Rollup merge of #57606 - oli-obk:shrink, r=RalfJung

Get rid of the fake stack frame for reading from constants

r? @RalfJung

fixes the ice in https://github.com/rust-lang/rust/issues/53708 but still keeps around the wrong "non-exhaustive match" error

cc @varkor

12 files changed, 93 insertions, 103 deletions

diff --git a/src/librustc_codegen_ssa/mir/constant.rs b/src/librustc_codegen_ssa/mir/constant.rs
index e6d6ef1d7a3..56d4342e6e1 100644
--- a/src/librustc_codegen_ssa/mir/constant.rs
+++ b/src/librustc_codegen_ssa/mir/constant.rs
@@ -59,7 +59,6 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     let field = const_field(
                         bx.tcx(),
                         ty::ParamEnv::reveal_all(),
-                        self.instance,
                         None,
                         mir::Field::new(field as usize),
                         c,
diff --git a/src/librustc_mir/const_eval.rs b/src/librustc_mir/const_eval.rs
index 105856fecc7..45c6c1b4249 100644
--- a/src/librustc_mir/const_eval.rs
+++ b/src/librustc_mir/const_eval.rs
@@ -10,11 +10,10 @@ use rustc::hir::{self, def_id::DefId};
 use rustc::hir::def::Def;
 use rustc::mir::interpret::{ConstEvalErr, ErrorHandled};
 use rustc::mir;
-use rustc::ty::{self, TyCtxt, Instance, query::TyCtxtAt};
+use rustc::ty::{self, TyCtxt, query::TyCtxtAt};
 use rustc::ty::layout::{self, LayoutOf, TyLayout, VariantIdx};
 use rustc::ty::subst::Subst;
 use rustc::traits::Reveal;
-use rustc_data_structures::indexed_vec::IndexVec;
 use rustc_data_structures::fx::FxHashMap;
 use rustc::util::common::ErrorReported;
 
@@ -35,56 +34,6 @@ const STEPS_UNTIL_DETECTOR_ENABLED: isize = 1_000_000;
 /// Should be a power of two for performance reasons.
 const DETECTOR_SNAPSHOT_PERIOD: isize = 256;
 
-/// Warning: do not use this function if you expect to start interpreting the given `Mir`.
-/// The `EvalContext` is only meant to be used to query values from constants and statics.
-///
-/// This function is used during const propagation. We cannot use `mk_eval_cx`, because copy
-/// propagation happens *during* the computation of the MIR of the current function. So if we
-/// tried to call the `optimized_mir` query, we'd get a cycle error because we are (transitively)
-/// inside the `optimized_mir` query of the `Instance` given.
-///
-/// Since we are looking at the MIR of the function in an abstract manner, we don't have a
-/// `ParamEnv` available to us. This function creates a `ParamEnv` for the given instance.
-pub fn mk_borrowck_eval_cx<'a, 'mir, 'tcx>(
-    tcx: TyCtxt<'a, 'tcx, 'tcx>,
-    instance: Instance<'tcx>,
-    mir: &'mir mir::Mir<'tcx>,
-    span: Span,
-) -> EvalResult<'tcx, CompileTimeEvalContext<'a, 'mir, 'tcx>> {
-    debug!("mk_borrowck_eval_cx: {:?}", instance);
-    let param_env = tcx.param_env(instance.def_id());
-    mk_eval_cx_inner(tcx, instance, mir, span, param_env)
-}
-
-/// This is just a helper function to reduce code duplication between `mk_borrowck_eval_cx` and
-/// `mk_eval_cx`. Do not call this function directly.
-fn mk_eval_cx_inner<'a, 'mir, 'tcx>(
-    tcx: TyCtxt<'a, 'tcx, 'tcx>,
-    instance: Instance<'tcx>,
-    mir: &'mir mir::Mir<'tcx>,
-    span: Span,
-    param_env: ty::ParamEnv<'tcx>,
-) -> EvalResult<'tcx, CompileTimeEvalContext<'a, 'mir, 'tcx>> {
-    let mut ecx = EvalContext::new(tcx.at(span), param_env, CompileTimeInterpreter::new());
-    // Insert a stack frame so any queries have the correct substs.
-    // We also avoid all the extra work performed by push_stack_frame,
-    // like initializing local variables
-    ecx.stack.push(interpret::Frame {
-        block: mir::START_BLOCK,
-        locals: IndexVec::new(),
-        local_layouts: IndexVec::new(),
-        instance,
-        span,
-        mir,
-        return_place: None,
-        return_to_block: StackPopCleanup::Goto(None), // never pop
-        stmt: 0,
-        extra: (),
-    });
-    Ok(ecx)
-}
-
-/// Warning: do not use this function if you expect to start interpreting the given `Mir`.
 /// The `EvalContext` is only meant to be used to do field and index projections into constants for
 /// `simd_shuffle` and const patterns in match arms.
 ///
@@ -92,15 +41,13 @@ fn mk_eval_cx_inner<'a, 'mir, 'tcx>(
 /// that inform us about the generic bounds of the constant. E.g. using an associated constant
 /// of a function's generic parameter will require knowledge about the bounds on the generic
 /// parameter. These bounds are passed to `mk_eval_cx` via the `ParamEnv` argument.
-fn mk_eval_cx<'a, 'tcx>(
+pub(crate) fn mk_eval_cx<'a, 'mir, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
-    instance: Instance<'tcx>,
+    span: Span,
     param_env: ty::ParamEnv<'tcx>,
-) -> EvalResult<'tcx, CompileTimeEvalContext<'a, 'tcx, 'tcx>> {
-    debug!("mk_eval_cx: {:?}, {:?}", instance, param_env);
-    let span = tcx.def_span(instance.def_id());
-    let mir = tcx.optimized_mir(instance.def.def_id());
-    mk_eval_cx_inner(tcx, instance, mir, span, param_env)
+) -> CompileTimeEvalContext<'a, 'mir, 'tcx> {
+    debug!("mk_eval_cx: {:?}", param_env);
+    EvalContext::new(tcx.at(span), param_env, CompileTimeInterpreter::new())
 }
 
