diff --git a/src/librustc/middle/mem_categorization.rs b/src/librustc/middle/mem_categorization.rs
index 524d5ddf929..723a4f39055 100644
--- a/src/librustc/middle/mem_categorization.rs
+++ b/src/librustc/middle/mem_categorization.rs
@@ -896,7 +896,7 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
 
         // Always promote `[T; 0]` (even when e.g., borrowed mutably).
         let promotable = match expr_ty.sty {
-            ty::Array(_, len) if len.assert_usize(self.tcx) == Some(0) => true,
+            ty::Array(_, len) if len.try_eval_usize(self.tcx) == Some(0) => true,
             _ => promotable,
         };
 
diff --git a/src/librustc/mir/tcx.rs b/src/librustc/mir/tcx.rs
index e2f5d192281..63f81aa97e7 100644
--- a/src/librustc/mir/tcx.rs
+++ b/src/librustc/mir/tcx.rs
@@ -90,7 +90,7 @@ impl<'tcx> PlaceTy<'tcx> {
             ProjectionElem::Subslice { from, to } => {
                 PlaceTy::from_ty(match self.ty.sty {
                     ty::Array(inner, size) => {
-                        let size = size.unwrap_usize(tcx);
+                        let size = size.eval_usize(tcx);
                         let len = size - (from as u64) - (to as u64);
                         tcx.mk_array(inner, len)
                     }
diff --git a/src/librustc/traits/error_reporting.rs b/src/librustc/traits/error_reporting.rs
index 14a288f5af1..04e3dad85ea 100644
--- a/src/librustc/traits/error_reporting.rs
+++ b/src/librustc/traits/error_reporting.rs
@@ -417,7 +417,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     Some(format!("[{}]", self.tcx.type_of(def.did).to_string())),
                 ));
                 let tcx = self.tcx;
-                if let Some(len) = len.assert_usize(tcx) {
+                if let Some(len) = len.try_eval_usize(tcx) {
                     flags.push((
                         sym::_Self,
                         Some(format!("[{}; {}]", self.tcx.type_of(def.did).to_string(), len)),
diff --git a/src/librustc/ty/error.rs b/src/librustc/ty/error.rs
index b8bdde4a787..46b6500e3cd 100644
--- a/src/librustc/ty/error.rs
+++ b/src/librustc/ty/error.rs
@@ -194,7 +194,7 @@ impl<'tcx> ty::TyS<'tcx> {
             ty::Foreign(def_id) => format!("extern type `{}`", tcx.def_path_str(def_id)).into(),
             ty::Array(_, n) => {
                 let n = tcx.lift_to_global(&n).unwrap();
-                match n.assert_usize(tcx) {
+                match n.try_eval_usize(tcx) {
                     Some(n) => format!("array of {} elements", n).into(),
                     None => "array".into(),
                 }
diff --git a/src/librustc/ty/inhabitedness/mod.rs b/src/librustc/ty/inhabitedness/mod.rs
index 0d96e5ea625..3aeae11685c 100644
--- a/src/librustc/ty/inhabitedness/mod.rs
+++ b/src/librustc/ty/inhabitedness/mod.rs
@@ -190,7 +190,7 @@ impl<'tcx> TyS<'tcx> {
                 }))
             }
 
-            Array(ty, len) => match len.assert_usize(tcx) {
+            Array(ty, len) => match len.try_eval_usize(tcx) {
                 // If the array is definitely non-empty, it's uninhabited if
                 // the type of its elements is uninhabited.
                 Some(n) if n != 0 => ty.uninhabited_from(tcx),
diff --git a/src/librustc/ty/layout.rs b/src/librustc/ty/layout.rs
index e0e70f41abe..7354ad241f9 100644
--- a/src/librustc/ty/layout.rs
+++ b/src/librustc/ty/layout.rs
@@ -594,7 +594,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     }
                 }
 
