Rollup merge of #104564 - RalfJung:either, r=oli-obk

interpret: use Either over Result when it is not representing an error condition

r? `@oli-obk`

17 files changed, 128 insertions, 102 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 267fa69485c..fb6140e74fe 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3433,6 +3433,7 @@ dependencies = [
 name = "rustc_const_eval"
 version = "0.0.0"
 dependencies = [
+ "either",
  "rustc_apfloat",
  "rustc_ast",
  "rustc_attr",
@@ -3927,6 +3928,7 @@ dependencies = [
 name = "rustc_mir_build"
 version = "0.0.0"
 dependencies = [
+ "either",
  "rustc_apfloat",
  "rustc_arena",
  "rustc_ast",
@@ -3973,6 +3975,7 @@ name = "rustc_mir_transform"
 version = "0.0.0"
 dependencies = [
  "coverage_test_macros",
+ "either",
  "itertools",
  "rustc_ast",
  "rustc_attr",
diff --git a/compiler/rustc_const_eval/Cargo.toml b/compiler/rustc_const_eval/Cargo.toml
index e09a6d1d6f5..51489e29360 100644
--- a/compiler/rustc_const_eval/Cargo.toml
+++ b/compiler/rustc_const_eval/Cargo.toml
@@ -7,6 +7,7 @@ edition = "2021"
 
 [dependencies]
 tracing = "0.1"
+either = "1"
 rustc_apfloat = { path = "../rustc_apfloat" }
 rustc_ast = { path = "../rustc_ast" }
 rustc_attr = { path = "../rustc_attr" }
diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
index f5942deaf9f..c777a840f3f 100644
--- a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -1,10 +1,7 @@
-use super::{CompileTimeEvalContext, CompileTimeInterpreter, ConstEvalErr};
-use crate::interpret::eval_nullary_intrinsic;
-use crate::interpret::{
-    intern_const_alloc_recursive, Allocation, ConstAlloc, ConstValue, CtfeValidationMode, GlobalId,
-    Immediate, InternKind, InterpCx, InterpError, InterpResult, MPlaceTy, MemoryKind, OpTy,
-    RefTracking, StackPopCleanup,
-};
+use std::borrow::Cow;
+use std::convert::TryInto;
+
+use either::{Left, Right};
 
 use rustc_hir::def::DefKind;
 use rustc_middle::mir;
@@ -16,8 +13,14 @@ use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::source_map::Span;
 use rustc_target::abi::{self, Abi};
-use std::borrow::Cow;
-use std::convert::TryInto;
+
+use super::{CompileTimeEvalContext, CompileTimeInterpreter, ConstEvalErr};
+use crate::interpret::eval_nullary_intrinsic;
+use crate::interpret::{
+    intern_const_alloc_recursive, Allocation, ConstAlloc, ConstValue, CtfeValidationMode, GlobalId,
+    Immediate, InternKind, InterpCx, InterpError, InterpResult, MPlaceTy, MemoryKind, OpTy,
+    RefTracking, StackPopCleanup,
+};
 
 const NOTE_ON_UNDEFINED_BEHAVIOR_ERROR: &str = "The rules on what exactly is undefined behavior aren't clear, \
      so this check might be overzealous. Please open an issue on the rustc \
@@ -135,14 +138,14 @@ pub(super) fn op_to_const<'tcx>(
         _ => false,
     };
     let immediate = if try_as_immediate {
-        Err(ecx.read_immediate(op).expect("normalization works on validated constants"))
+        Right(ecx.read_immediate(op).expect("normalization works on validated constants"))
     } else {
         // It is guaranteed that any non-slice scalar pair is actually ByRef here.
         // When we come back from raw const eval, we are always by-ref. The only way our op here is
         // by-val is if we are in destructure_mir_constant, i.e., if this is (a field of) something that we
         // "tried to make immediate" before. We wouldn't do that for non-slice scalar pairs or
         // structs containing such.
-        op.try_as_mplace()
+        op.as_mplace_or_imm()
     };
 
     debug!(?immediate);
@@ -168,9 +171,9 @@ pub(super) fn op_to_const<'tcx>(
         }
     };
     match immediate {
-        Ok(ref mplace) => to_const_value(mplace),
+        Left(ref mplace) => to_const_value(mplace),
         // see comment on `let try_as_immediate` above
-        Err(imm) => match *imm {
+        Right(imm) => match *imm {
             _ if imm.layout.is_zst() => ConstValue::ZeroSized,
             Immediate::Scalar(x) => ConstValue::Scalar(x),
             Immediate::ScalarPair(a, b) => {
diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index f59f34b874c..79450fccfc4 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -2,6 +2,8 @@ use std::cell::Cell;
 use std::fmt;
 use std::mem;
 
