Rollup merge of #71993 - ecstatic-morse:cleanup-old-liveness, r=jonas-schievink

Remove old `util/liveness.rs` module

The liveness dataflow analysis now lives in the `dataflow` module, so this one is no longer necessary. I've copied the relevant bits of the module docs for `util::liveness` to `MaybeLiveLocals`. The example in the docs is now a `mir-dataflow` test: https://github.com/rust-lang/rust/blob/a08c47310c7d49cbdc5d7afb38408ba519967ecd/src/test/ui/mir-dataflow/liveness-ptr.rs#L6-L26

The borrow-checker used the same notion of "defs" and "uses", so I've moved it into a submodule. I would have moved it to `util/def_use.rs`, since it seems generally useful, but there's already a slightly [different version](https://github.com/rust-lang/rust/blob/master/src/librustc_mir/util/def_use.rs) of the same abstraction needed for copy propagation.

Author: Dylan-DPC

src/librustc_mir/borrow_check/def_use.rs

@@ -0,0 +1,78 @@
+use rustc_middle::mir::visit::{
+    MutatingUseContext, NonMutatingUseContext, NonUseContext, PlaceContext,
+};
+
+#[derive(Eq, PartialEq, Clone)]
+pub enum DefUse {
+    Def,
+    Use,
+    Drop,
+}
+
+pub fn categorize(context: PlaceContext) -> Option<DefUse> {
+    match context {
+        ///////////////////////////////////////////////////////////////////////////
+        // DEFS
+
+        PlaceContext::MutatingUse(MutatingUseContext::Store) |
+
+        // This is potentially both a def and a use...
+        PlaceContext::MutatingUse(MutatingUseContext::AsmOutput) |
+
+        // We let Call define the result in both the success and
+        // unwind cases. This is not really correct, however it
+        // does not seem to be observable due to the way that we
+        // generate MIR. To do things properly, we would apply
+        // the def in call only to the input from the success
+        // path and not the unwind path. -nmatsakis
+        PlaceContext::MutatingUse(MutatingUseContext::Call) |
+        PlaceContext::MutatingUse(MutatingUseContext::Yield) |
+
+        // Storage live and storage dead aren't proper defines, but we can ignore
+        // values that come before them.
+        PlaceContext::NonUse(NonUseContext::StorageLive) |
+        PlaceContext::NonUse(NonUseContext::StorageDead) => Some(DefUse::Def),
+
+        ///////////////////////////////////////////////////////////////////////////
+        // REGULAR USES
+        //
+        // These are uses that occur *outside* of a drop. For the
+        // purposes of NLL, these are special in that **all** the
+        // lifetimes appearing in the variable must be live for each regular use.
+
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) |
+        PlaceContext::MutatingUse(MutatingUseContext::Projection) |
+
+        // Borrows only consider their local used at the point of the borrow.
+        // This won't affect the results since we use this analysis for generators
+        // and we only care about the result at suspension points. Borrows cannot
+        // cross suspension points so this behavior is unproblematic.
+        PlaceContext::MutatingUse(MutatingUseContext::Borrow) |
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) |
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow) |
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow) |
+
+        PlaceContext::MutatingUse(MutatingUseContext::AddressOf) |
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf) |
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) |
+        PlaceContext::NonUse(NonUseContext::AscribeUserTy) |
+        PlaceContext::MutatingUse(MutatingUseContext::Retag) =>
+            Some(DefUse::Use),
+
+        ///////////////////////////////////////////////////////////////////////////
+        // DROP USES
+        //
+        // These are uses that occur in a DROP (a MIR drop, not a
+        // call to `std::mem::drop()`). For the purposes of NLL,
+        // uses in drop are special because `#[may_dangle]`
+        // attributes can affect whether lifetimes must be live.
+
+        PlaceContext::MutatingUse(MutatingUseContext::Drop) =>
+            Some(DefUse::Drop),
+
+        // Debug info is neither def nor use.
+        PlaceContext::NonUse(NonUseContext::VarDebugInfo) => None,
+    }
+}

src/librustc_mir/borrow_check/diagnostics/find_use.rs

@@ -2,10 +2,10 @@ use std::collections::VecDeque;
 use std::rc::Rc;
 