 pub(crate) fn eval_promoted<'a, 'mir, 'tcx>(
@@ -109,7 +56,8 @@ pub(crate) fn eval_promoted<'a, 'mir, 'tcx>(
     mir: &'mir mir::Mir<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
 ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
-    let mut ecx = mk_borrowck_eval_cx(tcx, cid.instance, mir, DUMMY_SP).unwrap();
+    let span = tcx.def_span(cid.instance.def_id());
+    let mut ecx = mk_eval_cx(tcx, span, param_env);
     eval_body_using_ecx(&mut ecx, cid, Some(mir), param_env)
 }
 
@@ -530,13 +478,12 @@ impl<'a, 'mir, 'tcx> interpret::Machine<'a, 'mir, 'tcx>
 pub fn const_field<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
-    instance: ty::Instance<'tcx>,
     variant: Option<VariantIdx>,
     field: mir::Field,
     value: ty::Const<'tcx>,
 ) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
-    trace!("const_field: {:?}, {:?}, {:?}", instance, field, value);
-    let ecx = mk_eval_cx(tcx, instance, param_env).unwrap();
+    trace!("const_field: {:?}, {:?}", field, value);
+    let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env);
     let result = (|| {
         // get the operand again
         let op = lazy_const_to_op(&ecx, ty::LazyConst::Evaluated(value), value.ty)?;
@@ -561,11 +508,10 @@ pub fn const_field<'a, 'tcx>(
 pub fn const_variant_index<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
-    instance: ty::Instance<'tcx>,
     val: ty::Const<'tcx>,
 ) -> EvalResult<'tcx, VariantIdx> {
-    trace!("const_variant_index: {:?}, {:?}", instance, val);
-    let ecx = mk_eval_cx(tcx, instance, param_env).unwrap();
+    trace!("const_variant_index: {:?}", val);
+    let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env);
     let op = lazy_const_to_op(&ecx, ty::LazyConst::Evaluated(val), val.ty)?;
     Ok(ecx.read_discriminant(op)?.1)
 }
@@ -585,7 +531,7 @@ fn validate_and_turn_into_const<'a, 'tcx>(
     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 ) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
     let cid = key.value;
-    let ecx = mk_eval_cx(tcx, cid.instance, key.param_env).unwrap();
+    let ecx = mk_eval_cx(tcx, tcx.def_span(key.value.instance.def_id()), key.param_env);
     let val = (|| {
         let op = ecx.raw_const_to_mplace(constant)?.into();
         // FIXME: Once the visitor infrastructure landed, change validation to
diff --git a/src/librustc_mir/hair/pattern/_match.rs b/src/librustc_mir/hair/pattern/_match.rs
index 188a1120442..9cc5c93de41 100644
--- a/src/librustc_mir/hair/pattern/_match.rs
+++ b/src/librustc_mir/hair/pattern/_match.rs
@@ -427,13 +427,24 @@ pub enum Constructor<'tcx> {
 }
 
