast_size.rs:63:5
    |
 LL |     9_999_999_999_999_999usize as f64;

tests/ui/cast_size.r64bit.stderr

@@ -13,7 +13,7 @@ LL -     1isize as i8;
 LL +     i8::try_from(1isize);
    |
 
-error: casting `isize` to `f32` causes a loss of precision (`isize` is 32 or 64 bits wide, but `f32`'s mantissa is only 23 bits wide)
+error: casting `isize` to `f32` may cause a loss of precision (`isize` can be up to 64 bits wide depending on the target architecture, but `f32`'s mantissa is only 23 bits wide)
   --> tests/ui/cast_size.rs:24:5
    |
 LL |     x0 as f32;
@@ -22,19 +22,19 @@ LL |     x0 as f32;
    = note: `-D clippy::cast-precision-loss` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::cast_precision_loss)]`
 
-error: casting `usize` to `f32` causes a loss of precision (`usize` is 32 or 64 bits wide, but `f32`'s mantissa is only 23 bits wide)
+error: casting `usize` to `f32` may cause a loss of precision (`usize` can be up to 64 bits wide depending on the target architecture, but `f32`'s mantissa is only 23 bits wide)
   --> tests/ui/cast_size.rs:26:5
    |
 LL |     x1 as f32;
    |     ^^^^^^^^^
 
-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)
+error: casting `isize` to `f64` may cause a loss of precision (`isize` can be up to 64 bits wide depending on the target architecture, but `f64`'s mantissa is only 52 bits wide)
   --> tests/ui/cast_size.rs:28:5
    |
 LL |     x0 as f64;
    |     ^^^^^^^^^
 
-error: casting `usize` to `f64` causes a loss of precision on targets with 64-bit wide pointers (`usize` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
+error: casting `usize` to `f64` may cause a loss of precision (`usize` can be up to 64 bits wide depending on the target architecture, but `f64`'s mantissa is only 52 bits wide)
   --> tests/ui/cast_size.rs:30:5
    |
 LL |     x1 as f64;
@@ -165,13 +165,13 @@ error: casting `u32` to `isize` may wrap around the value on targets with 32-bit
 LL |     1u32 as isize;
    |     ^^^^^^^^^^^^^
 
-error: casting `i32` to `f32` causes a loss of precision (`i32` is 32 bits wide, but `f32`'s mantissa is only 23 bits wide)
+error: casting `i32` to `f32` may cause a loss of precision (`i32` is 32 bits wide, but `f32`'s mantissa is only 23 bits wide)
   --> tests/ui/cast_size.rs:61:5
    |
 LL |     999_999_999 as f32;
    |     ^^^^^^^^^^^^^^^^^^
 
-error: casting `usize` to `f64` causes a loss of precision on targets with 64-bit wide pointers (`usize` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
+error: casting `usize` to `f64` may cause a loss of precision (`usize` can be up to 64 bits wide depending on the target architecture, but `f64`'s mantissa is only 52 bits wide)
   --> tests/ui/cast_size.rs:63:5
    |
 LL |     9_999_999_999_999_999usize as f64;