-                let count = count.assert_usize(tcx).ok_or(LayoutError::Unknown(ty))?;
+                let count = count.try_eval_usize(tcx).ok_or(LayoutError::Unknown(ty))?;
                 let element = self.layout_of(element)?;
                 let size = element.size.checked_mul(count, dl)
                     .ok_or(LayoutError::SizeOverflow(ty))?;
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index e71f8d94e7c..e2834cd92ec 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -2368,7 +2368,7 @@ impl<'tcx> AdtDef {
         match tcx.const_eval(param_env.and(cid)) {
             Ok(val) => {
                 // FIXME: Find the right type and use it instead of `val.ty` here
-                if let Some(b) = val.assert_bits(tcx.global_tcx(), val.ty) {
+                if let Some(b) = val.try_eval_bits(tcx.global_tcx(), val.ty) {
                     trace!("discriminants: {} ({:?})", b, repr_type);
                     Some(Discr {
                         val: b,
diff --git a/src/librustc/ty/print/obsolete.rs b/src/librustc/ty/print/obsolete.rs
index c12402a57cc..84c45898470 100644
--- a/src/librustc/ty/print/obsolete.rs
+++ b/src/librustc/ty/print/obsolete.rs
@@ -89,7 +89,7 @@ impl DefPathBasedNames<'tcx> {
             ty::Array(inner_type, len) => {
                 output.push('[');
                 self.push_type_name(inner_type, output, debug);
-                write!(output, "; {}", len.unwrap_usize(self.tcx)).unwrap();
+                write!(output, "; {}", len.eval_usize(self.tcx)).unwrap();
                 output.push(']');
             }
             ty::Slice(inner_type) => {
diff --git a/src/librustc/ty/print/pretty.rs b/src/librustc/ty/print/pretty.rs
index e889f2edef2..41f85942faa 100644
--- a/src/librustc/ty/print/pretty.rs
+++ b/src/librustc/ty/print/pretty.rs
@@ -696,7 +696,7 @@ pub trait PrettyPrinter<'tcx>:
             },
             ty::Array(ty, sz) => {
                 p!(write("["), print(ty), write("; "));
-                if let Some(n) = sz.assert_usize(self.tcx()) {
+                if let Some(n) = sz.try_eval_usize(self.tcx()) {
                     p!(write("{}", n));
                 } else {
                     p!(write("_"));
@@ -915,7 +915,7 @@ pub trait PrettyPrinter<'tcx>:
         if let ty::Ref(_, ref_ty, _) = ct.ty.sty {
             let byte_str = match (ct.val, &ref_ty.sty) {
                 (ConstValue::Scalar(Scalar::Ptr(ptr)), ty::Array(t, n)) if *t == u8 => {
-                    let n = n.unwrap_usize(self.tcx());
+                    let n = n.eval_usize(self.tcx());
                     Some(self.tcx()
                         .alloc_map.lock()
                         .unwrap_memory(ptr.alloc_id)
diff --git a/src/librustc/ty/relate.rs b/src/librustc/ty/relate.rs
index ca54f63b83a..25ff71cca86 100644
--- a/src/librustc/ty/relate.rs
+++ b/src/librustc/ty/relate.rs
@@ -466,7 +466,7 @@ pub fn super_relate_tys<R: TypeRelation<'tcx>>(
                 Err(err) => {
                     // Check whether the lengths are both concrete/known values,
                     // but are unequal, for better diagnostics.
-                    match (sz_a.assert_usize(tcx), sz_b.assert_usize(tcx)) {
+                    match (sz_a.try_eval_usize(tcx), sz_b.try_eval_usize(tcx)) {
                         (Some(sz_a_val), Some(sz_b_val)) => {
                             Err(TypeError::FixedArraySize(
                                 expected_found(relation, &sz_a_val, &sz_b_val)
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index 7849ae3d516..7bc903e8c4a 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -15,7 +15,7 @@ use crate::ty::{self, AdtDef, Discr, DefIdTree, TypeFlags, Ty, TyCtxt, TypeFolda
 use crate::ty::{List, TyS, ParamEnvAnd, ParamEnv};
 use crate::ty::layout::VariantIdx;
 use crate::util::captures::Captures;
-use crate::mir::interpret::Scalar;
+use crate::mir::interpret::{Scalar, GlobalId};
 
 use smallvec::SmallVec;
 use std::borrow::Cow;
@@ -1726,7 +1726,7 @@ impl<'tcx> TyS<'tcx> {
                 ty.expect_ty().conservative_is_privately_uninhabited(tcx)
             }),
             ty::Array(ty, len) => {
-                match len.assert_usize(tcx) {
+                match len.try_eval_usize(tcx) {
                     // If the array is definitely non-empty, it's uninhabited if
                     // the type of its elements is uninhabited.
                     Some(n) if n != 0 => ty.conservative_is_privately_uninhabited(tcx),
@@ -2291,16 +2291,32 @@ impl<'tcx> Const<'tcx> {
     }
 