 impl<'tcx> Constructor<'tcx> {
-    fn variant_index_for_adt(&self, adt: &'tcx ty::AdtDef) -> VariantIdx {
+    fn variant_index_for_adt<'a>(
+        &self,
+        cx: &MatchCheckCtxt<'a, 'tcx>,
+        adt: &'tcx ty::AdtDef,
+    ) -> VariantIdx {
         match self {
             &Variant(vid) => adt.variant_index_with_id(vid),
             &Single => {
                 assert!(!adt.is_enum());
                 VariantIdx::new(0)
             }
+            &ConstantValue(c) => {
+                ::const_eval::const_variant_index(
+                    cx.tcx,
+                    cx.param_env,
+                    c,
+                ).unwrap()
+            },
             _ => bug!("bad constructor {:?} for adt {:?}", self, adt)
         }
     }
@@ -567,7 +578,7 @@ impl<'tcx> Witness<'tcx> {
                             PatternKind::Variant {
                                 adt_def: adt,
                                 substs,
-                                variant_index: ctor.variant_index_for_adt(adt),
+                                variant_index: ctor.variant_index_for_adt(cx, adt),
                                 subpatterns: pats
                             }
                         } else {
@@ -1329,7 +1340,7 @@ fn pat_constructors<'tcx>(cx: &mut MatchCheckCtxt<'_, 'tcx>,
 ///
 /// For instance, a tuple pattern (_, 42, Some([])) has the arity of 3.
 /// A struct pattern's arity is the number of fields it contains, etc.
-fn constructor_arity(_cx: &MatchCheckCtxt, ctor: &Constructor, ty: Ty) -> u64 {
+fn constructor_arity(cx: &MatchCheckCtxt<'a, 'tcx>, ctor: &Constructor<'tcx>, ty: Ty<'tcx>) -> u64 {
     debug!("constructor_arity({:#?}, {:?})", ctor, ty);
     match ty.sty {
         ty::Tuple(ref fs) => fs.len() as u64,
@@ -1340,7 +1351,7 @@ fn constructor_arity(_cx: &MatchCheckCtxt, ctor: &Constructor, ty: Ty) -> u64 {
         },
         ty::Ref(..) => 1,
         ty::Adt(adt, _) => {
-            adt.variants[ctor.variant_index_for_adt(adt)].fields.len() as u64
+            adt.variants[ctor.variant_index_for_adt(cx, adt)].fields.len() as u64
         }
         _ => 0
     }
@@ -1351,7 +1362,7 @@ fn constructor_arity(_cx: &MatchCheckCtxt, ctor: &Constructor, ty: Ty) -> u64 {
 ///
 /// For instance, a tuple pattern (43u32, 'a') has sub pattern types [u32, char].
 fn constructor_sub_pattern_tys<'a, 'tcx: 'a>(cx: &MatchCheckCtxt<'a, 'tcx>,
-                                             ctor: &Constructor,
+                                             ctor: &Constructor<'tcx>,
                                              ty: Ty<'tcx>) -> Vec<Ty<'tcx>>
 {
     debug!("constructor_sub_pattern_tys({:#?}, {:?})", ctor, ty);
@@ -1368,7 +1379,7 @@ fn constructor_sub_pattern_tys<'a, 'tcx: 'a>(cx: &MatchCheckCtxt<'a, 'tcx>,
                 // Use T as the sub pattern type of Box<T>.
                 vec![substs.type_at(0)]
             } else {
-                adt.variants[ctor.variant_index_for_adt(adt)].fields.iter().map(|field| {
+                adt.variants[ctor.variant_index_for_adt(cx, adt)].fields.iter().map(|field| {
                     let is_visible = adt.is_enum()
                         || field.vis.is_accessible_from(cx.module, cx.tcx);
                     if is_visible {
diff --git a/src/librustc_mir/hair/pattern/mod.rs b/src/librustc_mir/hair/pattern/mod.rs
index 8991a90737c..cdaffe5d456 100644
--- a/src/librustc_mir/hair/pattern/mod.rs
+++ b/src/librustc_mir/hair/pattern/mod.rs
@@ -885,7 +885,7 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
         let adt_subpattern = |i, variant_opt| {
             let field = Field::new(i);
             let val = const_field(
-                self.tcx, self.param_env, instance,
+                self.tcx, self.param_env,
                 variant_opt, field, cv,
             ).expect("field access failed");
             self.const_to_pat(instance, val, id, span)
@@ -928,7 +928,7 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
             },
             ty::Adt(adt_def, substs) if adt_def.is_enum() => {
                 let variant_index = const_variant_index(
-                    self.tcx, self.param_env, instance, cv
+                    self.tcx, self.param_env, cv
                 ).expect("const_variant_index failed");
                 let subpatterns = adt_subpatterns(
                     adt_def.variants[variant_index].fields.len(),
diff --git a/src/librustc_mir/interpret/cast.rs b/src/librustc_mir/interpret/cast.rs
index 190a381cf52..c3b71be8354 100644
--- a/src/librustc_mir/interpret/cast.rs
+++ b/src/librustc_mir/interpret/cast.rs
@@ -109,11 +109,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
                 // The src operand does not matter, just its type
                 match src.layout.ty.sty {
                     ty::Closure(def_id, substs) => {
-                        let substs = self.tcx.subst_and_normalize_erasing_regions(
-                            self.substs(),
-                            ty::ParamEnv::reveal_all(),
-                            &substs,
-                        );
+                        let substs = self.subst_and_normalize_erasing_regions(substs)?;
                         let instance = ty::Instance::resolve_closure(
                             *self.tcx,
                             def_id,
diff --git a/src/librustc_mir/interpret/eval_context.rs b/src/librustc_mir/interpret/eval_context.rs
index b2d3328a73f..132b753eb9a 100644
--- a/src/librustc_mir/interpret/eval_context.rs
+++ b/src/librustc_mir/interpret/eval_context.rs
@@ -216,11 +216,21 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
         self.frame().mir
     }
 
