Rework definition of MIR phases to more closely reflect semantic concerns

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -41,7 +41,7 @@ pub struct PromoteTemps<'tcx> {
 
 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
     fn phase_change(&self) -> Option<MirPhase> {
-        Some(MirPhase::ConstsPromoted)
+        Some(MirPhase::Analysis(AnalysisPhase::Initial))
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
@@ -964,7 +964,7 @@ pub fn promote_candidates<'tcx>(
         let mut scope = body.source_scopes[body.source_info(candidate.location).scope].clone();
         scope.parent_scope = None;
 
-        let promoted = Body::new(
+        let mut promoted = Body::new(
             body.source, // `promoted` gets filled in below
             IndexVec::new(),
             IndexVec::from_elem_n(scope, 1),
@@ -976,6 +976,7 @@ pub fn promote_candidates<'tcx>(
             body.generator_kind(),
             body.tainted_by_errors,
         );
+        promoted.phase = MirPhase::Analysis(AnalysisPhase::Initial);
 
         let promoter = Promoter {
             promoted,

compiler/rustc_const_eval/src/transform/validate.rs

@@ -8,8 +8,8 @@ use rustc_middle::mir::visit::NonUseContext::VarDebugInfo;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
 use rustc_middle::mir::{
     traversal, AggregateKind, BasicBlock, BinOp, Body, BorrowKind, CastKind, Local, Location,
-    MirPass, MirPhase, Operand, Place, PlaceElem, PlaceRef, ProjectionElem, Rvalue, SourceScope,
-    Statement, StatementKind, Terminator, TerminatorKind, UnOp, START_BLOCK,
+    MirPass, MirPhase, Operand, Place, PlaceElem, PlaceRef, ProjectionElem, RuntimePhase, Rvalue,
+    SourceScope, Statement, StatementKind, Terminator, TerminatorKind, UnOp, START_BLOCK,
 };
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::subst::Subst;
@@ -221,7 +221,8 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
 
     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
         // This check is somewhat expensive, so only run it when -Zvalidate-mir is passed.
-        if self.tcx.sess.opts.unstable_opts.validate_mir && self.mir_phase < MirPhase::DropsLowered
+        if self.tcx.sess.opts.unstable_opts.validate_mir
+            && self.mir_phase < MirPhase::Runtime(RuntimePhase::Initial)
         {
             // `Operand::Copy` is only supposed to be used with `Copy` types.
             if let Operand::Copy(place) = operand {
@@ -252,7 +253,9 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     self.fail(location, format!("bad index ({:?} != usize)", index_ty))
                 }
             }
-            ProjectionElem::Deref if self.mir_phase >= MirPhase::GeneratorsLowered => {
+            ProjectionElem::Deref
+                if self.mir_phase >= MirPhase::Runtime(RuntimePhase::PostCleanup) =>
+            {
                 let base_ty = Place::ty_from(local, proj_base, &self.body.local_decls, self.tcx).ty;
 
                 if base_ty.is_box() {
@@ -360,7 +363,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
         // Set off any `bug!`s in the type computation code
         let _ = place.ty(&self.body.local_decls, self.tcx);
 
