Replace ExpressionOperandId with enum Operand

Because the three kinds of operand are now distinguished explicitly, we no
longer need fiddly code to disambiguate counter IDs and expression IDs based on
the total number of counters/expressions in a function.

This does increase the size of operands from 4 bytes to 8 bytes, but that
shouldn't be a big deal since they are mostly stored inside boxed structures,
and the current coverage code is not particularly size-optimized anyway.

compiler/rustc_middle/src/mir/coverage.rs

@@ -5,32 +5,6 @@ use rustc_span::Symbol;
 
 use std::fmt::{self, Debug, Formatter};
 
-rustc_index::newtype_index! {
-    /// An ExpressionOperandId value is assigned directly from either a
-    /// CounterValueReference.as_u32() (which ascend from 1) or an ExpressionOperandId.as_u32()
-    /// (which _*descend*_ from u32::MAX). Id value `0` (zero) represents a virtual counter with a
-    /// constant value of `0`.
-    #[derive(HashStable)]
-    #[max = 0xFFFF_FFFF]
-    #[debug_format = "ExpressionOperandId({})"]
-    pub struct ExpressionOperandId {
-    }
-}
-
-impl ExpressionOperandId {
-    /// An expression operand for a "zero counter", as described in the following references:
-    ///
-    /// * <https://github.com/rust-lang/llvm-project/blob/rustc/13.0-2021-09-30/llvm/docs/CoverageMappingFormat.rst#counter>
-    /// * <https://github.com/rust-lang/llvm-project/blob/rustc/13.0-2021-09-30/llvm/docs/CoverageMappingFormat.rst#tag>
-    /// * <https://github.com/rust-lang/llvm-project/blob/rustc/13.0-2021-09-30/llvm/docs/CoverageMappingFormat.rst#counter-expressions>
-    ///
-    /// This operand can be used to count two or more separate code regions with a single counter,
-    /// if they run sequentially with no branches, by injecting the `Counter` in a `BasicBlock` for
-    /// one of the code regions, and inserting `CounterExpression`s ("add ZERO to the counter") in
-    /// the coverage map for the other code regions.
-    pub const ZERO: Self = Self::from_u32(0);
-}
-
 rustc_index::newtype_index! {
     #[derive(HashStable)]
     #[max = 0xFFFF_FFFF]
@@ -39,7 +13,7 @@ rustc_index::newtype_index! {
 }
 
 impl CounterValueReference {
-    /// Counters start at 1 to reserve 0 for ExpressionOperandId::ZERO.
+    /// Counters start at 1 for historical reasons.
     pub const START: Self = Self::from_u32(1);
 
     /// Returns explicitly-requested zero-based version of the counter id, used
@@ -52,8 +26,6 @@ impl CounterValueReference {
 }
 
 rustc_index::newtype_index! {
-    /// InjectedExpressionId.as_u32() converts to ExpressionOperandId.as_u32()
-    ///
     /// Values descend from u32::MAX.
     #[derive(HashStable)]
     #[max = 0xFFFF_FFFF]
@@ -62,8 +34,6 @@ rustc_index::newtype_index! {
 }
 
 rustc_index::newtype_index! {
-    /// InjectedExpressionIndex.as_u32() translates to u32::MAX - ExpressionOperandId.as_u32()
-    ///
     /// Values ascend from 0.
     #[derive(HashStable)]
     #[max = 0xFFFF_FFFF]
@@ -81,17 +51,25 @@ rustc_index::newtype_index! {
     pub struct MappedExpressionIndex {}
 }
 
-impl From<CounterValueReference> for ExpressionOperandId {
-    #[inline]
-    fn from(v: CounterValueReference) -> ExpressionOperandId {
-        ExpressionOperandId::from(v.as_u32())
-    }
+/// Operand of a coverage-counter expression.
+///
+/// Operands can be a constant zero value, an actual coverage counter, or another
+/// expression. Counter/expression operands are referred to by ID.
+#[derive(Copy, Clone, PartialEq, Eq)]
+#[derive(TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
+pub enum Operand {
+    Zero,
+    Counter(CounterValueReference),
+    Expression(InjectedExpressionId),
 }
 
-impl From<InjectedExpressionId> for ExpressionOperandId {
-    #[inline]
-    fn from(v: InjectedExpressionId) -> ExpressionOperandId {
-        ExpressionOperandId::from(v.as_u32())
+impl Debug for Operand {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::Zero => write!(f, "Zero"),
+            Self::Counter(id) => f.debug_tuple("Counter").field(&id.as_u32()).finish(),
+            Self::Expression(id) => f.debug_tuple("Expression").field(&id.as_u32()).finish(),
+        }
     }
 }
 
@@ -107,19 +85,19 @@ pub enum CoverageKind {
         /// ID of this coverage-counter expression within its enclosing function.
         /// Other expressions in the same function can refer to it as an operand.
         id: InjectedExpressionId,
-        lhs: ExpressionOperandId,
+        lhs: Operand,
         op: Op,
-        rhs: ExpressionOperandId,
+        rhs: Operand,
     },
     Unreachable,
 }
 
 impl CoverageKind {
-    pub fn as_operand_id(&self) -> ExpressionOperandId {
+    pub fn as_operand(&self) -> Operand {
         use CoverageKind::*;
         match *self {
-            Counter { id, .. } => ExpressionOperandId::from(id),
-            Expression { id, .. } => ExpressionOperandId::from(id),
+            Counter { id, .. } => Operand::Counter(id),
+            Expression { id, .. } => Operand::Expression(id),
             Unreachable => bug!("Unreachable coverage cannot be part of an expression"),
         }
     }
@@ -136,14 +114,14 @@ impl Debug for CoverageKind {
             Counter { id, .. } => write!(fmt, "Counter({:?})", id.index()),
             Expression { id, lhs, op, rhs } => write!(
                 fmt,
-                "Expression({:?}) = {} {} {}",
+                "Expression({:?}) = {:?} {} {:?}",
                 id.index(),
-                lhs.index(),
+                lhs,
                 match op {
                     Op::Add => "+",
                     Op::Subtract => "-",
                 },
-                rhs.index(),
+                rhs,
             ),
             Unreachable => write!(fmt, "Unreachable"),
         }

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -470,7 +470,6 @@ TrivialTypeTraversalAndLiftImpls! {
     ::rustc_hir::Unsafety,
     ::rustc_target::asm::InlineAsmRegOrRegClass,
     ::rustc_target::spec::abi::Abi,
-    crate::mir::coverage::ExpressionOperandId,
     crate::mir::coverage::CounterValueReference,
     crate::mir::coverage::InjectedExpressionId,
     crate::mir::coverage::InjectedExpressionIndex,