diff options
| -rw-r--r-- | compiler/rustc_middle/src/mir/mod.rs | 24 |
1 files changed, 23 insertions, 1 deletions
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs index a01261c543b..4d124da43ec 100644 --- a/compiler/rustc_middle/src/mir/mod.rs +++ b/compiler/rustc_middle/src/mir/mod.rs @@ -127,12 +127,24 @@ pub trait MirPass<'tcx> { /// The various "big phases" that MIR goes through. /// /// These phases all describe dialects of MIR. Since all MIR uses the same datastructures, the -/// dialects forbid certain variants or values in certain phases. +/// dialects forbid certain variants or values in certain phases. The sections below summarize the +/// changes, but do not document them thoroughly. The full documentation is found in the appropriate +/// documentation for the thing the change is affecting. /// /// Warning: ordering of variants is significant. #[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, PartialOrd, Ord)] #[derive(HashStable)] pub enum MirPhase { + /// The dialect of MIR used during all phases before `DropsLowered` is the same. This is also + /// the MIR that analysis such as borrowck uses. + /// + /// One important thing to remember about the behavior of this section of MIR is that drop terminators + /// (including drop and replace) are *conditional*. The elaborate drops pass will then replace each + /// instance of a drop terminator with a nop, an unconditional drop, or a drop conditioned on a drop + /// flag. Of course, this means that it is important that the drop elaboration can accurately recognize + /// when things are initialized and when things are de-initialized. That means any code running on this + /// version of MIR must be sure to produce output that drop elaboration can reason about. See the + /// section on the drop terminatorss for more details. Built = 0, // FIXME(oli-obk): it's unclear whether we still need this phase (and its corresponding query). // We used to have this for pre-miri MIR based const eval. @@ -162,6 +174,16 @@ pub enum MirPhase { /// And the following variant is allowed: /// * [`StatementKind::SetDiscriminant`] Deaggregated = 4, + /// Before this phase, generators are in the "source code" form, featuring `yield` statements + /// and such. With this phase change, they are transformed into a proper state machine. Running + /// optimizations before this change can be potentially dangerous because the source code is to + /// some extent a "lie." In particular, `yield` terminators effectively make the value of all + /// locals visible to the caller. This means that dead store elimination before them, or code + /// motion across them, is not correct in general. This is also exasperated by type checking + /// having pre-computed a list of the types that it thinks are ok to be live across a yield + /// point - this is necessary to decide eg whether autotraits are implemented. Introducing new + /// types across a yield point will lead to ICEs becaues of this. + /// /// Beginning with this phase, the following variants are disallowed: /// * [`TerminatorKind::Yield`](terminator::TerminatorKind::Yield) /// * [`TerminatorKind::GeneratorDrop](terminator::TerminatorKind::GeneratorDrop) |