use uX::from instead of _ as uX in non - const contexts

compiler/rustc_mir_transform/src/check_unnecessary_transmutes.rs

@@ -30,6 +30,7 @@ impl<'a, 'tcx> UnnecessaryTransmuteChecker<'a, 'tcx> {
         function: &Operand<'tcx>,
         arg: String,
         span: Span,
+        is_in_const: bool,
     ) -> Option<Error> {
         let fn_sig = function.ty(self.body, self.tcx).fn_sig(self.tcx).skip_binder();
         let [input] = fn_sig.inputs() else { return None };
@@ -97,8 +98,14 @@ impl<'a, 'tcx> UnnecessaryTransmuteChecker<'a, 'tcx> {
             )),
             // uNN → fNN
             (Uint(_), Float(ty)) => err(format!("{}::from_bits({arg})", ty.name_str())),
-            // bool → { x8 }
-            (Bool, Int(..) | Uint(..)) => err(format!("({arg}) as {}", fn_sig.output())),
+            // bool → { x8 } in const context since `From::from` is not const yet
+            // FIXME: is it possible to know when the parentheses arent necessary?
+            // FIXME(const_traits): Remove this when From::from is constified?
+            (Bool, Int(..) | Uint(..)) if is_in_const => {
+                err(format!("({arg}) as {}", fn_sig.output()))
+            }
+            // " using `x8::from`
+            (Bool, Int(..) | Uint(..)) => err(format!("{}::from({arg})", fn_sig.output())),
             _ => return None,
         })
     }
@@ -114,7 +121,13 @@ impl<'tcx> Visitor<'tcx> for UnnecessaryTransmuteChecker<'_, 'tcx> {
             && self.tcx.is_intrinsic(func_def_id, sym::transmute)
             && let span = self.body.source_info(location).span
             && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(arg)
-            && let Some(lint) = self.is_unnecessary_transmute(func, snippet, span)
+            && let def_id = self.body.source.def_id()
+            && let Some(lint) = self.is_unnecessary_transmute(
+                func,
+                snippet,
+                span,
+                self.tcx.hir_body_const_context(def_id.expect_local()).is_some(),
+            )
             && let Some(hir_id) = terminator.source_info.scope.lint_root(&self.body.source_scopes)
         {
             self.tcx.emit_node_span_lint(UNNECESSARY_TRANSMUTES, hir_id, span, lint);

tests/ui/transmute/unnecessary-transmutation.fixed

@@ -8,7 +8,27 @@ pub fn bytes_at_home(x: u32) -> [u8; 4] {
     //~^ ERROR
 }
 
+pub const fn intinator_const(from: bool) -> u8 {
+    unsafe { (from) as u8 }
+    //~^ ERROR
+}
+
+pub static X: u8 = unsafe { (true) as u8 };
+//~^ ERROR
+pub const Y: u8 = unsafe { (true) as u8 };
+//~^ ERROR
+
+pub struct Z {}
+impl Z {
+    pub const fn intinator_assoc(x: bool) -> u8 {
+        unsafe { (x) as u8 }
+        //~^ ERROR
+    }
+}
+
 fn main() {
+    const { unsafe { (true) as u8 } };
+    //~^ ERROR
     unsafe {
         let x: u16 = u16::from_ne_bytes(*b"01");
         //~^ ERROR
@@ -83,12 +103,12 @@ fn main() {
 
         let z: bool = transmute(1u8);
         // clippy
-        let z: u8 = (z) as u8;
+        let z: u8 = u8::from(z);
         //~^ ERROR
 
         let z: bool = transmute(1i8);
         // clippy
-        let z: i8 = (z) as i8;
+        let z: i8 = i8::from(z);
         //~^ ERROR
     }
 }

tests/ui/transmute/unnecessary-transmutation.rs

@@ -8,7 +8,27 @@ pub fn bytes_at_home(x: u32) -> [u8; 4] {
     //~^ ERROR
 }
 
+pub const fn intinator_const(from: bool) -> u8 {
+    unsafe { transmute(from) }
+    //~^ ERROR
+}
+
+pub static X: u8 = unsafe { transmute(true) };
+//~^ ERROR
+pub const Y: u8 = unsafe { transmute(true) };
+//~^ ERROR
+
+pub struct Z {}
+impl Z {
+    pub const fn intinator_assoc(x: bool) -> u8 {
+        unsafe { transmute(x) }
+        //~^ ERROR
+    }
+}
+
 fn main() {
+    const { unsafe { transmute::<_, u8>(true) } };
+    //~^ ERROR
     unsafe {
         let x: u16 = transmute(*b"01");
         //~^ ERROR