-    pub fn substs(&self) -> &'tcx Substs<'tcx> {
-        if let Some(frame) = self.stack.last() {
-            frame.instance.substs
-        } else {
-            Substs::empty()
+    pub(super) fn subst_and_normalize_erasing_regions<T: TypeFoldable<'tcx>>(
+        &self,
+        substs: T,
+    ) -> EvalResult<'tcx, T> {
+        match self.stack.last() {
+            Some(frame) => Ok(self.tcx.subst_and_normalize_erasing_regions(
+                frame.instance.substs,
+                self.param_env,
+                &substs,
+            )),
+            None => if substs.needs_subst() {
+                err!(TooGeneric).into()
+            } else {
+                Ok(substs)
+            },
         }
     }
 
@@ -230,13 +240,9 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
         substs: &'tcx Substs<'tcx>
     ) -> EvalResult<'tcx, ty::Instance<'tcx>> {
         trace!("resolve: {:?}, {:#?}", def_id, substs);
-        trace!("substs: {:#?}", self.substs());
         trace!("param_env: {:#?}", self.param_env);
-        let substs = self.tcx.subst_and_normalize_erasing_regions(
-            self.substs(),
-            self.param_env,
-            &substs,
-        );
+        let substs = self.subst_and_normalize_erasing_regions(substs)?;
+        trace!("substs: {:#?}", substs);
         ty::Instance::resolve(
             *self.tcx,
             self.param_env,
@@ -276,7 +282,21 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
         }
     }
 
-    pub fn monomorphize<T: TypeFoldable<'tcx> + Subst<'tcx>>(
+    pub(super) fn monomorphize<T: TypeFoldable<'tcx> + Subst<'tcx>>(
+        &self,
+        t: T,
+    ) -> EvalResult<'tcx, T> {
+        match self.stack.last() {
+            Some(frame) => Ok(self.monomorphize_with_substs(t, frame.instance.substs)),
+            None => if t.needs_subst() {
+                err!(TooGeneric).into()
+            } else {
+                Ok(t)
+            },
+        }
+    }
+
+    fn monomorphize_with_substs<T: TypeFoldable<'tcx> + Subst<'tcx>>(
         &self,
         t: T,
         substs: &'tcx Substs<'tcx>
@@ -295,7 +315,7 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
         let cell = &frame.local_layouts[local];
         if cell.get().is_none() {
             let local_ty = frame.mir.local_decls[local].ty;
-            let local_ty = self.monomorphize(local_ty, frame.instance.substs);
+            let local_ty = self.monomorphize_with_substs(local_ty, frame.instance.substs);
             let layout = self.layout_of(local_ty)?;
             cell.set(Some(layout));
         }
diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index b2648480f20..8741571342f 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -508,7 +508,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
 