+use either::{Either, Left, Right};
+
 use rustc_hir::{self as hir, def_id::DefId, definitions::DefPathData};
 use rustc_index::vec::IndexVec;
 use rustc_middle::mir;
@@ -121,13 +123,12 @@ pub struct Frame<'mir, 'tcx, Prov: Provenance = AllocId, Extra = ()> {
     ////////////////////////////////////////////////////////////////////////////////
     // Current position within the function
     ////////////////////////////////////////////////////////////////////////////////
-    /// If this is `Err`, we are not currently executing any particular statement in
+    /// If this is `Right`, we are not currently executing any particular statement in
     /// this frame (can happen e.g. during frame initialization, and during unwinding on
     /// frames without cleanup code).
-    /// We basically abuse `Result` as `Either`.
     ///
     /// Needs to be public because ConstProp does unspeakable things to it.
-    pub loc: Result<mir::Location, Span>,
+    pub loc: Either<mir::Location, Span>,
 }
 
 /// What we store about a frame in an interpreter backtrace.
@@ -227,25 +228,24 @@ impl<'mir, 'tcx, Prov: Provenance> Frame<'mir, 'tcx, Prov> {
 impl<'mir, 'tcx, Prov: Provenance, Extra> Frame<'mir, 'tcx, Prov, Extra> {
     /// Get the current location within the Frame.
     ///
-    /// If this is `Err`, we are not currently executing any particular statement in
+    /// If this is `Left`, we are not currently executing any particular statement in
     /// this frame (can happen e.g. during frame initialization, and during unwinding on
     /// frames without cleanup code).
-    /// We basically abuse `Result` as `Either`.
     ///
     /// Used by priroda.
-    pub fn current_loc(&self) -> Result<mir::Location, Span> {
+    pub fn current_loc(&self) -> Either<mir::Location, Span> {
         self.loc
     }
 
     /// Return the `SourceInfo` of the current instruction.
     pub fn current_source_info(&self) -> Option<&mir::SourceInfo> {
-        self.loc.ok().map(|loc| self.body.source_info(loc))
+        self.loc.left().map(|loc| self.body.source_info(loc))
     }
 
     pub fn current_span(&self) -> Span {
         match self.loc {
-            Ok(loc) => self.body.source_info(loc).span,
-            Err(span) => span,
+            Left(loc) => self.body.source_info(loc).span,
+            Right(span) => span,
         }
     }
 }
@@ -679,7 +679,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // first push a stack frame so we have access to the local substs
         let pre_frame = Frame {
             body,
-            loc: Err(body.span), // Span used for errors caused during preamble.
+            loc: Right(body.span), // Span used for errors caused during preamble.
             return_to_block,
             return_place: return_place.clone(),
             // empty local array, we fill it in below, after we are inside the stack frame and
@@ -713,7 +713,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // done
         self.frame_mut().locals = locals;
         M::after_stack_push(self)?;
-        self.frame_mut().loc = Ok(mir::Location::START);
+        self.frame_mut().loc = Left(mir::Location::START);
 
         let span = info_span!("frame", "{}", instance);
         self.frame_mut().tracing_span.enter(span);
@@ -724,7 +724,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// Jump to the given block.
     #[inline]
     pub fn go_to_block(&mut self, target: mir::BasicBlock) {
-        self.frame_mut().loc = Ok(mir::Location { block: target, statement_index: 0 });
+        self.frame_mut().loc = Left(mir::Location { block: target, statement_index: 0 });
     }
 
