Auto merge of #64470 - ecstatic-morse:split-promotion-and-validation, r=eddyb,oli-obk

Implement dataflow-based const validation

This PR adds a separate, dataflow-enabled pass that checks the bodies of `const`s, `static`s and `const fn`s for [const safety](https://github.com/rust-rfcs/const-eval/blob/master/const.md). This is based on my work in #63860, which tried to integrate into the existing pass in [`qualify_consts.rs`](https://github.com/rust-lang/rust/blob/master/src/librustc_mir/transform/qualify_consts.rs). However, the resulting pass was even more unwieldy than the original. Unlike its predecessor, this PR is designed to be combined with #63812 to replace the existing pass completely.

The new checker lives in [`librustc_mir/transform/check_consts`](https://github.com/ecstatic-morse/rust/tree/split-promotion-and-validation/src/librustc_mir/transform/check_consts).

[`qualifs.rs`](https://github.com/ecstatic-morse/rust/blob/split-promotion-and-validation/src/librustc_mir/transform/check_consts/qualifs.rs) contains small modifications to the existing `Qualif` trait and its implementors, but is mostly unchanged except for the removal of `IsNotPromotable` and `IsNotImplicitlyPromotable`, which are only necessary for promotion.

[`resolver.rs`](https://github.com/ecstatic-morse/rust/blob/split-promotion-and-validation/src/librustc_mir/transform/check_consts/resolver.rs) contains the dataflow analysis used to propagate qualifs between locals.

Finally, [`validation.rs`](https://github.com/ecstatic-morse/rust/blob/split-promotion-and-validation/src/librustc_mir/transform/check_consts/validation.rs) contains a refactored version of the existing [`Visitor`](https://github.com/rust-lang/rust/blob/ca3766e2e58f462a20922e42c821a37eaf0e13db/src/librustc_mir/transform/qualify_consts.rs#L1024) in `qualfy_consts.rs`. All errors have been associated with a `struct` to make [comparison with the existing pass](https://github.com/ecstatic-morse/rust/blob/1c19f2d540ca0a964900449d79a5d5181b43146d/src/librustc_mir/transform/qualify_consts.rs#L1006) simple.

The existing validation logic in [`qualify_consts`](https://github.com/rust-lang/rust/blob/master/src/librustc_mir/transform/qualify_consts.rs) has been modified to allow it to run in parallel with the new validator. If [`use_new_validator`](https://github.com/rust-lang/rust/pull/64470/files#diff-c2552a106550d05b69d5e07612f0f812R950) is not set, the old validation will be responsible for actually generating the errors, but those errors can be compared with the ones from the new validator.

Author: bors

src/librustc/session/config.rs

@@ -1359,6 +1359,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "describes how to render the `rendered` field of json diagnostics"),
     unleash_the_miri_inside_of_you: bool = (false, parse_bool, [TRACKED],
         "take the breaks off const evaluation. NOTE: this is unsound"),
+    suppress_const_validation_back_compat_ice: bool = (false, parse_bool, [TRACKED],
+        "silence ICE triggered when the new const validator disagrees with the old"),
     osx_rpath_install_name: bool = (false, parse_bool, [TRACKED],
         "pass `-install_name @rpath/...` to the macOS linker"),
     sanitizer: Option<Sanitizer> = (None, parse_sanitizer, [TRACKED],

