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incr.comp.: Remove outdated comment about TraitSelect dep-node.

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Michael Woerister 2017-11-08 11:34:09 +01:00
parent 13bc7ad476
commit 702ce8f0a6
1 changed files with 0 additions and 25 deletions
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src/librustc/dep_graph/dep_node.rs
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@ -539,31 +539,6 @@ define_dep_nodes!( <'tcx>
[input] AllLocalTraitImpls,
// Trait selection cache is a little funny. Given a trait
// reference like `Foo: SomeTrait<Bar>`, there could be
// arbitrarily many def-ids to map on in there (e.g., `Foo`,
// `SomeTrait`, `Bar`). We could have a vector of them, but it
// requires heap-allocation, and trait sel in general can be a
// surprisingly hot path. So instead we pick two def-ids: the
// trait def-id, and the first def-id in the input types. If there
// is no def-id in the input types, then we use the trait def-id
// again. So for example:
//
// - `i32: Clone` -> `TraitSelect { trait_def_id: Clone, self_def_id: Clone }`
// - `u32: Clone` -> `TraitSelect { trait_def_id: Clone, self_def_id: Clone }`
// - `Clone: Clone` -> `TraitSelect { trait_def_id: Clone, self_def_id: Clone }`
// - `Vec<i32>: Clone` -> `TraitSelect { trait_def_id: Clone, self_def_id: Vec }`
// - `String: Clone` -> `TraitSelect { trait_def_id: Clone, self_def_id: String }`
// - `Foo: Trait<Bar>` -> `TraitSelect { trait_def_id: Trait, self_def_id: Foo }`
// - `Foo: Trait<i32>` -> `TraitSelect { trait_def_id: Trait, self_def_id: Foo }`
// - `(Foo, Bar): Trait` -> `TraitSelect { trait_def_id: Trait, self_def_id: Foo }`
// - `i32: Trait<Foo>` -> `TraitSelect { trait_def_id: Trait, self_def_id: Foo }`
//
// You can see that we map many trait refs to the same
// trait-select node. This is not a problem, it just means
// imprecision in our dep-graph tracking. The important thing is
// that for any given trait-ref, we always map to the **same**
// trait-select node.
[anon] TraitSelect,
[] ParamEnv(DefId),