     #[inline]
-    pub fn assert_bits(&self, tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Option<u128> {
+    pub fn try_eval_bits(&self, tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Option<u128> {
         assert_eq!(self.ty, ty);
-        let ty = tcx.lift_to_global(&ty).unwrap();
+        // FIXME(eddyb, oli-obk) get the right param_env.
         let size = tcx.layout_of(ParamEnv::empty().and(ty)).ok()?.size;
-        self.val.try_to_bits(size)
+        match self.val {
+            // FIXME(const_generics): this doesn't work right now,
+            // because it tries to relate an `Infer` to a `Param`.
+            ConstValue::Unevaluated(did, substs) => {
+                let substs = tcx.lift_to_global(&substs).unwrap();
+                let instance = ty::Instance::resolve(tcx, ParamEnv::empty(), did, substs)?;
+                let gid = GlobalId {
+                    instance,
+                    promoted: None,
+                };
+                let evaluated = tcx.const_eval(ParamEnv::empty().and(gid)).ok()?;
+                evaluated.val.try_to_bits(size)
+            },
+            // FIXME(const_generics): try to evaluate generic consts with a given param env?
+            // E.g. when you have an associated constant whose value depends on a generic const
+            _ => self.val.try_to_bits(size),
+        }
     }
 
     #[inline]
-    pub fn assert_bool(&self, tcx: TyCtxt<'tcx>) -> Option<bool> {
-        self.assert_bits(tcx, tcx.types.bool).and_then(|v| match v {
+    pub fn try_eval_bool(&self, tcx: TyCtxt<'tcx>) -> Option<bool> {
+        self.try_eval_bits(tcx, tcx.types.bool).and_then(|v| match v {
             0 => Some(false),
             1 => Some(true),
             _ => None,
@@ -2308,19 +2324,19 @@ impl<'tcx> Const<'tcx> {
     }
 
     #[inline]
-    pub fn assert_usize(&self, tcx: TyCtxt<'tcx>) -> Option<u64> {
-        self.assert_bits(tcx, tcx.types.usize).map(|v| v as u64)
+    pub fn try_eval_usize(&self, tcx: TyCtxt<'tcx>) -> Option<u64> {
+        self.try_eval_bits(tcx, tcx.types.usize).map(|v| v as u64)
     }
 
     #[inline]
-    pub fn unwrap_bits(&self, tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> u128 {
-        self.assert_bits(tcx, ty).unwrap_or_else(||
+    pub fn eval_bits(&self, tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> u128 {
+        self.try_eval_bits(tcx, ty).unwrap_or_else(||
             bug!("expected bits of {}, got {:#?}", ty, self))
     }
 
     #[inline]
-    pub fn unwrap_usize(&self, tcx: TyCtxt<'tcx>) -> u64 {
-        self.unwrap_bits(tcx, tcx.types.usize) as u64
+    pub fn eval_usize(&self, tcx: TyCtxt<'tcx>) -> u64 {
+        self.eval_bits(tcx, tcx.types.usize) as u64
     }
 }
 
diff --git a/src/librustc_codegen_llvm/debuginfo/metadata.rs b/src/librustc_codegen_llvm/debuginfo/metadata.rs
index f0bdb0018ef..f07f2dc3aa7 100644
--- a/src/librustc_codegen_llvm/debuginfo/metadata.rs
+++ b/src/librustc_codegen_llvm/debuginfo/metadata.rs
@@ -341,7 +341,7 @@ fn fixed_vec_metadata(
     let (size, align) = cx.size_and_align_of(array_or_slice_type);
 
     let upper_bound = match array_or_slice_type.sty {
-        ty::Array(_, len) => len.unwrap_usize(cx.tcx) as c_longlong,
+        ty::Array(_, len) => len.eval_usize(cx.tcx) as c_longlong,
         _ => -1
     };
 
