Rollup merge of #121260 - gurry:constprop-lint-cleanup, r=oli-obk

Remove const_prop.rs

Removed const_prop.rs and moved its contents to const_prop_lint.rs and dataflow_const_prop.rs where they are used.

const_prop.rs does not actually do any const propagation any more. After #116012 all it contains is code that is used by const_prop_lint.rs and one macro that is used by dataflow_const_prop.rs. So it made sense to just move it to those two places and remove this file.

compiler/rustc_mir_transform/src/const_prop_lint.rs

@@ -9,17 +9,14 @@ use rustc_const_eval::interpret::{
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def::DefKind;
 use rustc_hir::HirId;
-use rustc_index::bit_set::BitSet;
-use rustc_index::{Idx, IndexVec};
-use rustc_middle::mir::visit::Visitor;
+use rustc_index::{bit_set::BitSet, Idx, IndexVec};
+use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
 use rustc_middle::ty::{self, ConstInt, ParamEnv, ScalarInt, Ty, TyCtxt, TypeVisitableExt};
 use rustc_span::Span;
 use rustc_target::abi::{Abi, FieldIdx, HasDataLayout, Size, TargetDataLayout, VariantIdx};
 
-use crate::const_prop::CanConstProp;
-use crate::const_prop::ConstPropMode;
 use crate::dataflow_const_prop::DummyMachine;
 use crate::errors::{AssertLint, AssertLintKind};
 use crate::MirLint;
@@ -849,3 +846,128 @@ impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
         }
     }
 }
+
+/// The maximum number of bytes that we'll allocate space for a local or the return value.
+/// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
+/// Severely regress performance.
+const MAX_ALLOC_LIMIT: u64 = 1024;
+
+/// The mode that `ConstProp` is allowed to run in for a given `Local`.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum ConstPropMode {
+    /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
+    FullConstProp,
+    /// The `Local` can only be propagated into and from its own block.
+    OnlyInsideOwnBlock,
+    /// The `Local` cannot be part of propagation at all. Any statement
+    /// referencing it either for reading or writing will not get propagated.
+    NoPropagation,
+}
+
+pub struct CanConstProp {
+    can_const_prop: IndexVec<Local, ConstPropMode>,
+    // False at the beginning. Once set, no more assignments are allowed to that local.
+    found_assignment: BitSet<Local>,
+}
+
+impl CanConstProp {
+    /// Returns true if `local` can be propagated
+    pub fn check<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        param_env: ParamEnv<'tcx>,
+        body: &Body<'tcx>,
+    ) -> IndexVec<Local, ConstPropMode> {
+        let mut cpv = CanConstProp {
+            can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
+            found_assignment: BitSet::new_empty(body.local_decls.len()),
+        };
+        for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
+            let ty = body.local_decls[local].ty;
+            match tcx.layout_of(param_env.and(ty)) {
+                Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
+                // Either the layout fails to compute, then we can't use this local anyway
+                // or the local is too large, then we don't want to.
+                _ => {
+                    *val = ConstPropMode::NoPropagation;
+                    continue;
+                }
+            }
+        }
+        // Consider that arguments are assigned on entry.
+        for arg in body.args_iter() {
+            cpv.found_assignment.insert(arg);
+        }
+        cpv.visit_body(body);
+        cpv.can_const_prop
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for CanConstProp {
+    fn visit_place(&mut self, place: &Place<'tcx>, mut context: PlaceContext, loc: Location) {
+        use rustc_middle::mir::visit::PlaceContext::*;
+
+        // Dereferencing just read the addess of `place.local`.
+        if place.projection.first() == Some(&PlaceElem::Deref) {
+            context = NonMutatingUse(NonMutatingUseContext::Copy);
+        }
+
+        self.visit_local(place.local, context, loc);
+        self.visit_projection(place.as_ref(), context, loc);
+    }
+
+    fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
+        use rustc_middle::mir::visit::PlaceContext::*;
+        match context {
+            // These are just stores, where the storing is not propagatable, but there may be later
+            // mutations of the same local via `Store`
+            | MutatingUse(MutatingUseContext::Call)
+            | MutatingUse(MutatingUseContext::AsmOutput)
+            | MutatingUse(MutatingUseContext::Deinit)
+            // Actual store that can possibly even propagate a value
+            | MutatingUse(MutatingUseContext::Store)
+            | MutatingUse(MutatingUseContext::SetDiscriminant) => {
+                if !self.found_assignment.insert(local) {
+                    match &mut self.can_const_prop[local] {
+                        // If the local can only get propagated in its own block, then we don't have
+                        // to worry about multiple assignments, as we'll nuke the const state at the
+                        // end of the block anyway, and inside the block we overwrite previous
+                        // states as applicable.
+                        ConstPropMode::OnlyInsideOwnBlock => {}
+                        ConstPropMode::NoPropagation => {}
+                        other @ ConstPropMode::FullConstProp => {
+                            trace!(
+                                "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
+                                local, other,
+                            );
+                            *other = ConstPropMode::OnlyInsideOwnBlock;
+                        }
+                    }
+                }
+            }
+            // Reading constants is allowed an arbitrary number of times
+            NonMutatingUse(NonMutatingUseContext::Copy)
+            | NonMutatingUse(NonMutatingUseContext::Move)
+            | NonMutatingUse(NonMutatingUseContext::Inspect)
+            | NonMutatingUse(NonMutatingUseContext::PlaceMention)
+            | NonUse(_) => {}
+
+            // These could be propagated with a smarter analysis or just some careful thinking about
+            // whether they'd be fine right now.
+            MutatingUse(MutatingUseContext::Yield)
+            | MutatingUse(MutatingUseContext::Drop)
+            | MutatingUse(MutatingUseContext::Retag)
+            // These can't ever be propagated under any scheme, as we can't reason about indirect
+            // mutation.
+            | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
+            | NonMutatingUse(NonMutatingUseContext::FakeBorrow)
+            | NonMutatingUse(NonMutatingUseContext::AddressOf)
+            | MutatingUse(MutatingUseContext::Borrow)
+            | MutatingUse(MutatingUseContext::AddressOf) => {
+                trace!("local {:?} can't be propagated because it's used: {:?}", local, context);
+                self.can_const_prop[local] = ConstPropMode::NoPropagation;
+            }
+            MutatingUse(MutatingUseContext::Projection)
+            | NonMutatingUse(NonMutatingUseContext::Projection) => bug!("visit_place should not pass {context:?} for {local:?}"),
+        }
+    }
+}