     /// *Return* to the given `target` basic block.
@@ -750,8 +750,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// unwinding, and doing so is UB.
     pub fn unwind_to_block(&mut self, target: StackPopUnwind) -> InterpResult<'tcx> {
         self.frame_mut().loc = match target {
-            StackPopUnwind::Cleanup(block) => Ok(mir::Location { block, statement_index: 0 }),
-            StackPopUnwind::Skip => Err(self.frame_mut().body.span),
+            StackPopUnwind::Cleanup(block) => Left(mir::Location { block, statement_index: 0 }),
+            StackPopUnwind::Skip => Right(self.frame_mut().body.span),
             StackPopUnwind::NotAllowed => {
                 throw_ub_format!("unwinding past a stack frame that does not allow unwinding")
             }
@@ -783,8 +783,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         assert_eq!(
             unwinding,
             match self.frame().loc {
-                Ok(loc) => self.body().basic_blocks[loc.block].is_cleanup,
-                Err(_) => true,
+                Left(loc) => self.body().basic_blocks[loc.block].is_cleanup,
+                Right(_) => true,
             }
         );
         if unwinding && self.frame_idx() == 0 {
diff --git a/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs b/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs
index 0e3867557ad..7d94a22c43d 100644
--- a/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs
+++ b/compiler/rustc_const_eval/src/interpret/intrinsics/caller_location.rs
@@ -19,8 +19,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             debug!("find_closest_untracked_caller_location: checking frame {:?}", frame.instance);
 
             // Assert that the frame we look at is actually executing code currently
-            // (`loc` is `Err` when we are unwinding and the frame does not require cleanup).
-            let loc = frame.loc.unwrap();
+            // (`loc` is `Right` when we are unwinding and the frame does not require cleanup).
+            let loc = frame.loc.left().unwrap();
 
             // This could be a non-`Call` terminator (such as `Drop`), or not a terminator at all
             // (such as `box`). Use the normal span by default.
diff --git a/compiler/rustc_const_eval/src/interpret/operand.rs b/compiler/rustc_const_eval/src/interpret/operand.rs
index cf9202540c7..3eb2b3a0b1b 100644
--- a/compiler/rustc_const_eval/src/interpret/operand.rs
+++ b/compiler/rustc_const_eval/src/interpret/operand.rs
@@ -1,6 +1,8 @@
 //! Functions concerning immediate values and operands, and reading from operands.
 //! All high-level functions to read from memory work on operands as sources.
 
+use either::{Either, Left, Right};
+
 use rustc_hir::def::Namespace;
 use rustc_middle::ty::layout::{LayoutOf, PrimitiveExt, TyAndLayout};
 use rustc_middle::ty::print::{FmtPrinter, PrettyPrinter};
@@ -261,9 +263,9 @@ impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
         layout: TyAndLayout<'tcx>,
         cx: &impl HasDataLayout,
     ) -> InterpResult<'tcx, Self> {
-        match self.try_as_mplace() {
-            Ok(mplace) => Ok(mplace.offset_with_meta(offset, meta, layout, cx)?.into()),
-            Err(imm) => {
+        match self.as_mplace_or_imm() {
+            Left(mplace) => Ok(mplace.offset_with_meta(offset, meta, layout, cx)?.into()),
+            Right(imm) => {
                 assert!(
                     matches!(*imm, Immediate::Uninit),
                     "Scalar/ScalarPair cannot be offset into"
@@ -353,8 +355,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
     /// Try returning an immediate for the operand. If the layout does not permit loading this as an
     /// immediate, return where in memory we can find the data.
-    /// Note that for a given layout, this operation will either always fail or always
-    /// succeed!  Whether it succeeds depends on whether the layout can be represented
+    /// Note that for a given layout, this operation will either always return Left or Right!
+    /// succeed!  Whether it returns Left depends on whether the layout can be represented
     /// in an `Immediate`, not on which data is stored there currently.
     ///
     /// This is an internal function that should not usually be used; call `read_immediate` instead.
@@ -362,16 +364,16 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     pub fn read_immediate_raw(
         &self,
         src: &OpTy<'tcx, M::Provenance>,
-    ) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::Provenance>, MPlaceTy<'tcx, M::Provenance>>> {
-        Ok(match src.try_as_mplace() {
-            Ok(ref mplace) => {
+    ) -> InterpResult<'tcx, Either<MPlaceTy<'tcx, M::Provenance>, ImmTy<'tcx, M::Provenance>>> {
+        Ok(match src.as_mplace_or_imm() {
+            Left(ref mplace) => {
                 if let Some(val) = self.read_immediate_from_mplace_raw(mplace)? {
-                    Ok(val)
+                    Right(val)
                 } else {
-                    Err(*mplace)
+                    Left(*mplace)
                 }
             }
-            Err(val) => Ok(val),
+            Right(val) => Right(val),
         })
     }
 