-        if self.mir_phase >= MirPhase::Derefered
+        if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial)
             && place.projection.len() > 1
             && cntxt != PlaceContext::NonUse(VarDebugInfo)
             && place.projection[1..].contains(&ProjectionElem::Deref)
@@ -384,8 +387,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
             Rvalue::Aggregate(agg_kind, _) => {
                 let disallowed = match **agg_kind {
                     AggregateKind::Array(..) => false,
-                    AggregateKind::Generator(..) => self.mir_phase >= MirPhase::GeneratorsLowered,
-                    _ => self.mir_phase >= MirPhase::Deaggregated,
+                    _ => self.mir_phase >= MirPhase::Runtime(RuntimePhase::PostCleanup),
                 };
                 if disallowed {
                     self.fail(
@@ -395,10 +397,10 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
             }
             Rvalue::Ref(_, BorrowKind::Shallow, _) => {
-                if self.mir_phase >= MirPhase::DropsLowered {
+                if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(
                         location,
-                        "`Assign` statement with a `Shallow` borrow should have been removed after drop lowering phase",
+                        "`Assign` statement with a `Shallow` borrow should have been removed in runtime MIR",
                     );
                 }
             }
@@ -612,15 +614,15 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
             }
             StatementKind::AscribeUserType(..) => {
-                if self.mir_phase >= MirPhase::DropsLowered {
+                if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(
                         location,
                         "`AscribeUserType` should have been removed after drop lowering phase",
                     );
                 }
             }
             StatementKind::FakeRead(..) => {
-                if self.mir_phase >= MirPhase::DropsLowered {
+                if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(
                         location,
                         "`FakeRead` should have been removed after drop lowering phase",
@@ -664,7 +666,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
             }
             StatementKind::SetDiscriminant { place, .. } => {
-                if self.mir_phase < MirPhase::Deaggregated {
+                if self.mir_phase < MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(location, "`SetDiscriminant`is not allowed until deaggregation");
                 }
                 let pty = place.ty(&self.body.local_decls, self.tcx).ty.kind();
@@ -679,7 +681,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
             }
             StatementKind::Deinit(..) => {
-                if self.mir_phase < MirPhase::Deaggregated {
+                if self.mir_phase < MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(location, "`Deinit`is not allowed until deaggregation");
                 }
             }
@@ -759,7 +761,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
             }
             TerminatorKind::DropAndReplace { target, unwind, .. } => {
-                if self.mir_phase >= MirPhase::DropsLowered {
+                if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(
                         location,
                         "`DropAndReplace` should have been removed during drop elaboration",
@@ -830,7 +832,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 if self.body.generator.is_none() {
                     self.fail(location, "`Yield` cannot appear outside generator bodies");
                 }
-                if self.mir_phase >= MirPhase::GeneratorsLowered {
+                if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(location, "`Yield` should have been replaced by generator lowering");
                 }
                 self.check_edge(location, *resume, EdgeKind::Normal);
@@ -839,7 +841,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
             }
             TerminatorKind::FalseEdge { real_target, imaginary_target } => {
-                if self.mir_phase >= MirPhase::DropsLowered {
+                if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(
                         location,
                         "`FalseEdge` should have been removed after drop elaboration",
@@ -849,7 +851,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 self.check_edge(location, *imaginary_target, EdgeKind::Normal);
             }
             TerminatorKind::FalseUnwind { real_target, unwind } => {
-                if self.mir_phase >= MirPhase::DropsLowered {
+                if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(
                         location,
                         "`FalseUnwind` should have been removed after drop elaboration",
@@ -872,7 +874,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 if self.body.generator.is_none() {
                     self.fail(location, "`GeneratorDrop` cannot appear outside generator bodies");
                 }
-                if self.mir_phase >= MirPhase::GeneratorsLowered {
+                if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(
                         location,
                         "`GeneratorDrop` should have been replaced by generator lowering",

compiler/rustc_middle/src/mir/mod.rs

@@ -128,8 +128,20 @@ pub trait MirPass<'tcx> {
 
 impl MirPhase {
     /// Gets the index of the current MirPhase within the set of all `MirPhase`s.
+    ///
+    /// FIXME(JakobDegen): Return a `(usize, usize)` instead.
     pub fn phase_index(&self) -> usize {
-        *self as usize
+        const BUILT_PHASE_COUNT: usize = 1;
+        const ANALYSIS_PHASE_COUNT: usize = 2;
+        match self {
+            MirPhase::Built => 1,
+            MirPhase::Analysis(analysis_phase) => {
+                1 + BUILT_PHASE_COUNT + (*analysis_phase as usize)
+            }
+            MirPhase::Runtime(runtime_phase) => {
+                1 + BUILT_PHASE_COUNT + ANALYSIS_PHASE_COUNT + (*runtime_phase as usize)
+            }
+        }
     }
 }
 

compiler/rustc_middle/src/mir/syntax.rs

@@ -23,75 +23,110 @@ use rustc_span::symbol::Symbol;
 use rustc_span::Span;
 use rustc_target::asm::InlineAsmRegOrRegClass;
 
