Remove allow(unused_variables) for rustc_transmute.

This was hiding some genuine sins, including unused arguments in
numerous functions/methods (incl. trait methods), and some unnecessary
computation.

Author: nnethercote

compiler/rustc_next_trait_solver/src/delegate.rs

@@ -102,7 +102,6 @@ pub trait SolverDelegate: Deref<Target = Self::Infcx> + Sized {
 
     fn is_transmutable(
         &self,
-        param_env: <Self::Interner as Interner>::ParamEnv,
         dst: <Self::Interner as Interner>::Ty,
         src: <Self::Interner as Interner>::Ty,
         assume: <Self::Interner as Interner>::Const,

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -1037,12 +1037,11 @@ where
 
     pub(super) fn is_transmutable(
         &mut self,
-        param_env: I::ParamEnv,
         dst: I::Ty,
         src: I::Ty,
         assume: I::Const,
     ) -> Result<Certainty, NoSolution> {
-        self.delegate.is_transmutable(param_env, dst, src, assume)
+        self.delegate.is_transmutable(dst, src, assume)
     }
 }
 

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -617,7 +617,6 @@ where
             )?;
 
             let certainty = ecx.is_transmutable(
-                goal.param_env,
                 goal.predicate.trait_ref.args.type_at(0),
                 goal.predicate.trait_ref.args.type_at(1),
                 assume,

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -2530,9 +2530,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 return GetSafeTransmuteErrorAndReason::Silent;
             };
 
-            let Some(assume) =
-                rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, assume)
-            else {
+            let Some(assume) = rustc_transmute::Assume::from_const(self.infcx.tcx, assume) else {
                 self.dcx().span_delayed_bug(
                     span,
                     "Unable to construct rustc_transmute::Assume where it was previously possible",
@@ -2544,11 +2542,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             let src = trait_pred.trait_ref.args.type_at(1);
             let err_msg = format!("`{src}` cannot be safely transmuted into `{dst}`");
 
-            match rustc_transmute::TransmuteTypeEnv::new(self.infcx.tcx).is_transmutable(
-                obligation.cause,
-                src_and_dst,
-                assume,
-            ) {
+            match rustc_transmute::TransmuteTypeEnv::new(self.infcx.tcx)
+                .is_transmutable(src_and_dst, assume)
+            {
                 Answer::No(reason) => {
                     let safe_transmute_explanation = match reason {
                         rustc_transmute::Reason::SrcIsNotYetSupported => {

compiler/rustc_trait_selection/src/solve/delegate.rs

@@ -7,7 +7,6 @@ use rustc_infer::infer::canonical::{
     Canonical, CanonicalExt as _, CanonicalQueryInput, CanonicalVarInfo, CanonicalVarValues,
 };
 use rustc_infer::infer::{InferCtxt, RegionVariableOrigin, TyCtxtInferExt};
-use rustc_infer::traits::ObligationCause;
 use rustc_infer::traits::solve::Goal;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt as _};
@@ -222,7 +221,6 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
     // register candidates. We probably need to register >1 since we may have an OR of ANDs.
     fn is_transmutable(
         &self,
-        param_env: ty::ParamEnv<'tcx>,
         dst: Ty<'tcx>,
         src: Ty<'tcx>,
         assume: ty::Const<'tcx>,
@@ -231,16 +229,14 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
         // which will ICE for region vars.
         let (dst, src) = self.tcx.erase_regions((dst, src));
 
-        let Some(assume) = rustc_transmute::Assume::from_const(self.tcx, param_env, assume) else {
+        let Some(assume) = rustc_transmute::Assume::from_const(self.tcx, assume) else {
             return Err(NoSolution);
         };
 