@@ -390,7 +392,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         ) {
             span_bug!(self.cur_span(), "primitive read not possible for type: {:?}", op.layout.ty);
         }
-        let imm = self.read_immediate_raw(op)?.unwrap();
+        let imm = self.read_immediate_raw(op)?.right().unwrap();
         if matches!(*imm, Immediate::Uninit) {
             throw_ub!(InvalidUninitBytes(None));
         }
@@ -432,9 +434,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // Basically we just transmute this place into an array following simd_size_and_type.
         // This only works in memory, but repr(simd) types should never be immediates anyway.
         assert!(op.layout.ty.is_simd());
-        match op.try_as_mplace() {
-            Ok(mplace) => self.mplace_to_simd(&mplace),
-            Err(imm) => match *imm {
+        match op.as_mplace_or_imm() {
+            Left(mplace) => self.mplace_to_simd(&mplace),
+            Right(imm) => match *imm {
                 Immediate::Uninit => {
                     throw_ub!(InvalidUninitBytes(None))
                 }
diff --git a/compiler/rustc_const_eval/src/interpret/place.rs b/compiler/rustc_const_eval/src/interpret/place.rs
index 29d2312612e..c47cfe8bb69 100644
--- a/compiler/rustc_const_eval/src/interpret/place.rs
+++ b/compiler/rustc_const_eval/src/interpret/place.rs
@@ -2,6 +2,8 @@
 //! into a place.
 //! All high-level functions to write to memory work on places as destinations.
 
+use either::{Either, Left, Right};
+
 use rustc_ast::Mutability;
 use rustc_middle::mir;
 use rustc_middle::ty;
@@ -252,36 +254,36 @@ impl<'tcx, Prov: Provenance> MPlaceTy<'tcx, Prov> {
 // These are defined here because they produce a place.
 impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
     #[inline(always)]
-    pub fn try_as_mplace(&self) -> Result<MPlaceTy<'tcx, Prov>, ImmTy<'tcx, Prov>> {
+    pub fn as_mplace_or_imm(&self) -> Either<MPlaceTy<'tcx, Prov>, ImmTy<'tcx, Prov>> {
         match **self {
             Operand::Indirect(mplace) => {
-                Ok(MPlaceTy { mplace, layout: self.layout, align: self.align.unwrap() })
+                Left(MPlaceTy { mplace, layout: self.layout, align: self.align.unwrap() })
             }
-            Operand::Immediate(imm) => Err(ImmTy::from_immediate(imm, self.layout)),
+            Operand::Immediate(imm) => Right(ImmTy::from_immediate(imm, self.layout)),
         }
     }
 
     #[inline(always)]
     #[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
     pub fn assert_mem_place(&self) -> MPlaceTy<'tcx, Prov> {
-        self.try_as_mplace().unwrap()
+        self.as_mplace_or_imm().left().unwrap()
     }
 }
 
 impl<'tcx, Prov: Provenance> PlaceTy<'tcx, Prov> {
     /// A place is either an mplace or some local.
     #[inline]
-    pub fn try_as_mplace(&self) -> Result<MPlaceTy<'tcx, Prov>, (usize, mir::Local)> {
+    pub fn as_mplace_or_local(&self) -> Either<MPlaceTy<'tcx, Prov>, (usize, mir::Local)> {
         match **self {
-            Place::Ptr(mplace) => Ok(MPlaceTy { mplace, layout: self.layout, align: self.align }),
-            Place::Local { frame, local } => Err((frame, local)),
+            Place::Ptr(mplace) => Left(MPlaceTy { mplace, layout: self.layout, align: self.align }),
+            Place::Local { frame, local } => Right((frame, local)),
         }
     }
 