-/// The various "big phases" that MIR goes through.
+/// Represents the "flavors" of MIR.
 ///
-/// These phases all describe dialects of MIR. Since all MIR uses the same datastructures, the
-/// 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.
+/// All flavors of MIR use the same data structure, but there are some important differences. These
+/// differences come in two forms: Dialects and phases.
 ///
-/// Warning: ordering of variants is significant.
+/// Dialects represent a stronger distinction than phases. This is because the transitions between
+/// dialects are semantic changes, and therefore technically *lowerings* between distinct IRs. In
+/// other words, the same [`Body`](crate::mir::Body) might be well-formed for multiple dialects, but
+/// have different semantic meaning and different behavior at runtime.
+///
+/// Each dialect additionally has a number of phases. However, phase changes never involve semantic
+/// changes. If some MIR is well-formed both before and after a phase change, it is also guaranteed
+/// that it has the same semantic meaning. In this sense, phase changes can only add additional
+/// restrictions on what MIR is well-formed.
+///
+/// When adding phases, remember to update [`MirPhase::phase_index`].
 #[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.
-    Const = 1,
-    /// This phase checks the MIR for promotable elements and takes them out of the main MIR body
-    /// by creating a new MIR body per promoted element. After this phase (and thus the termination
-    /// of the `mir_promoted` query), these promoted elements are available in the `promoted_mir`
-    /// query.
-    ConstsPromoted = 2,
-    /// After this projections may only contain deref projections as the first element.
-    Derefered = 3,
-    /// Beginning with this phase, the following variants are disallowed:
-    /// * [`TerminatorKind::DropAndReplace`]
+    /// The MIR that is generated by MIR building.
+    ///
+    /// The only things that operate on this dialect are unsafeck, the various MIR lints, and const
+    /// qualifs.
+    ///
+    /// This has no distinct phases.
+    Built,
+    /// The MIR used for most analysis.
+    ///
+    /// The only semantic change between analysis and built MIR is constant promotion. In built MIR,
+    /// sequences of statements that would generally be subject to constant promotion are
+    /// semantically constants, while in analysis MIR all constants are explicit.
+    ///
+    /// The result of const promotion is available from the `mir_promoted` and `promoted_mir` queries.
+    ///
+    /// This is the version of MIR used by borrowck and friends.
+    Analysis(AnalysisPhase),
+    /// The MIR used for CTFE, optimizations, and codegen.
+    ///
+    /// The semantic changes that occur in the lowering from analysis to runtime MIR are as follows:
+    ///
+    ///  - Drops: In analysis MIR, `Drop` terminators represent *conditional* drops; roughly speaking,
+    ///    if dataflow analysis determines that the place being dropped is uninitialized, the drop will
+    ///    not be executed. The exact semantics of this aren't written down anywhere, which means they
+    ///    are essentially "what drop elaboration does." In runtime MIR, the drops are unconditional;
+    ///    when a `Drop` terminator is reached, if the type has drop glue that drop glue is always
+    ///    executed. This may be UB if the underlying place is not initialized.
+    ///  - Packed drops: Places might in general be misaligned - in most cases this is UB, the exception
+    ///    is fields of packed structs. In analysis MIR, `Drop(P)` for a `P` that might be misaligned
+    ///    for this reason implicitly moves `P` to a temporary before dropping. Runtime MIR has no such
+    ///    rules, and dropping a misaligned place is simply UB.
+    ///  - Unwinding: in analysis MIR, unwinding from a function which may not unwind aborts. In runtime