src/librustc_mir/dataflow/impls/indirect_mutation.rs

@@ -0,0 +1,148 @@
+use rustc::mir::visit::Visitor;
+use rustc::mir::{self, Local, Location};
+use rustc::ty::{self, TyCtxt};
+use rustc_data_structures::bit_set::BitSet;
+use syntax_pos::DUMMY_SP;
+
+use crate::dataflow::{self, GenKillSet};
+
+/// Whether a borrow to a `Local` has been created that could allow that `Local` to be mutated
+/// indirectly. This could either be a mutable reference (`&mut`) or a shared borrow if the type of
+/// that `Local` allows interior mutability. Operations that can mutate local's indirectly include:
+/// assignments through a pointer (`*p = 42`), function calls, drop terminators and inline assembly.
+///
+/// If this returns false for a `Local` at a given statement (or terminator), that `Local` could
+/// not possibly have been mutated indirectly prior to that statement.
+#[derive(Copy, Clone)]
+pub struct IndirectlyMutableLocals<'mir, 'tcx> {
+    body: &'mir mir::Body<'tcx>,
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'mir, 'tcx> IndirectlyMutableLocals<'mir, 'tcx> {
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        body: &'mir mir::Body<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Self {
+        IndirectlyMutableLocals { body, tcx, param_env }
+    }
+
+    fn transfer_function<'a>(
+        &self,
+        trans: &'a mut GenKillSet<Local>,
+    ) -> TransferFunction<'a, 'mir, 'tcx> {
+        TransferFunction {
+            body: self.body,
+            tcx: self.tcx,
+            param_env: self.param_env,
+            trans
+        }
+    }
+}
+
+impl<'mir, 'tcx> dataflow::BitDenotation<'tcx> for IndirectlyMutableLocals<'mir, 'tcx> {
+    type Idx = Local;
+
+    fn name() -> &'static str { "mut_borrowed_locals" }
+
+    fn bits_per_block(&self) -> usize {
+        self.body.local_decls.len()
+    }
+
+    fn start_block_effect(&self, _entry_set: &mut BitSet<Local>) {
+        // Nothing is borrowed on function entry
+    }
+
+    fn statement_effect(
+        &self,
+        trans: &mut GenKillSet<Local>,
+        loc: Location,
+    ) {
+        let stmt = &self.body[loc.block].statements[loc.statement_index];
+        self.transfer_function(trans).visit_statement(stmt, loc);
+    }
+
+    fn terminator_effect(
+        &self,
+        trans: &mut GenKillSet<Local>,
+        loc: Location,
+    ) {
+        let terminator = self.body[loc.block].terminator();
+        self.transfer_function(trans).visit_terminator(terminator, loc);
+    }
+
+    fn propagate_call_return(
+        &self,
+        _in_out: &mut BitSet<Local>,
+        _call_bb: mir::BasicBlock,
+        _dest_bb: mir::BasicBlock,
+        _dest_place: &mir::Place<'tcx>,
+    ) {
+        // Nothing to do when a call returns successfully
+    }
+}
+
+impl<'mir, 'tcx> dataflow::BottomValue for IndirectlyMutableLocals<'mir, 'tcx> {
+    // bottom = unborrowed
+    const BOTTOM_VALUE: bool = false;
+}
+
+/// A `Visitor` that defines the transfer function for `IndirectlyMutableLocals`.
+struct TransferFunction<'a, 'mir, 'tcx> {
+    trans: &'a mut GenKillSet<Local>,
+    body: &'mir mir::Body<'tcx>,
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'tcx> Visitor<'tcx> for TransferFunction<'_, '_, 'tcx> {
+    fn visit_rvalue(
+        &mut self,
+        rvalue: &mir::Rvalue<'tcx>,
+        location: Location,
+    ) {
+        if let mir::Rvalue::Ref(_, kind, ref borrowed_place) = *rvalue {
+            let is_mut = match kind {
+                mir::BorrowKind::Mut { .. } => true,
+
+                | mir::BorrowKind::Shared
+                | mir::BorrowKind::Shallow
+                | mir::BorrowKind::Unique
+                => {
+                    !borrowed_place
+                        .ty(self.body, self.tcx)
+                        .ty
+                        .is_freeze(self.tcx, self.param_env, DUMMY_SP)
+                }
+            };
+
+            if is_mut {
+                match borrowed_place.base {
+                    mir::PlaceBase::Local(borrowed_local) if !borrowed_place.is_indirect()
+                        => self.trans.gen(borrowed_local),
+
+                    _ => (),
+                }
+            }
+        }
+
+        self.super_rvalue(rvalue, location);
+    }
+
+
+    fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
+        // This method purposely does nothing except call `super_terminator`. It exists solely to
+        // document the subtleties around drop terminators.
+
+        self.super_terminator(terminator, location);
+
+        if let mir::TerminatorKind::Drop { location: _, .. }
+             | mir::TerminatorKind::DropAndReplace { location: _, .. } = &terminator.kind
+        {
+            // Although drop terminators mutably borrow the location being dropped, that borrow
+            // cannot live beyond the drop terminator because the dropped location is invalidated.
+        }
+    }
+}

src/librustc_mir/dataflow/impls/mod.rs

@@ -18,13 +18,13 @@ use super::drop_flag_effects_for_function_entry;
 use super::drop_flag_effects_for_location;
 use super::on_lookup_result_bits;
 
-mod storage_liveness;
-
-pub use self::storage_liveness::*;
-
 mod borrowed_locals;
+mod indirect_mutation;
+mod storage_liveness;
 
 pub use self::borrowed_locals::*;
+pub use self::indirect_mutation::IndirectlyMutableLocals;
+pub use self::storage_liveness::*;
 
 pub(super) mod borrows;
 

src/librustc_mir/dataflow/mod.rs

@@ -23,6 +23,7 @@ pub use self::impls::DefinitelyInitializedPlaces;
 pub use self::impls::EverInitializedPlaces;
 pub use self::impls::borrows::Borrows;
 pub use self::impls::HaveBeenBorrowedLocals;
+pub use self::impls::IndirectlyMutableLocals;
 pub use self::at_location::{FlowAtLocation, FlowsAtLocation};
 pub(crate) use self::drop_flag_effects::*;
 

src/librustc_mir/transform/check_consts/mod.rs

@@ -0,0 +1,52 @@
+//! Check the bodies of `const`s, `static`s and `const fn`s for illegal operations.
+//!
+//! This module will eventually replace the parts of `qualify_consts.rs` that check whether a local
+//! has interior mutability or needs to be dropped, as well as the visitor that emits errors when
+//! it finds operations that are invalid in a certain context.
+
+use rustc::hir::def_id::DefId;
+use rustc::mir;
+use rustc::ty::{self, TyCtxt};
+
+pub use self::qualifs::Qualif;
+
+pub mod ops;
+mod qualifs;
+mod resolver;
+pub mod validation;
+
+/// Information about the item currently being validated, as well as a reference to the global
+/// context.
+pub struct Item<'mir, 'tcx> {
+    body: &'mir mir::Body<'tcx>,
+    tcx: TyCtxt<'tcx>,
+    def_id: DefId,
+    param_env: ty::ParamEnv<'tcx>,
+    mode: validation::Mode,
+}
+
+impl Item<'mir, 'tcx> {
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        def_id: DefId,
+        body: &'mir mir::Body<'tcx>,
+    ) -> Self {
+        let param_env = tcx.param_env(def_id);
+        let mode = validation::Mode::for_item(tcx, def_id)
+            .expect("const validation must only be run inside a const context");
+
+        Item {
+            body,
+            tcx,
+            def_id,
+            param_env,
+            mode,
+        }
+    }
+}
+
+
+fn is_lang_panic_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
+    Some(def_id) == tcx.lang_items().panic_fn() ||
+    Some(def_id) == tcx.lang_items().begin_panic_fn()
+}