diff --git a/src/librustc_codegen_ssa/base.rs b/src/librustc_codegen_ssa/base.rs
index fc04976f511..7db971bc4b8 100644
--- a/src/librustc_codegen_ssa/base.rs
+++ b/src/librustc_codegen_ssa/base.rs
@@ -132,7 +132,7 @@ pub fn unsized_info<'tcx, Cx: CodegenMethods<'tcx>>(
         cx.tcx().struct_lockstep_tails_erasing_lifetimes(source, target, cx.param_env());
     match (&source.sty, &target.sty) {
         (&ty::Array(_, len), &ty::Slice(_)) => {
-            cx.const_usize(len.unwrap_usize(cx.tcx()))
+            cx.const_usize(len.eval_usize(cx.tcx()))
         }
         (&ty::Dynamic(..), &ty::Dynamic(..)) => {
             // For now, upcasts are limited to changes in marker
diff --git a/src/librustc_codegen_ssa/debuginfo/type_names.rs b/src/librustc_codegen_ssa/debuginfo/type_names.rs
index 8f0bb6ee198..0c1def33ad3 100644
--- a/src/librustc_codegen_ssa/debuginfo/type_names.rs
+++ b/src/librustc_codegen_ssa/debuginfo/type_names.rs
@@ -89,7 +89,7 @@ pub fn push_debuginfo_type_name<'tcx>(
         ty::Array(inner_type, len) => {
             output.push('[');
             push_debuginfo_type_name(tcx, inner_type, true, output, visited);
-            output.push_str(&format!("; {}", len.unwrap_usize(tcx)));
+            output.push_str(&format!("; {}", len.eval_usize(tcx)));
             output.push(']');
         },
         ty::Slice(inner_type) => {
diff --git a/src/librustc_codegen_ssa/mir/constant.rs b/src/librustc_codegen_ssa/mir/constant.rs
index d6951b923bf..a51deafa9d7 100644
--- a/src/librustc_codegen_ssa/mir/constant.rs
+++ b/src/librustc_codegen_ssa/mir/constant.rs
@@ -41,7 +41,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             .map(|c| {
                 let field_ty = c.ty.builtin_index().unwrap();
                 let fields = match c.ty.sty {
-                    ty::Array(_, n) => n.unwrap_usize(bx.tcx()),
+                    ty::Array(_, n) => n.eval_usize(bx.tcx()),
                     _ => bug!("invalid simd shuffle type: {}", c.ty),
                 };
                 let values: Vec<_> = (0..fields).map(|field| {
diff --git a/src/librustc_codegen_ssa/mir/rvalue.rs b/src/librustc_codegen_ssa/mir/rvalue.rs
index 202cf147f1f..40f2a000289 100644
--- a/src/librustc_codegen_ssa/mir/rvalue.rs
+++ b/src/librustc_codegen_ssa/mir/rvalue.rs
@@ -521,7 +521,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         } = *place {
             if let LocalRef::Operand(Some(op)) = self.locals[index] {
                 if let ty::Array(_, n) = op.layout.ty.sty {
-                    let n = n.unwrap_usize(bx.cx().tcx());
+                    let n = n.eval_usize(bx.cx().tcx());
                     return bx.cx().const_usize(n);
                 }
             }
diff --git a/src/librustc_codegen_utils/symbol_names/v0.rs b/src/librustc_codegen_utils/symbol_names/v0.rs
index 8a54fb6bbc4..cce228ff63b 100644
--- a/src/librustc_codegen_utils/symbol_names/v0.rs
+++ b/src/librustc_codegen_utils/symbol_names/v0.rs
@@ -512,7 +512,7 @@ impl Printer<'tcx> for SymbolMangler<'tcx> {
         }
         self = ct.ty.print(self)?;
 