+    ///    MIR, this is UB.
+    ///  - Retags: If `-Zmir-emit-retag` is enabled, analysis MIR has "implicit" retags in the same way
+    ///    that Rust itself has them. Where exactly these are is generally subject to change, and so we
+    ///    don't document this here. Runtime MIR has all retags explicit.
+    ///  - Generator bodies: In analysis MIR, locals may actually be behind a pointer that user code has
+    ///    access to. This occurs in generator bodies. Such locals do not behave like other locals,
+    ///    because they eg may be aliased in surprising ways. Runtime MIR has no such special locals -
+    ///    all generator bodies are lowered and so all places that look like locals really are locals.
+    ///  - Const prop lints: The lint pass which reports eg `200_u8 + 200_u8` as an error is run as a
+    ///    part of analysis to runtime MIR lowering. This means that transformations which may supress
+    ///    such errors may not run on analysis MIR.
+    Runtime(RuntimePhase),
+}
+
+/// See [`MirPhase::Analysis`].
+#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(HashStable)]
+pub enum AnalysisPhase {
+    Initial = 0,
+    /// Beginning in this phase, the following variants are disallowed:
     /// * [`TerminatorKind::FalseUnwind`]
     /// * [`TerminatorKind::FalseEdge`]
     /// * [`StatementKind::FakeRead`]
     /// * [`StatementKind::AscribeUserType`]
     /// * [`Rvalue::Ref`] with `BorrowKind::Shallow`
     ///
-    /// And the following variant is allowed:
-    /// * [`StatementKind::Retag`]
-    ///
-    /// Furthermore, `Drop` now uses explicit drop flags visible in the MIR and reaching a `Drop`
-    /// terminator means that the auto-generated drop glue will be invoked. Also, `Copy` operands
-    /// are allowed for non-`Copy` types.
-    DropsLowered = 4,
-    /// Beginning with this phase, the following variant is disallowed:
+    /// Furthermore, `Deref` projections must be the first projection within any place (if they
+    /// appear at all)
+    PostCleanup = 1,
+}
+
+/// See [`MirPhase::Runtime`].
+#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(HashStable)]
+pub enum RuntimePhase {
+    /// In addition to the semantic changes, beginning with this phase, the following variants are
+    /// disallowed:
+    /// * [`TerminatorKind::DropAndReplace`]
+    /// * [`TerminatorKind::Yield`]
+    /// * [`TerminatorKind::GeneratorDrop`]
     /// * [`Rvalue::Aggregate`] for any `AggregateKind` except `Array`
     ///
-    /// And the following variant is allowed:
+    /// And the following variants are allowed:
+    /// * [`StatementKind::Retag`]
     /// * [`StatementKind::SetDiscriminant`]
-    Deaggregated = 5,
-    /// 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`]
-    /// * [`TerminatorKind::GeneratorDrop`]
+    /// * [`StatementKind::Deinit`]
+    ///
+    /// Furthermore, `Copy` operands are allowed for non-`Copy` types.
+    Initial = 0,
+    /// Beginning with this phase, the following variant is disallowed:
     /// * [`ProjectionElem::Deref`] of `Box`
-    GeneratorsLowered = 6,
-    Optimized = 7,
+    PostCleanup = 1,
+    Optimized = 2,
 }
 
 ///////////////////////////////////////////////////////////////////////////

compiler/rustc_mir_transform/src/deaggregator.rs

@@ -6,10 +6,6 @@ use rustc_middle::ty::TyCtxt;
 pub struct Deaggregator;
 
 impl<'tcx> MirPass<'tcx> for Deaggregator {
-    fn phase_change(&self) -> Option<MirPhase> {
-        Some(MirPhase::Deaggregated)
-    }
-
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
         for bb in basic_blocks {

compiler/rustc_mir_transform/src/deref_separator.rs

@@ -82,6 +82,5 @@ pub fn deref_finder<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 impl<'tcx> MirPass<'tcx> for Derefer {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         deref_finder(tcx, body);
-        body.phase = MirPhase::Derefered;
     }
 }