Rollup merge of #140431 - bend-n:dont_handle_bool_transmute, r=Nadrieril

dont handle bool transmute

removes `transmute(u8) -> bool` suggestion due to ambiguity, leave it for clippy

elaboration on ambiguity in question:
`transmute::<u8, bool>(x)` will codegen to an `assume(u8 < 2)`;
`_ == 1` or `_ != 0` or `_ % 2 == 0` would remove that assumption
`match _ { x @ (0 | 1) => x == 1, _ => std::hint::unreachable_unchecked() }` is very verbose

`@rustbot` label L-unnecessary_transmutes

Author: matthiaskrgr

compiler/rustc_mir_transform/src/check_unnecessary_transmutes.rs

@@ -9,6 +9,7 @@ use crate::errors::UnnecessaryTransmute as Error;
 
 /// Check for transmutes that overlap with stdlib methods.
 /// For example, transmuting `[u8; 4]` to `u32`.
+/// We chose not to lint u8 -> bool transmutes, see #140431
 pub(super) struct CheckUnnecessaryTransmutes;
 
 impl<'tcx> crate::MirLint<'tcx> for CheckUnnecessaryTransmutes {
@@ -98,8 +99,6 @@ impl<'a, 'tcx> UnnecessaryTransmuteChecker<'a, 'tcx> {
             (Uint(_), Float(ty)) => err(format!("{}::from_bits({arg})", ty.name_str())),
             // bool → { x8 }
             (Bool, Int(..) | Uint(..)) => err(format!("({arg}) as {}", fn_sig.output())),
-            // u8 → bool
-            (Uint(_), Bool) => err(format!("({arg} == 1)")),
             _ => return None,
         })
     }

tests/ui/transmute/unnecessary-transmutation.fixed

@@ -81,13 +81,13 @@ fn main() {
         let y: i64 = f64::to_bits(1f64).cast_signed();
         //~^ ERROR
 
-        let z: bool = (1u8 == 1);
-        //~^ ERROR
+        let z: bool = transmute(1u8);
+        // clippy
         let z: u8 = (z) as u8;
         //~^ ERROR
 
         let z: bool = transmute(1i8);
-        // no error!
+        // clippy
         let z: i8 = (z) as i8;
         //~^ ERROR
     }

tests/ui/transmute/unnecessary-transmutation.rs

@@ -82,12 +82,12 @@ fn main() {
         //~^ ERROR
 
         let z: bool = transmute(1u8);
-        //~^ ERROR
+        // clippy
         let z: u8 = transmute(z);
         //~^ ERROR
 
         let z: bool = transmute(1i8);
-        // no error!
+        // clippy
         let z: i8 = transmute(z);
         //~^ ERROR
     }

tests/ui/transmute/unnecessary-transmutation.stderr

@@ -239,12 +239,6 @@ error: unnecessary transmute
 LL |         let y: i64 = transmute(1f64);
    |                      ^^^^^^^^^^^^^^^ help: replace this with: `f64::to_bits(1f64).cast_signed()`
 
-error: unnecessary transmute
-  --> $DIR/unnecessary-transmutation.rs:84:23
-   |
-LL |         let z: bool = transmute(1u8);
-   |                       ^^^^^^^^^^^^^^ help: replace this with: `(1u8 == 1)`
-
 error: unnecessary transmute
   --> $DIR/unnecessary-transmutation.rs:86:21
    |
@@ -257,5 +251,5 @@ error: unnecessary transmute
 LL |         let z: i8 = transmute(z);
    |                     ^^^^^^^^^^^^ help: replace this with: `(z) as i8`
 
-error: aborting due to 36 previous errors
+error: aborting due to 35 previous errors