     #[inline(always)]
     #[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
     pub fn assert_mem_place(&self) -> MPlaceTy<'tcx, Prov> {
-        self.try_as_mplace().unwrap()
+        self.as_mplace_or_local().left().unwrap()
     }
 }
 
@@ -569,9 +571,9 @@ where
     }
 
     pub fn write_uninit(&mut self, dest: &PlaceTy<'tcx, M::Provenance>) -> InterpResult<'tcx> {
-        let mplace = match dest.try_as_mplace() {
-            Ok(mplace) => mplace,
-            Err((frame, local)) => {
+        let mplace = match dest.as_mplace_or_local() {
+            Left(mplace) => mplace,
+            Right((frame, local)) => {
                 match M::access_local_mut(self, frame, local)? {
                     Operand::Immediate(local) => {
                         *local = Immediate::Uninit;
@@ -639,7 +641,7 @@ where
         // Let us see if the layout is simple so we take a shortcut,
         // avoid force_allocation.
         let src = match self.read_immediate_raw(src)? {
-            Ok(src_val) => {
+            Right(src_val) => {
                 // FIXME(const_prop): Const-prop can possibly evaluate an
                 // unsized copy operation when it thinks that the type is
                 // actually sized, due to a trivially false where-clause
@@ -669,7 +671,7 @@ where
                     )
                 };
             }
-            Err(mplace) => mplace,
+            Left(mplace) => mplace,
         };
         // Slow path, this does not fit into an immediate. Just memcpy.
         trace!("copy_op: {:?} <- {:?}: {}", *dest, src, dest.layout.ty);
diff --git a/compiler/rustc_const_eval/src/interpret/projection.rs b/compiler/rustc_const_eval/src/interpret/projection.rs
index 6b2e2bb8aca..4966fd6ea80 100644
--- a/compiler/rustc_const_eval/src/interpret/projection.rs
+++ b/compiler/rustc_const_eval/src/interpret/projection.rs
@@ -7,6 +7,8 @@
 //! but we still need to do bounds checking and adjust the layout. To not duplicate that with MPlaceTy, we actually
 //! implement the logic on OpTy, and MPlaceTy calls that.
 
+use either::{Left, Right};
+
 use rustc_middle::mir;
 use rustc_middle::ty;
 use rustc_middle::ty::layout::LayoutOf;
@@ -84,13 +86,13 @@ where
         base: &OpTy<'tcx, M::Provenance>,
         field: usize,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
-        let base = match base.try_as_mplace() {
-            Ok(ref mplace) => {
+        let base = match base.as_mplace_or_imm() {
+            Left(ref mplace) => {
                 // We can reuse the mplace field computation logic for indirect operands.
                 let field = self.mplace_field(mplace, field)?;
                 return Ok(field.into());
             }
-            Err(value) => value,
+            Right(value) => value,
         };
 
         let field_layout = base.layout.field(self, field);
diff --git a/compiler/rustc_const_eval/src/interpret/step.rs b/compiler/rustc_const_eval/src/interpret/step.rs
index 3c286fa61be..73f8bf4362e 100644
--- a/compiler/rustc_const_eval/src/interpret/step.rs
+++ b/compiler/rustc_const_eval/src/interpret/step.rs
@@ -2,6 +2,8 @@
 //!
 //! The main entry point is the `step` method.
 
