diff options
| -rw-r--r-- | compiler/rustc_middle/src/mir/syntax.rs | 30 |
1 files changed, 19 insertions, 11 deletions
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs index 9cec8d832dd..dfd40a9535b 100644 --- a/compiler/rustc_middle/src/mir/syntax.rs +++ b/compiler/rustc_middle/src/mir/syntax.rs @@ -1015,22 +1015,30 @@ impl TerminatorKind<'_> { #[derive(Debug, Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq)] pub struct SwitchTargets { - /// Possible values. The locations to branch to in each case - /// are found in the corresponding indices from the `targets` vector. + /// Possible values. For each value, the location to branch to is found in + /// the corresponding element in the `targets` vector. pub(super) values: SmallVec<[Pu128; 1]>, - /// Possible branch sites. The last element of this vector is used - /// for the otherwise branch, so targets.len() == values.len() + 1 - /// should hold. + /// Possible branch targets. The last element of this vector is used for + /// the "otherwise" branch, so `targets.len() == values.len() + 1` always + /// holds. // - // This invariant is quite non-obvious and also could be improved. - // One way to make this invariant is to have something like this instead: + // Note: This invariant is non-obvious and easy to violate. This would be a + // more rigorous representation: // - // branches: Vec<(ConstInt, BasicBlock)>, - // otherwise: Option<BasicBlock> // exhaustive if None + // normal: SmallVec<[(Pu128, BasicBlock); 1]>, + // otherwise: BasicBlock, // - // However we’ve decided to keep this as-is until we figure a case - // where some other approach seems to be strictly better than other. + // But it's important to have the targets in a sliceable type, because + // target slices show up elsewhere. E.g. `TerminatorKind::InlineAsm` has a + // boxed slice, and `TerminatorKind::FalseEdge` has a single target that + // can be converted to a slice with `slice::from_ref`. + // + // Why does this matter? In functions like `TerminatorKind::successors` we + // return `impl Iterator` and a non-slice-of-targets representation here + // causes problems because multiple different concrete iterator types would + // be involved and we would need a boxed trait object, which requires an + // allocation, which is expensive if done frequently. pub(super) targets: SmallVec<[BasicBlock; 2]>, } |