         // FIXME(transmutability): This really should be returning nested goals for `Answer::If*`
-        match rustc_transmute::TransmuteTypeEnv::new(self.0.tcx).is_transmutable(
-            ObligationCause::dummy(),
-            rustc_transmute::Types { src, dst },
-            assume,
-        ) {
+        match rustc_transmute::TransmuteTypeEnv::new(self.0.tcx)
+            .is_transmutable(rustc_transmute::Types { src, dst }, assume)
+        {
             rustc_transmute::Answer::Yes => Ok(Certainty::Yes),
             rustc_transmute::Answer::No(_) | rustc_transmute::Answer::If(_) => Err(NoSolution),
         }

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -414,9 +414,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         if self.tcx().features().generic_const_exprs() {
             assume = crate::traits::evaluate_const(self.infcx, assume, obligation.param_env)
         }
-        let Some(assume) =
-            rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, assume)
-        else {
+        let Some(assume) = rustc_transmute::Assume::from_const(self.infcx.tcx, assume) else {
             return Err(Unimplemented);
         };
 
@@ -425,11 +423,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         debug!(?src, ?dst);
         let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx.tcx);
-        let maybe_transmutable = transmute_env.is_transmutable(
-            obligation.cause.clone(),
-            rustc_transmute::Types { dst, src },
-            assume,
-        );
+        let maybe_transmutable =
+            transmute_env.is_transmutable(rustc_transmute::Types { dst, src }, assume);
 
         let fully_flattened = match maybe_transmutable {
             Answer::No(_) => Err(Unimplemented)?,

compiler/rustc_transmute/src/layout/tree.rs

@@ -237,7 +237,7 @@ pub(crate) mod rustc {
 
                 ty::Tuple(members) => Self::from_tuple((ty, layout), members, cx),
 
-                ty::Array(inner_ty, len) => {
+                ty::Array(inner_ty, _len) => {
                     let FieldsShape::Array { stride, count } = &layout.fields else {
                         return Err(Err::NotYetSupported);
                     };
@@ -282,7 +282,6 @@ pub(crate) mod rustc {
                 FieldsShape::Primitive => {
                     assert_eq!(members.len(), 1);
                     let inner_ty = members[0];
-                    let inner_layout = layout_of(cx, inner_ty)?;
                     Self::from_ty(inner_ty, cx)
                 }
                 FieldsShape::Arbitrary { offsets, .. } => {
@@ -345,7 +344,7 @@ pub(crate) mod rustc {
                     // the enum delegates its layout to the variant at `index`.
                     layout_of_variant(*index, None)
                 }
-                Variants::Multiple { tag, tag_encoding, tag_field, .. } => {
+                Variants::Multiple { tag: _, tag_encoding, tag_field, .. } => {
                     // `Variants::Multiple` denotes an enum with multiple
                     // variants. The layout of such an enum is the disjunction
                     // of the layouts of its tagged variants.
@@ -356,7 +355,7 @@ pub(crate) mod rustc {
 
                     let variants = def.discriminants(cx.tcx()).try_fold(
                         Self::uninhabited(),
-                        |variants, (idx, ref discriminant)| {
+                        |variants, (idx, _discriminant)| {
                             let variant = layout_of_variant(idx, Some(tag_encoding.clone()))?;
                             Result::<Self, Err>::Ok(variants.or(variant))
                         },
@@ -414,7 +413,7 @@ pub(crate) mod rustc {
 
             // Append the fields, in memory order, to the layout.
             let inverse_memory_index = memory_index.invert_bijective_mapping();
-            for (memory_idx, &field_idx) in inverse_memory_index.iter_enumerated() {
+            for &field_idx in inverse_memory_index.iter() {
                 // Add interfield padding.
                 let padding_needed = offsets[field_idx] - size;
                 let padding = Self::padding(padding_needed.bytes_usize());
@@ -468,15 +467,14 @@ pub(crate) mod rustc {
 