+use either::Either;
+
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::{InterpResult, Scalar};
 use rustc_middle::ty::layout::LayoutOf;
@@ -46,7 +48,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             return Ok(false);
         }
 
-        let Ok(loc) = self.frame().loc else {
+        let Either::Left(loc) = self.frame().loc else {
             // We are unwinding and this fn has no cleanup code.
             // Just go on unwinding.
             trace!("unwinding: skipping frame");
@@ -61,7 +63,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             // Make sure we are not updating `statement_index` of the wrong frame.
             assert_eq!(old_frames, self.frame_idx());
             // Advance the program counter.
-            self.frame_mut().loc.as_mut().unwrap().statement_index += 1;
+            self.frame_mut().loc.as_mut().left().unwrap().statement_index += 1;
             return Ok(true);
         }
 
@@ -305,7 +307,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
         self.eval_terminator(terminator)?;
         if !self.stack().is_empty() {
-            if let Ok(loc) = self.frame().loc {
+            if let Either::Left(loc) = self.frame().loc {
                 info!("// executing {:?}", loc.block);
             }
         }
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index 8aa56c275d9..cd7a472c0f0 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -8,6 +8,8 @@ use std::convert::TryFrom;
 use std::fmt::{Display, Write};
 use std::num::NonZeroUsize;
 
+use either::{Left, Right};
+
 use rustc_ast::Mutability;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
@@ -852,9 +854,9 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
                     return Ok(());
                 }
                 // Now that we definitely have a non-ZST array, we know it lives in memory.
-                let mplace = match op.try_as_mplace() {
-                    Ok(mplace) => mplace,
-                    Err(imm) => match *imm {
+                let mplace = match op.as_mplace_or_imm() {
+                    Left(mplace) => mplace,
+                    Right(imm) => match *imm {
                         Immediate::Uninit =>
                             throw_validation_failure!(self.path, { "uninitialized bytes" }),
                         Immediate::Scalar(..) | Immediate::ScalarPair(..) =>
diff --git a/compiler/rustc_middle/src/mir/interpret/allocation.rs b/compiler/rustc_middle/src/mir/interpret/allocation.rs
index 68024070346..f8a69f3c7d5 100644
--- a/compiler/rustc_middle/src/mir/interpret/allocation.rs
+++ b/compiler/rustc_middle/src/mir/interpret/allocation.rs
@@ -11,6 +11,8 @@ use std::hash;
 use std::ops::Range;
 use std::ptr;
 
+use either::{Left, Right};
+
 use rustc_ast::Mutability;
 use rustc_data_structures::intern::Interned;
 use rustc_span::DUMMY_SP;
@@ -503,11 +505,11 @@ impl<Prov: Provenance, Extra> Allocation<Prov, Extra> {
         // `to_bits_or_ptr_internal` is the right method because we just want to store this data
         // as-is into memory.
         let (bytes, provenance) = match val.to_bits_or_ptr_internal(range.size)? {
-            Err(val) => {
-                let (provenance, offset) = val.into_parts();
+            Right(ptr) => {
+                let (provenance, offset) = ptr.into_parts();
                 (u128::from(offset.bytes()), Some(provenance))
             }
-            Ok(data) => (data, None),
+            Left(data) => (data, None),
         };
 
         let endian = cx.data_layout().endian;
diff --git a/compiler/rustc_middle/src/mir/interpret/value.rs b/compiler/rustc_middle/src/mir/interpret/value.rs
index ac5fddb7ad1..770c3ed05e8 100644
--- a/compiler/rustc_middle/src/mir/interpret/value.rs
+++ b/compiler/rustc_middle/src/mir/interpret/value.rs
@@ -1,6 +1,8 @@
 use std::convert::{TryFrom, TryInto};
 use std::fmt;
 