             Constant(ref constant) => {
                 let layout = from_known_layout(layout, || {
-                    let ty = self.monomorphize(mir_op.ty(self.mir(), *self.tcx), self.substs());
+                    let ty = self.monomorphize(mir_op.ty(self.mir(), *self.tcx))?;
                     self.layout_of(ty)
                 })?;
                 let op = self.const_value_to_op(*constant.literal)?;
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 962e0b7a742..f3a948a6ca3 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/src/librustc_mir/interpret/place.rs
@@ -9,6 +9,7 @@ use rustc::hir;
 use rustc::mir;
 use rustc::ty::{self, Ty};
 use rustc::ty::layout::{self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx};
+use rustc::ty::TypeFoldable;
 
 use super::{
     GlobalId, AllocId, Allocation, Scalar, EvalResult, Pointer, PointerArithmetic,
@@ -583,8 +584,8 @@ where
             }
 
             Static(ref static_) => {
-                let ty = self.monomorphize(static_.ty, self.substs());
-                let layout = self.layout_of(ty)?;
+                assert!(!static_.ty.needs_subst());
+                let layout = self.layout_of(static_.ty)?;
                 let instance = ty::Instance::mono(*self.tcx, static_.def_id);
                 let cid = GlobalId {
                     instance,
diff --git a/src/librustc_mir/interpret/step.rs b/src/librustc_mir/interpret/step.rs
index 2de8b3c1afd..25f3e4c1f77 100644
--- a/src/librustc_mir/interpret/step.rs
+++ b/src/librustc_mir/interpret/step.rs
@@ -248,7 +248,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
             }
 
             NullaryOp(mir::NullOp::SizeOf, ty) => {
-                let ty = self.monomorphize(ty, self.substs());
+                let ty = self.monomorphize(ty)?;
                 let layout = self.layout_of(ty)?;
                 assert!(!layout.is_unsized(),
                         "SizeOf nullary MIR operator called for unsized type");
@@ -260,7 +260,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
             }
 
             Cast(kind, ref operand, cast_ty) => {
-                debug_assert_eq!(self.monomorphize(cast_ty, self.substs()), dest.layout.ty);
+                debug_assert_eq!(self.monomorphize(cast_ty)?, dest.layout.ty);
                 let src = self.eval_operand(operand, None)?;
                 self.cast(src, kind, dest)?;
             }
diff --git a/src/librustc_mir/transform/const_prop.rs b/src/librustc_mir/transform/const_prop.rs
index 03d6d3868c9..dc556a15cd8 100644
--- a/src/librustc_mir/transform/const_prop.rs
+++ b/src/librustc_mir/transform/const_prop.rs
@@ -20,7 +20,7 @@ use rustc::ty::layout::{
 
 use interpret::{self, EvalContext, ScalarMaybeUndef, Immediate, OpTy, MemoryKind};
 use const_eval::{
-    CompileTimeInterpreter, error_to_const_error, eval_promoted, mk_borrowck_eval_cx,
+    CompileTimeInterpreter, error_to_const_error, eval_promoted, mk_eval_cx,
     lazy_const_to_op,
 };
 use transform::{MirPass, MirSource};
@@ -110,9 +110,7 @@ impl<'a, 'mir, 'tcx> ConstPropagator<'a, 'mir, 'tcx> {
         source: MirSource,
     ) -> ConstPropagator<'a, 'mir, 'tcx> {
         let param_env = tcx.param_env(source.def_id);
-        let substs = Substs::identity_for_item(tcx, source.def_id);
-        let instance = Instance::new(source.def_id, substs);
-        let ecx = mk_borrowck_eval_cx(tcx, instance, mir, DUMMY_SP).unwrap();
+        let ecx = mk_eval_cx(tcx, tcx.def_span(source.def_id), param_env);
         ConstPropagator {
             ecx,
             mir,
diff --git a/src/test/ui/consts/match_ice.rs b/src/test/ui/consts/match_ice.rs
new file mode 100644
index 00000000000..53c5782a4c7
--- /dev/null
+++ b/src/test/ui/consts/match_ice.rs
@@ -0,0 +1,10 @@
+// https://github.com/rust-lang/rust/issues/53708
+
+struct S;
+
+fn main() {
+    const C: &S = &S;
+    match C { //~ ERROR non-exhaustive
+        C => {} // this is a common bug around constants and references in patterns
+    }
+}
diff --git a/src/test/ui/consts/match_ice.stderr b/src/test/ui/consts/match_ice.stderr
new file mode 100644
index 00000000000..e6e04e2c462
--- /dev/null
+++ b/src/test/ui/consts/match_ice.stderr
@@ -0,0 +1,9 @@
+error[E0004]: non-exhaustive patterns: `&S` not covered
+  --> $DIR/match_ice.rs:7:11
+   |
+LL |     match C { //~ ERROR non-exhaustive
+   |           ^ pattern `&S` not covered
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0004`.