-        if let Some(bits) = ct.assert_bits(self.tcx, ty::ParamEnv::empty().and(ct.ty)) {
+        if let Some(bits) = ct.try_eval_bits(self.tcx, ct.ty) {
             let _ = write!(self.out, "{:x}_", bits);
         } else {
             // NOTE(eddyb) despite having the path, we need to
diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index 4cccaa942b7..c717d269b58 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -208,7 +208,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
                     }
                     has_emitted
                 }
-                ty::Array(ty, len) => match len.assert_usize(cx.tcx) {
+                ty::Array(ty, len) => match len.try_eval_usize(cx.tcx) {
                     // If the array is definitely non-empty, we can do `#[must_use]` checking.
                     Some(n) if n != 0 => {
                         let descr_pre = &format!(
diff --git a/src/librustc_mir/borrow_check/nll/type_check/mod.rs b/src/librustc_mir/borrow_check/nll/type_check/mod.rs
index b27be43dda2..7cfb8f552ad 100644
--- a/src/librustc_mir/borrow_check/nll/type_check/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/type_check/mod.rs
@@ -669,7 +669,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
             ProjectionElem::Subslice { from, to } => PlaceTy::from_ty(
                 match base_ty.sty {
                     ty::Array(inner, size) => {
-                        let size = size.unwrap_usize(tcx);
+                        let size = size.eval_usize(tcx);
                         let min_size = (from as u64) + (to as u64);
                         if let Some(rest_size) = size.checked_sub(min_size) {
                             tcx.mk_array(inner, rest_size)
diff --git a/src/librustc_mir/borrow_check/places_conflict.rs b/src/librustc_mir/borrow_check/places_conflict.rs
index b2a03147ecf..631e7e0309a 100644
--- a/src/librustc_mir/borrow_check/places_conflict.rs
+++ b/src/librustc_mir/borrow_check/places_conflict.rs
@@ -339,7 +339,7 @@ fn place_base_conflict<'tcx>(
                 (StaticKind::Promoted(promoted_1), StaticKind::Promoted(promoted_2)) => {
                     if promoted_1 == promoted_2 {
                         if let ty::Array(_, len) = s1.ty.sty {
-                            if let Some(0) = len.assert_usize(tcx) {
+                            if let Some(0) = len.try_eval_usize(tcx) {
                                 // Ignore conflicts with promoted [T; 0].
                                 debug!("place_element_conflict: IGNORE-LEN-0-PROMOTED");
                                 return Overlap::Disjoint;
diff --git a/src/librustc_mir/build/matches/test.rs b/src/librustc_mir/build/matches/test.rs
index 45efbfe9b1a..930b0c93082 100644
--- a/src/librustc_mir/build/matches/test.rs
+++ b/src/librustc_mir/build/matches/test.rs
@@ -111,7 +111,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             PatternKind::Constant { value } => {
                 indices.entry(value)
                        .or_insert_with(|| {
-                           options.push(value.unwrap_bits(self.hir.tcx(), switch_ty));
+                           options.push(value.eval_bits(self.hir.tcx(), switch_ty));
                            options.len() - 1
                        });
                 true
diff --git a/src/librustc_mir/hair/cx/expr.rs b/src/librustc_mir/hair/cx/expr.rs
index 242afcf5abb..caa77f2eb0e 100644
--- a/src/librustc_mir/hair/cx/expr.rs
+++ b/src/librustc_mir/hair/cx/expr.rs
@@ -556,7 +556,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(
             };
             let span = cx.tcx.def_span(def_id);
             let count = match cx.tcx.at(span).const_eval(cx.param_env.and(global_id)) {
-                Ok(cv) => cv.unwrap_usize(cx.tcx),
+                Ok(cv) => cv.eval_usize(cx.tcx),
                 Err(ErrorHandled::Reported) => 0,
                 Err(ErrorHandled::TooGeneric) => {
                     cx.tcx.sess.span_err(span, "array lengths can't depend on generic parameters");
diff --git a/src/librustc_mir/hair/pattern/_match.rs b/src/librustc_mir/hair/pattern/_match.rs
index 8baefc3162b..2ef1b76979b 100644
--- a/src/librustc_mir/hair/pattern/_match.rs
+++ b/src/librustc_mir/hair/pattern/_match.rs
@@ -228,7 +228,7 @@ impl LiteralExpander<'tcx> {
                 ConstValue::Slice {
                     data: self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id),
                     start: p.offset.bytes().try_into().unwrap(),
-                    end: n.unwrap_usize(self.tcx).try_into().unwrap(),
+                    end: n.eval_usize(self.tcx).try_into().unwrap(),
                 }
             },
             // fat pointers stay the same
@@ -646,8 +646,8 @@ fn all_constructors<'a, 'tcx>(
