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

1 files changed, 1 insertions, 2 deletions

diff --git a/compiler/rustc_mir_transform/src/check_unnecessary_transmutes.rs b/compiler/rustc_mir_transform/src/check_unnecessary_transmutes.rs
index 8da17a056e3..0514de3ea13 100644
--- a/compiler/rustc_mir_transform/src/check_unnecessary_transmutes.rs
+++ b/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,
         })
     }