+use either::{Either, Left, Right};
+
 use rustc_apfloat::{
     ieee::{Double, Single},
     Float,
@@ -293,10 +295,10 @@ impl<Prov> Scalar<Prov> {
     pub fn to_bits_or_ptr_internal(
         self,
         target_size: Size,
-    ) -> Result<Result<u128, Pointer<Prov>>, ScalarSizeMismatch> {
+    ) -> Result<Either<u128, Pointer<Prov>>, ScalarSizeMismatch> {
         assert_ne!(target_size.bytes(), 0, "you should never look at the bits of a ZST");
         Ok(match self {
-            Scalar::Int(int) => Ok(int.to_bits(target_size).map_err(|size| {
+            Scalar::Int(int) => Left(int.to_bits(target_size).map_err(|size| {
                 ScalarSizeMismatch { target_size: target_size.bytes(), data_size: size.bytes() }
             })?),
             Scalar::Ptr(ptr, sz) => {
@@ -306,7 +308,7 @@ impl<Prov> Scalar<Prov> {
                         data_size: sz.into(),
                     });
                 }
-                Err(ptr)
+                Right(ptr)
             }
         })
     }
@@ -318,8 +320,8 @@ impl<'tcx, Prov: Provenance> Scalar<Prov> {
             .to_bits_or_ptr_internal(cx.pointer_size())
             .map_err(|s| err_ub!(ScalarSizeMismatch(s)))?
         {
-            Err(ptr) => Ok(ptr.into()),
-            Ok(bits) => {
+            Right(ptr) => Ok(ptr.into()),
+            Left(bits) => {
                 let addr = u64::try_from(bits).unwrap();
                 Ok(Pointer::from_addr(addr))
             }
diff --git a/compiler/rustc_mir_build/Cargo.toml b/compiler/rustc_mir_build/Cargo.toml
index c726fa3a33f..2baa3bfcb64 100644
--- a/compiler/rustc_mir_build/Cargo.toml
+++ b/compiler/rustc_mir_build/Cargo.toml
@@ -8,6 +8,7 @@ edition = "2021"
 [dependencies]
 rustc_arena = { path = "../rustc_arena" }
 tracing = "0.1"
+either = "1"
 rustc_middle = { path = "../rustc_middle" }
 rustc_apfloat = { path = "../rustc_apfloat" }
 rustc_attr = { path = "../rustc_attr" }
diff --git a/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs b/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs
index 595abc8f668..d60e8722cb6 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs
+++ b/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs
@@ -42,16 +42,17 @@
 //! wildcards, see [`SplitWildcard`]; for integer ranges, see [`SplitIntRange`]; for slices, see
 //! [`SplitVarLenSlice`].
 
-use self::Constructor::*;
-use self::SliceKind::*;
+use std::cell::Cell;
+use std::cmp::{self, max, min, Ordering};
+use std::fmt;
+use std::iter::{once, IntoIterator};
+use std::ops::RangeInclusive;
 
-use super::compare_const_vals;
-use super::usefulness::{MatchCheckCtxt, PatCtxt};
+use smallvec::{smallvec, SmallVec};
 
 use rustc_data_structures::captures::Captures;
-use rustc_index::vec::Idx;
-
 use rustc_hir::{HirId, RangeEnd};
+use rustc_index::vec::Idx;
 use rustc_middle::mir::{self, Field};
 use rustc_middle::thir::{FieldPat, Pat, PatKind, PatRange};
 use rustc_middle::ty::layout::IntegerExt;
@@ -61,12 +62,11 @@ use rustc_session::lint;
 use rustc_span::{Span, DUMMY_SP};
 use rustc_target::abi::{Integer, Size, VariantIdx};
 
-use smallvec::{smallvec, SmallVec};
-use std::cell::Cell;
-use std::cmp::{self, max, min, Ordering};
-use std::fmt;
-use std::iter::{once, IntoIterator};
-use std::ops::RangeInclusive;
+use self::Constructor::*;
+use self::SliceKind::*;
+
+use super::compare_const_vals;
+use super::usefulness::{MatchCheckCtxt, PatCtxt};
 