                 ConstantValue(ty::Const::from_bool(cx.tcx, b))
             }).collect()
         }
-        ty::Array(ref sub_ty, len) if len.assert_usize(cx.tcx).is_some() => {
-            let len = len.unwrap_usize(cx.tcx);
+        ty::Array(ref sub_ty, len) if len.try_eval_usize(cx.tcx).is_some() => {
+            let len = len.eval_usize(cx.tcx);
             if len != 0 && cx.is_uninhabited(sub_ty) {
                 vec![]
             } else {
@@ -789,7 +789,7 @@ where
                 match (value.val, &value.ty.sty) {
                     (_, ty::Array(_, n)) => max_fixed_len = cmp::max(
                         max_fixed_len,
-                        n.unwrap_usize(cx.tcx),
+                        n.eval_usize(cx.tcx),
                     ),
                     (ConstValue::Slice{ start, end, .. }, ty::Slice(_)) => max_fixed_len = cmp::max(
                         max_fixed_len,
@@ -856,7 +856,7 @@ impl<'tcx> IntRange<'tcx> {
             }
             ConstantValue(val) if is_integral(val.ty) => {
                 let ty = val.ty;
-                if let Some(val) = val.assert_bits(tcx, ty) {
+                if let Some(val) = val.try_eval_bits(tcx, ty) {
                     let bias = IntRange::signed_bias(tcx, ty);
                     let val = val ^ bias;
                     Some(IntRange { range: val..=val, ty })
@@ -873,8 +873,8 @@ impl<'tcx> IntRange<'tcx> {
             match pat.kind {
                 box PatternKind::Constant { value } => break ConstantValue(value),
                 box PatternKind::Range(PatternRange { lo, hi, ty, end }) => break ConstantRange(
-                    lo.unwrap_bits(tcx, ty),
-                    hi.unwrap_bits(tcx, ty),
+                    lo.eval_bits(tcx, ty),
+                    hi.eval_bits(tcx, ty),
                     ty,
                     end,
                 ),
@@ -1327,14 +1327,14 @@ fn pat_constructors<'tcx>(cx: &mut MatchCheckCtxt<'_, 'tcx>,
         PatternKind::Constant { value } => Some(vec![ConstantValue(value)]),
         PatternKind::Range(PatternRange { lo, hi, ty, end }) =>
             Some(vec![ConstantRange(
-                lo.unwrap_bits(cx.tcx, ty),
-                hi.unwrap_bits(cx.tcx, ty),
+                lo.eval_bits(cx.tcx, ty),
+                hi.eval_bits(cx.tcx, ty),
                 ty,
                 end,
             )]),
         PatternKind::Array { .. } => match pcx.ty.sty {
             ty::Array(_, length) => Some(vec![
-                Slice(length.unwrap_usize(cx.tcx))
+                Slice(length.eval_usize(cx.tcx))
             ]),
             _ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty)
         },
@@ -1402,7 +1402,7 @@ fn constructor_sub_pattern_tys<'a, 'tcx>(
                         match ty.sty {
                             // If the field type returned is an array of an unknown
                             // size return an TyErr.
-                            ty::Array(_, len) if len.assert_usize(cx.tcx).is_none() =>
+                            ty::Array(_, len) if len.try_eval_usize(cx.tcx).is_none() =>
                                 cx.tcx.types.err,
                             _ => ty,
                         }
@@ -1436,7 +1436,7 @@ fn slice_pat_covered_by_const<'tcx>(
     let data: &[u8] = match (const_val.val, &const_val.ty.sty) {
         (ConstValue::ByRef { offset, alloc, .. }, ty::Array(t, n)) => {
             assert_eq!(*t, tcx.types.u8);
-            let n = n.assert_usize(tcx).unwrap();
+            let n = n.eval_usize(tcx);
             let ptr = Pointer::new(AllocId(0), offset);
             alloc.get_bytes(&tcx, ptr, Size::from_bytes(n)).unwrap()
         },
@@ -1464,7 +1464,7 @@ fn slice_pat_covered_by_const<'tcx>(
     {
         match pat.kind {
             box PatternKind::Constant { value } => {
-                let b = value.unwrap_bits(tcx, pat.ty);
+                let b = value.eval_bits(tcx, pat.ty);
                 assert_eq!(b as u8 as u128, b);
                 if b as u8 != *ch {
                     return Ok(false);
@@ -1760,7 +1760,7 @@ fn specialize<'p, 'a: 'p, 'tcx>(
                                 ConstValue::ByRef { offset, alloc, .. } => (
                                     alloc,
                                     offset,
-                                    n.unwrap_usize(cx.tcx),
+                                    n.eval_usize(cx.tcx),
                                     t,
                                 ),
                                 _ => span_bug!(
diff --git a/src/librustc_mir/hair/pattern/mod.rs b/src/librustc_mir/hair/pattern/mod.rs
index 745266a6485..0614b0b2993 100644
--- a/src/librustc_mir/hair/pattern/mod.rs
+++ b/src/librustc_mir/hair/pattern/mod.rs
@@ -728,7 +728,7 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
 