 use crate::borrow_check::{
+    def_use::{self, DefUse},
     nll::ToRegionVid,
     region_infer::{Cause, RegionInferenceContext},
 };
-use crate::util::liveness::{self, DefUse};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::mir::visit::{MirVisitable, PlaceContext, Visitor};
 use rustc_middle::mir::{Body, Local, Location};
@@ -117,7 +117,7 @@ impl<'cx, 'tcx> Visitor<'tcx> for DefUseVisitor<'cx, 'tcx> {
         });
 
         if found_it {
-            self.def_use_result = match liveness::categorize(context) {
+            self.def_use_result = match def_use::categorize(context) {
                 Some(DefUse::Def) => Some(DefUseResult::Def),
                 Some(DefUse::Use) => Some(DefUseResult::UseLive { local }),
                 Some(DefUse::Drop) => Some(DefUseResult::UseDrop { local }),

src/librustc_mir/borrow_check/mod.rs

@@ -51,6 +51,7 @@ use self::path_utils::*;
 mod borrow_set;
 mod constraint_generation;
 mod constraints;
+mod def_use;
 mod diagnostics;
 mod facts;
 mod invalidation;

src/librustc_mir/borrow_check/type_check/liveness/local_use_map.rs

@@ -3,8 +3,7 @@ use rustc_index::vec::IndexVec;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
 use rustc_middle::mir::{Body, Local, Location};
 
-use crate::util::liveness::{categorize, DefUse};
-
+use crate::borrow_check::def_use::{self, DefUse};
 use crate::borrow_check::region_infer::values::{PointIndex, RegionValueElements};
 
 /// A map that cross references each local with the locations where it
@@ -160,7 +159,7 @@ impl LocalUseMapBuild<'_> {
 impl Visitor<'tcx> for LocalUseMapBuild<'_> {
     fn visit_local(&mut self, &local: &Local, context: PlaceContext, location: Location) {
         if self.locals_with_use_data[local] {
-            match categorize(context) {
+            match def_use::categorize(context) {
                 Some(DefUse::Def) => self.insert_def(local, location),
                 Some(DefUse::Use) => self.insert_use(local, location),
                 Some(DefUse::Drop) => self.insert_drop(local, location),

src/librustc_mir/borrow_check/type_check/liveness/polonius.rs

@@ -1,7 +1,7 @@
+use crate::borrow_check::def_use::{self, DefUse};
 use crate::borrow_check::location::{LocationIndex, LocationTable};
 use crate::dataflow::indexes::MovePathIndex;
 use crate::dataflow::move_paths::{LookupResult, MoveData};
-use crate::util::liveness::{categorize, DefUse};
 use rustc_middle::mir::visit::{MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{Body, Local, Location, Place};
 use rustc_middle::ty::subst::GenericArg;
@@ -56,7 +56,7 @@ impl UseFactsExtractor<'_> {
 
 impl Visitor<'tcx> for UseFactsExtractor<'_> {
     fn visit_local(&mut self, &local: &Local, context: PlaceContext, location: Location) {
-        match categorize(context) {
+        match def_use::categorize(context) {
             Some(DefUse::Def) => self.insert_def(local, location),
             Some(DefUse::Use) => self.insert_use(local, location),
             Some(DefUse::Drop) => self.insert_drop_use(local, location),

src/librustc_mir/dataflow/impls/liveness.rs

@@ -6,6 +6,13 @@ use crate::dataflow::{AnalysisDomain, Backward, BottomValue, GenKill, GenKillAna
 
 /// A [live-variable dataflow analysis][liveness].
 ///
+/// This analysis considers references as being used only at the point of the
+/// borrow. In other words, this analysis does not track uses because of references that already
+/// exist. See [this `mir-datalow` test][flow-test] for an example. You almost never want to use
+/// this analysis without also looking at the results of [`MaybeBorrowedLocals`].
+///
+/// [`MaybeBorrowedLocals`]: ../struct.MaybeBorrowedLocals.html
+/// [flow-test]: https://github.com/rust-lang/rust/blob/a08c47310c7d49cbdc5d7afb38408ba519967ecd/src/test/ui/mir-dataflow/liveness-ptr.rs
 /// [liveness]: https://en.wikipedia.org/wiki/Live_variable_analysis
 pub struct MaybeLiveLocals;