             // This constructor does not support non-`FieldsShape::Union`
             // layouts. Fields of this shape are all placed at offset 0.
-            let FieldsShape::Union(fields) = layout.fields() else {
+            let FieldsShape::Union(_fields) = layout.fields() else {
                 return Err(Err::NotYetSupported);
             };
 
             let fields = &def.non_enum_variant().fields;
             let fields = fields.iter_enumerated().try_fold(
                 Self::uninhabited(),
-                |fields, (idx, field_def)| {
-                    let field_def = Def::Field(field_def);
+                |fields, (idx, _field_def)| {
                     let field_ty = ty_field(cx, (ty, layout), idx);
                     let field_layout = layout_of(cx, field_ty)?;
                     let field = Self::from_ty(field_ty, cx)?;

compiler/rustc_transmute/src/lib.rs

@@ -1,5 +1,4 @@
 // tidy-alphabetical-start
-#![allow(unused_variables)]
 #![feature(never_type)]
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
@@ -80,8 +79,7 @@ pub enum Reason<T> {
 #[cfg(feature = "rustc")]
 mod rustc {
     use rustc_hir::lang_items::LangItem;
-    use rustc_middle::traits::ObligationCause;
-    use rustc_middle::ty::{Const, ParamEnv, Ty, TyCtxt};
+    use rustc_middle::ty::{Const, Ty, TyCtxt};
 
     use super::*;
 
@@ -105,27 +103,19 @@ mod rustc {
 
         pub fn is_transmutable(
             &mut self,
-            cause: ObligationCause<'tcx>,
             types: Types<'tcx>,
             assume: crate::Assume,
         ) -> crate::Answer<crate::layout::rustc::Ref<'tcx>> {
             crate::maybe_transmutable::MaybeTransmutableQuery::new(
-                types.src,
-                types.dst,
-                assume,
-                self.tcx,
+                types.src, types.dst, assume, self.tcx,
             )
             .answer()
         }
     }
 
     impl Assume {
         /// Constructs an `Assume` from a given const-`Assume`.
-        pub fn from_const<'tcx>(
-            tcx: TyCtxt<'tcx>,
-            param_env: ParamEnv<'tcx>,
-            ct: Const<'tcx>,
-        ) -> Option<Self> {
+        pub fn from_const<'tcx>(tcx: TyCtxt<'tcx>, ct: Const<'tcx>) -> Option<Self> {
             use rustc_middle::ty::ScalarInt;
             use rustc_span::sym;
 

compiler/rustc_transmute/src/maybe_transmutable/mod.rs

@@ -79,12 +79,12 @@ where
     pub(crate) fn answer(self) -> Answer<<C as QueryContext>::Ref> {
         let Self { src, dst, assume, context } = self;
 
-        // Unconditionally all `Def` nodes from `src`, without pruning away the
+        // Unconditionally remove all `Def` nodes from `src`, without pruning away the
         // branches they appear in. This is valid to do for value-to-value
         // transmutations, but not for `&mut T` to `&mut U`; we will need to be
         // more sophisticated to handle transmutations between mutable
         // references.
-        let src = src.prune(&|def| false);
+        let src = src.prune(&|_def| false);
 
         if src.is_inhabited() && !dst.is_inhabited() {
             return Answer::No(Reason::DstUninhabited);
@@ -96,7 +96,7 @@ where
         let dst = if assume.safety {
             // ...if safety is assumed, don't check if they carry safety
             // invariants; retain all paths.
-            dst.prune(&|def| false)
+            dst.prune(&|_def| false)
         } else {
             // ...otherwise, prune away all paths with safety invariants from
             // the `Dst` layout.

compiler/rustc_transmute/src/maybe_transmutable/tests.rs

@@ -92,7 +92,6 @@ mod bool {
 
     #[test]
     fn should_permit_validity_expansion_and_reject_contraction() {
-        let un = layout::Tree::<Def, !>::uninhabited();
         let b0 = layout::Tree::<Def, !>::from_bits(0);
         let b1 = layout::Tree::<Def, !>::from_bits(1);
         let b2 = layout::Tree::<Def, !>::from_bits(2);