             ty::Array(_, len) => {
                 // fixed-length array
-                let len = len.unwrap_usize(self.tcx);
+                let len = len.eval_usize(self.tcx);
                 assert!(len >= prefix.len() as u64 + suffix.len() as u64);
                 PatternKind::Array { prefix: prefix, slice: slice, suffix: suffix }
             }
@@ -1123,7 +1123,7 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
             }
             ty::Array(_, n) => {
                 PatternKind::Array {
-                    prefix: (0..n.unwrap_usize(self.tcx))
+                    prefix: (0..n.eval_usize(self.tcx))
                         .map(|i| adt_subpattern(i as usize, None))
                         .collect(),
                     slice: None,
@@ -1206,7 +1206,7 @@ fn search_for_adt_without_structural_match<'tcx>(tcx: TyCtxt<'tcx>,
                     // (But still tell caller to continue search.)
                     return false;
                 }
-                ty::Array(_, n) if n.assert_usize(self.tcx) == Some(0) => {
+                ty::Array(_, n) if n.try_eval_usize(self.tcx) == Some(0) => {
                     // rust-lang/rust#62336: ignore type of contents
                     // for empty array.
                     return false;
@@ -1470,7 +1470,7 @@ pub fn compare_const_vals<'tcx>(
         return fallback();
     }
 
