Add SwitchTargetValue.

This is much clearer than `Option<u128>`.

compiler/rustc_middle/src/mir/basic_blocks.rs

@@ -27,7 +27,15 @@ type Predecessors = IndexVec<BasicBlock, SmallVec<[BasicBlock; 4]>>;
 /// `BasicBlocks::switch_sources`, which is only called by backwards analyses
 /// that do `SwitchInt` handling, and we don't have any of those, not even in
 /// tests. See #95120 and #94576.
-type SwitchSources = FxHashMap<(BasicBlock, BasicBlock), SmallVec<[Option<u128>; 1]>>;
+type SwitchSources = FxHashMap<(BasicBlock, BasicBlock), SmallVec<[SwitchTargetValue; 1]>>;
+
+#[derive(Debug, Clone, Copy)]
+pub enum SwitchTargetValue {
+    // A normal switch value.
+    Normal(u128),
+    // The final "otherwise" fallback value.
+    Otherwise,
+}
 
 #[derive(Clone, Default, Debug)]
 struct Cache {
@@ -89,9 +97,15 @@ impl<'tcx> BasicBlocks<'tcx> {
                 }) = &data.terminator
                 {
                     for (value, target) in targets.iter() {
-                        switch_sources.entry((target, bb)).or_default().push(Some(value));
+                        switch_sources
+                            .entry((target, bb))
+                            .or_default()
+                            .push(SwitchTargetValue::Normal(value));
                     }
-                    switch_sources.entry((targets.otherwise(), bb)).or_default().push(None);
+                    switch_sources
+                        .entry((targets.otherwise(), bb))
+                        .or_default()
+                        .push(SwitchTargetValue::Otherwise);
                 }
             }
             switch_sources

compiler/rustc_middle/src/mir/mod.rs

@@ -7,7 +7,7 @@ use std::fmt::{self, Debug, Formatter};
 use std::ops::{Index, IndexMut};
 use std::{iter, mem};
 
-pub use basic_blocks::BasicBlocks;
+pub use basic_blocks::{BasicBlocks, SwitchTargetValue};
 use either::Either;
 use polonius_engine::Atom;
 use rustc_abi::{FieldIdx, VariantIdx};

compiler/rustc_mir_dataflow/src/framework/direction.rs

@@ -1,6 +1,8 @@
 use std::ops::RangeInclusive;
 
-use rustc_middle::mir::{self, BasicBlock, CallReturnPlaces, Location, TerminatorEdges};
+use rustc_middle::mir::{
+    self, BasicBlock, CallReturnPlaces, Location, SwitchTargetValue, TerminatorEdges,
+};
 
 use super::visitor::ResultsVisitor;
 use super::{Analysis, Effect, EffectIndex, Results, SwitchIntTarget};
@@ -290,7 +292,8 @@ impl Direction for Forward {
                     let mut tmp = analysis.bottom_value(body);
                     for (value, target) in targets.iter() {
                         tmp.clone_from(exit_state);
-                        let si_target = SwitchIntTarget { value: Some(value), target };
+                        let si_target =
+                            SwitchIntTarget { value: SwitchTargetValue::Normal(value), target };
                         analysis.apply_switch_int_edge_effect(&mut data, &mut tmp, si_target);
                         propagate(target, &tmp);
                     }
@@ -302,7 +305,7 @@ impl Direction for Forward {
                     analysis.apply_switch_int_edge_effect(
                         &mut data,
                         exit_state,
-                        SwitchIntTarget { value: None, target: otherwise },
+                        SwitchIntTarget { value: SwitchTargetValue::Otherwise, target: otherwise },
                     );
                     propagate(otherwise, exit_state);
                 } else {

compiler/rustc_mir_dataflow/src/framework/mod.rs

@@ -38,7 +38,9 @@ use rustc_data_structures::work_queue::WorkQueue;
 use rustc_index::bit_set::{DenseBitSet, MixedBitSet};
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::bug;
-use rustc_middle::mir::{self, BasicBlock, CallReturnPlaces, Location, TerminatorEdges, traversal};
+use rustc_middle::mir::{
+    self, BasicBlock, CallReturnPlaces, Location, SwitchTargetValue, TerminatorEdges, traversal,
+};
 use rustc_middle::ty::TyCtxt;
 use tracing::error;
 
@@ -431,7 +433,7 @@ impl EffectIndex {
 }
 
 pub struct SwitchIntTarget {
-    pub value: Option<u128>,
+    pub value: SwitchTargetValue,
     pub target: BasicBlock,
 }
 

compiler/rustc_mir_dataflow/src/impls/initialized.rs

@@ -4,7 +4,9 @@ use rustc_abi::VariantIdx;
 use rustc_index::Idx;
 use rustc_index::bit_set::{DenseBitSet, MixedBitSet};
 use rustc_middle::bug;
-use rustc_middle::mir::{self, Body, CallReturnPlaces, Location, TerminatorEdges};
+use rustc_middle::mir::{
+    self, Body, CallReturnPlaces, Location, SwitchTargetValue, TerminatorEdges,
+};
 use rustc_middle::ty::util::Discr;
 use rustc_middle::ty::{self, TyCtxt};
 use tracing::{debug, instrument};
@@ -424,7 +426,7 @@ impl<'tcx> Analysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
         state: &mut Self::Domain,
         edge: SwitchIntTarget,
     ) {
-        if let Some(value) = edge.value {
+        if let SwitchTargetValue::Normal(value) = edge.value {
             // Kill all move paths that correspond to variants we know to be inactive along this
             // particular outgoing edge of a `SwitchInt`.
             drop_flag_effects::on_all_inactive_variants(
@@ -537,7 +539,7 @@ impl<'tcx> Analysis<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
         state: &mut Self::Domain,
         edge: SwitchIntTarget,
     ) {
-        if let Some(value) = edge.value {
+        if let SwitchTargetValue::Normal(value) = edge.value {
             // Mark all move paths that correspond to variants other than this one as maybe
             // uninitialized (in reality, they are *definitely* uninitialized).
             drop_flag_effects::on_all_inactive_variants(