 /// Recursively expand this pattern into its subpatterns. Only useful for or-patterns.
 fn expand_or_pat<'p, 'tcx>(pat: &'p Pat<'tcx>) -> Vec<&'p Pat<'tcx>> {
@@ -147,11 +147,7 @@ impl IntRange {
                         // straight to the result, after doing a bit of checking. (We
                         // could remove this branch and just fall through, which
                         // is more general but much slower.)
-                        if let Ok(Ok(bits)) = scalar.to_bits_or_ptr_internal(target_size) {
-                            return Some(bits);
-                        } else {
-                            return None;
-                        }
+                        return scalar.to_bits_or_ptr_internal(target_size).unwrap().left();
                     }
                     mir::ConstantKind::Ty(c) => match c.kind() {
                         ty::ConstKind::Value(_) => bug!(
diff --git a/compiler/rustc_mir_transform/Cargo.toml b/compiler/rustc_mir_transform/Cargo.toml
index 53545cff0cb..962536669e0 100644
--- a/compiler/rustc_mir_transform/Cargo.toml
+++ b/compiler/rustc_mir_transform/Cargo.toml
@@ -9,6 +9,7 @@ edition = "2021"
 itertools = "0.10.1"
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 tracing = "0.1"
+either = "1"
 rustc_ast = { path = "../rustc_ast" }
 rustc_attr = { path = "../rustc_attr" }
 rustc_data_structures = { path = "../rustc_data_structures" }
diff --git a/compiler/rustc_mir_transform/src/const_prop.rs b/compiler/rustc_mir_transform/src/const_prop.rs
index 3f5890040c3..b0514e03356 100644
--- a/compiler/rustc_mir_transform/src/const_prop.rs
+++ b/compiler/rustc_mir_transform/src/const_prop.rs
@@ -3,6 +3,8 @@
 
 use std::cell::Cell;
 
+use either::Right;
+
 use rustc_ast::Mutability;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def::DefKind;
@@ -429,7 +431,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         // Try to read the local as an immediate so that if it is representable as a scalar, we can
         // handle it as such, but otherwise, just return the value as is.
         Some(match self.ecx.read_immediate_raw(&op) {
-            Ok(Ok(imm)) => imm.into(),
+            Ok(Right(imm)) => imm.into(),
             _ => op,
         })
     }
@@ -743,7 +745,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         // FIXME> figure out what to do when read_immediate_raw fails
         let imm = self.use_ecx(|this| this.ecx.read_immediate_raw(value));
 
-        if let Some(Ok(imm)) = imm {
+        if let Some(Right(imm)) = imm {
             match *imm {
                 interpret::Immediate::Scalar(scalar) => {
                     *rval = Rvalue::Use(self.operand_from_scalar(
diff --git a/compiler/rustc_mir_transform/src/const_prop_lint.rs b/compiler/rustc_mir_transform/src/const_prop_lint.rs
index 786063d538c..0ab67228f3f 100644
--- a/compiler/rustc_mir_transform/src/const_prop_lint.rs
+++ b/compiler/rustc_mir_transform/src/const_prop_lint.rs
@@ -1,10 +1,10 @@
 //! Propagates constants for early reporting of statically known
 //! assertion failures
 
-use crate::const_prop::CanConstProp;
-use crate::const_prop::ConstPropMachine;
-use crate::const_prop::ConstPropMode;
-use crate::MirLint;
+use std::cell::Cell;
+
+use either::{Left, Right};
+
 use rustc_const_eval::interpret::Immediate;
 use rustc_const_eval::interpret::{
     self, InterpCx, InterpResult, LocalState, LocalValue, MemoryKind, OpTy, Scalar, StackPopCleanup,
@@ -26,12 +26,17 @@ use rustc_session::lint;
 use rustc_span::Span;
 use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
 use rustc_trait_selection::traits;
-use std::cell::Cell;
+
+use crate::const_prop::CanConstProp;
+use crate::const_prop::ConstPropMachine;
+use crate::const_prop::ConstPropMode;
+use crate::MirLint;
 
 /// The maximum number of bytes that we'll allocate space for a local or the return value.
 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
 /// Severely regress performance.
 const MAX_ALLOC_LIMIT: u64 = 1024;
+
 pub struct ConstProp;
 
 impl<'tcx> MirLint<'tcx> for ConstProp {
@@ -243,7 +248,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         // Try to read the local as an immediate so that if it is representable as a scalar, we can
         // handle it as such, but otherwise, just return the value as is.
         Some(match self.ecx.read_immediate_raw(&op) {
-            Ok(Ok(imm)) => imm.into(),
+            Ok(Left(imm)) => imm.into(),
             _ => op,
         })
     }
@@ -266,7 +271,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
     {
         // Overwrite the PC -- whatever the interpreter does to it does not make any sense anyway.
-        self.ecx.frame_mut().loc = Err(source_info.span);
+        self.ecx.frame_mut().loc = Right(source_info.span);
         match f(self) {
             Ok(val) => Some(val),
             Err(error) => {