-    if let (Some(a), Some(b)) = (a.assert_bits(tcx, ty), b.assert_bits(tcx, ty)) {
+    if let (Some(a), Some(b)) = (a.try_eval_bits(tcx, ty), b.try_eval_bits(tcx, ty)) {
         use ::rustc_apfloat::Float;
         return match ty.sty {
             ty::Float(ast::FloatTy::F32) => {
diff --git a/src/librustc_mir/interpret/cast.rs b/src/librustc_mir/interpret/cast.rs
index 8e3450656ab..5efcd1cbeeb 100644
--- a/src/librustc_mir/interpret/cast.rs
+++ b/src/librustc_mir/interpret/cast.rs
@@ -253,7 +253,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 // u64 cast is from usize to u64, which is always good
                 let val = Immediate::new_slice(
                     ptr,
-                    length.unwrap_usize(self.tcx.tcx),
+                    length.eval_usize(self.tcx.tcx),
                     self,
                 );
                 self.write_immediate(val, dest)
diff --git a/src/librustc_mir/interpret/intern.rs b/src/librustc_mir/interpret/intern.rs
index e9bba788911..e615645bb2a 100644
--- a/src/librustc_mir/interpret/intern.rs
+++ b/src/librustc_mir/interpret/intern.rs
@@ -176,7 +176,7 @@ for
                     (InternMode::ConstBase, hir::Mutability::MutMutable) |
                     (InternMode::Const, hir::Mutability::MutMutable) => {
                         match referenced_ty.sty {
-                            ty::Array(_, n) if n.unwrap_usize(self.ecx.tcx.tcx) == 0 => {}
+                            ty::Array(_, n) if n.eval_usize(self.ecx.tcx.tcx) == 0 => {}
                             ty::Slice(_)
                                 if value.to_meta().unwrap().unwrap().to_usize(self.ecx)? == 0 => {}
                             _ => bug!("const qualif failed to prevent mutable references"),
diff --git a/src/librustc_mir/shim.rs b/src/librustc_mir/shim.rs
index 32b964dbadf..5752a8ccec0 100644
--- a/src/librustc_mir/shim.rs
+++ b/src/librustc_mir/shim.rs
@@ -313,7 +313,7 @@ fn build_clone_shim<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, self_ty: Ty<'tcx>) -
     match self_ty.sty {
         _ if is_copy => builder.copy_shim(),
         ty::Array(ty, len) => {
-            let len = len.unwrap_usize(tcx);
+            let len = len.eval_usize(tcx);
             builder.array_shim(dest, src, ty, len)
         }
         ty::Closure(def_id, substs) => {
diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs
index ffeaf4e19c2..69ad4126077 100644
--- a/src/librustc_mir/transform/qualify_consts.rs
+++ b/src/librustc_mir/transform/qualify_consts.rs
@@ -367,7 +367,7 @@ impl Qualif for HasMutInterior {
                     } else if let ty::Array(_, len) = ty.sty {
                         // FIXME(eddyb) the `cx.mode == Mode::NonConstFn` condition
                         // seems unnecessary, given that this is merely a ZST.
-                        match len.assert_usize(cx.tcx) {
+                        match len.try_eval_usize(cx.tcx) {
                             Some(0) if cx.mode == Mode::NonConstFn => {},
                             _ => return true,
                         }
diff --git a/src/librustc_mir/transform/simplify_branches.rs b/src/librustc_mir/transform/simplify_branches.rs
index dd45595a4fe..160751aac59 100644
--- a/src/librustc_mir/transform/simplify_branches.rs
+++ b/src/librustc_mir/transform/simplify_branches.rs
@@ -26,7 +26,7 @@ impl MirPass for SimplifyBranches {
                 TerminatorKind::SwitchInt {
                     discr: Operand::Constant(ref c), switch_ty, ref values, ref targets, ..
                 } => {
-                    let constant = c.literal.assert_bits(tcx, switch_ty);
+                    let constant = c.literal.try_eval_bits(tcx, switch_ty);
                     if let Some(constant) = constant {
                         let (otherwise, targets) = targets.split_last().unwrap();
                         let mut ret = TerminatorKind::Goto { target: *otherwise };
@@ -43,7 +43,7 @@ impl MirPass for SimplifyBranches {
                 },
                 TerminatorKind::Assert {
                     target, cond: Operand::Constant(ref c), expected, ..
-                } if (c.literal.assert_bool(tcx) == Some(true)) == expected =>
+                } if (c.literal.try_eval_bool(tcx) == Some(true)) == expected =>
                     TerminatorKind::Goto { target },
                 TerminatorKind::FalseEdges { real_target, .. } => {
                     TerminatorKind::Goto { target: real_target }
diff --git a/src/librustc_mir/transform/uniform_array_move_out.rs b/src/librustc_mir/transform/uniform_array_move_out.rs
index 6aceeebaea1..2ad6eaa3ed9 100644
--- a/src/librustc_mir/transform/uniform_array_move_out.rs
+++ b/src/librustc_mir/transform/uniform_array_move_out.rs
@@ -68,7 +68,7 @@ impl<'a, 'tcx> Visitor<'tcx> for UniformArrayMoveOutVisitor<'a, 'tcx> {
                     let place_ty =
                         Place::ty_from(&src_place.base, &proj.base, self.body, self.tcx).ty;
                     if let ty::Array(item_ty, const_size) = place_ty.sty {
-                        if let Some(size) = const_size.assert_usize(self.tcx) {
+                        if let Some(size) = const_size.try_eval_usize(self.tcx) {
                             assert!(size <= u32::max_value() as u64,
                                     "uniform array move out doesn't supported
                                      for array bigger then u32");
@@ -219,7 +219,7 @@ impl MirPass for RestoreSubsliceArrayMoveOut {
                             let src_ty =
                                 Place::ty_from(src_place.base, src_place.projection, body, tcx).ty;
                             if let ty::Array(_, ref size_o) = src_ty.sty {
-                                size_o.assert_usize(tcx)
+                                size_o.try_eval_usize(tcx)
                             } else {
                                 None
                             }
diff --git a/src/librustc_mir/util/elaborate_drops.rs b/src/librustc_mir/util/elaborate_drops.rs
index 61ad2ba8f57..42f90b2ae40 100644
--- a/src/librustc_mir/util/elaborate_drops.rs
+++ b/src/librustc_mir/util/elaborate_drops.rs
@@ -821,7 +821,7 @@ where
                 self.complete_drop(Some(DropFlagMode::Deep), succ, unwind)
             }
             ty::Array(ety, size) => {
-                let size = size.assert_usize(self.tcx());
+                let size = size.try_eval_usize(self.tcx());
                 self.open_drop_for_array(ety, size)
             },
             ty::Slice(ety) => self.open_drop_for_array(ety, None),
diff --git a/src/librustc_typeck/check/_match.rs b/src/librustc_typeck/check/_match.rs
index e4dda20c822..9ae4e99ca6e 100644
--- a/src/librustc_typeck/check/_match.rs
+++ b/src/librustc_typeck/check/_match.rs
@@ -423,7 +423,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let expected_ty = self.structurally_resolved_type(pat.span, expected);
                 let (inner_ty, slice_ty) = match expected_ty.sty {
                     ty::Array(inner_ty, size) => {
-                        if let Some(size) = size.assert_usize(tcx) {
+                        if let Some(size) = size.try_eval_usize(tcx) {
                             let min_len = before.len() as u64 + after.len() as u64;
                             if slice.is_none() {
                                 if min_len != size {
diff --git a/src/librustc_typeck/check/expr.rs b/src/librustc_typeck/check/expr.rs
index 8d2160c0ec7..845022f506a 100644
--- a/src/librustc_typeck/check/expr.rs
+++ b/src/librustc_typeck/check/expr.rs
@@ -1386,7 +1386,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     }
                     ty::Array(_, len) => {
                         if let (Some(len), Ok(user_index)) = (
-                            len.assert_usize(self.tcx),
+                            len.try_eval_usize(self.tcx),
                             field.as_str().parse::<u64>()
                         ) {
                             let base = self.tcx.sess.source_map()