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -21,7 +21,32 @@ use rustc_span::def_id::DefId;
 use rustc_span::DUMMY_SP;
 use rustc_target::abi::{Abi, FieldIdx, Size, VariantIdx, FIRST_VARIANT};
 
-use crate::const_prop::throw_machine_stop_str;
+/// Macro for machine-specific `InterpError` without allocation.
+/// (These will never be shown to the user, but they help diagnose ICEs.)
+pub(crate) macro throw_machine_stop_str($($tt:tt)*) {{
+    // We make a new local type for it. The type itself does not carry any information,
+    // but its vtable (for the `MachineStopType` trait) does.
+    #[derive(Debug)]
+    struct Zst;
+    // Printing this type shows the desired string.
+    impl std::fmt::Display for Zst {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            write!(f, $($tt)*)
+        }
+    }
+
+    impl rustc_middle::mir::interpret::MachineStopType for Zst {
+        fn diagnostic_message(&self) -> rustc_errors::DiagnosticMessage {
+            self.to_string().into()
+        }
+
+        fn add_args(
+            self: Box<Self>,
+            _: &mut dyn FnMut(rustc_errors::DiagnosticArgName, rustc_errors::DiagnosticArgValue),
+        ) {}
+    }
+    throw_machine_stop!(Zst)
+}}
 
 // These constants are somewhat random guesses and have not been optimized.
 // If `tcx.sess.mir_opt_level() >= 4`, we ignore the limits (this can become very expensive).

compiler/rustc_mir_transform/src/lib.rs

@@ -59,7 +59,6 @@ mod remove_place_mention;
 mod add_subtyping_projections;
 pub mod cleanup_post_borrowck;
 mod const_debuginfo;
-mod const_prop;
 mod const_prop_lint;
 mod copy_prop;
 mod coroutine;