make relate's const ty assertion use semantic equality

Author: BoxyUwU

Diff for: compiler/rustc_infer/src/infer/combine.rs

@@ -31,6 +31,7 @@ use super::{InferCtxt, MiscVariable, TypeTrace};
 use crate::traits::{Obligation, PredicateObligations};
 use rustc_data_structures::sso::SsoHashMap;
 use rustc_hir::def_id::DefId;
+use rustc_middle::infer::canonical::OriginalQueryValues;
 use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
 use rustc_middle::traits::ObligationCause;
@@ -152,6 +153,33 @@ impl<'tcx> InferCtxt<'tcx> {
         let a = self.shallow_resolve(a);
         let b = self.shallow_resolve(b);
 
+        // We should never have to relate the `ty` field on `Const` as it is checked elsewhere that consts have the
+        // correct type for the generic param they are an argument for. However there have been a number of cases
+        // historically where asserting that the types are equal has found bugs in the compiler so this is valuable
+        // to check even if it is a bit nasty impl wise :(
+        //
+        // This probe is probably not strictly necessary but it seems better to be safe and not accidentally find
+        // ourselves with a check to find bugs being required for code to compile because it made inference progress.
+        self.probe(|_| {
+            if a.ty() == b.ty() {
+                return;
+            }
+
+            // We don't have access to trait solving machinery in `rustc_infer` so the logic for determining if the
+            // two const param's types are able to be equal has to go through a canonical query with the actual logic
+            // in `rustc_trait_selection`.
+            let canonical = self.canonicalize_query(
+                (relation.param_env(), a.ty(), b.ty()),
+                &mut OriginalQueryValues::default(),
+            );
+            if let Err(()) = self.tcx.check_const_param_definitely_unequal(canonical) {
+                self.tcx.sess.delay_span_bug(
+                    DUMMY_SP,
+                    &format!("cannot relate consts of different types (a={:?}, b={:?})", a, b,),
+                );
+            }
+        });
+
         match (a.kind(), b.kind()) {
             (
                 ty::ConstKind::Infer(InferConst::Var(a_vid)),

Diff for: compiler/rustc_middle/src/query/mod.rs

@@ -2168,4 +2168,11 @@ rustc_queries! {
         desc { "traits in scope for documentation links for a module" }
         separate_provide_extern
     }
+
+    /// Used in `super_combine_consts` to ICE if the type of the two consts are definitely not going to end up being
+    /// equal to eachother. This might return `Ok` even if the types are unequal, but will never return `Err` if
+    /// the types might be equal.
+    query check_const_param_definitely_unequal(arg: Canonical<'tcx, (ty::ParamEnv<'tcx>, Ty<'tcx>, Ty<'tcx>)>) -> Result<(), ()> {
+        desc { "check whether two const param are definitely not equal to eachother"}
+    }
 }

Diff for: compiler/rustc_middle/src/ty/relate.rs

@@ -9,7 +9,6 @@ use crate::ty::{self, Expr, ImplSubject, Term, TermKind, Ty, TyCtxt, TypeFoldabl
 use crate::ty::{GenericArg, GenericArgKind, SubstsRef};
 use rustc_hir as ast;
 use rustc_hir::def_id::DefId;
-use rustc_span::DUMMY_SP;
 use rustc_target::spec::abi;
 use std::iter;
 
@@ -594,25 +593,6 @@ pub fn super_relate_consts<'tcx, R: TypeRelation<'tcx>>(
     debug!("{}.super_relate_consts(a = {:?}, b = {:?})", relation.tag(), a, b);
     let tcx = relation.tcx();
 
-    let a_ty;
-    let b_ty;
-    if relation.tcx().features().adt_const_params {
-        a_ty = tcx.normalize_erasing_regions(relation.param_env(), a.ty());
-        b_ty = tcx.normalize_erasing_regions(relation.param_env(), b.ty());
-    } else {
-        a_ty = tcx.erase_regions(a.ty());
-        b_ty = tcx.erase_regions(b.ty());
-    }
-    if a_ty != b_ty {
-        relation.tcx().sess.delay_span_bug(
-            DUMMY_SP,
-            &format!(
-                "cannot relate constants ({:?}, {:?}) of different types: {} != {}",
-                a, b, a_ty, b_ty
-            ),
-        );
-    }
-
     // HACK(const_generics): We still need to eagerly evaluate consts when
     // relating them because during `normalize_param_env_or_error`,
     // we may relate an evaluated constant in a obligation against

Diff for: compiler/rustc_trait_selection/src/traits/misc.rs

@@ -1,12 +1,14 @@
 //! Miscellaneous type-system utilities that are too small to deserve their own modules.
 
-use crate::traits::{self, ObligationCause};
+use crate::traits::{self, ObligationCause, ObligationCtxt};
 
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir as hir;
+use rustc_infer::infer::canonical::Canonical;
 use rustc_infer::infer::{RegionResolutionError, TyCtxtInferExt};
 use rustc_infer::{infer::outlives::env::OutlivesEnvironment, traits::FulfillmentError};
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitable};
+use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt, TypeVisitable};
+use rustc_span::DUMMY_SP;
 
 use super::outlives_bounds::InferCtxtExt;
 
@@ -131,3 +133,19 @@ pub fn type_allowed_to_implement_copy<'tcx>(
 
     Ok(())
 }
+
+pub fn check_const_param_definitely_unequal<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    canonical: Canonical<'tcx, (ParamEnv<'tcx>, Ty<'tcx>, Ty<'tcx>)>,
+) -> Result<(), ()> {
+    let (infcx, (param_env, ty_a, ty_b), _) =
+        tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &canonical);
+    let ocx = ObligationCtxt::new(&infcx);
+
+    let result = ocx.eq(&ObligationCause::dummy(), param_env, ty_a, ty_b);
+    // use `select_where_possible` instead of `select_all_or_error` so that
+    // we don't get errors from obligations being ambiguous.
+    let errors = ocx.select_where_possible();
+
+    if errors.len() > 0 || result.is_err() { Err(()) } else { Ok(()) }
+}

Diff for: compiler/rustc_trait_selection/src/traits/mod.rs

@@ -554,6 +554,7 @@ pub fn provide(providers: &mut ty::query::Providers) {
         specialization_graph_of: specialize::specialization_graph_provider,
         specializes: specialize::specializes,
         subst_and_check_impossible_predicates,
+        check_const_param_definitely_unequal: misc::check_const_param_definitely_unequal,
         is_impossible_method,
         ..*providers
     };

Diff for: tests/ui/const-generics/generic_const_exprs/const_kind_expr/relate_ty_with_infer_1.rs

@@ -0,0 +1,30 @@
+// check-pass
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+
+// issue #107899
+// We end up relating `Const(ty: size_of<?0>, kind: Value(Branch([])))` with
+// `Const(ty: size_of<T>, kind: Value(Branch([])))` which if you were to `==`
+// the `ty` fields would return `false` and ICE. This test checks that we use
+// actual semantic equality that takes into account aliases and infer vars.
+
+use std::mem::size_of;
+
+trait X<T> {
+    fn f(self);
+    fn g(self);
+}
+
+struct Y;
+
+impl<T> X<T> for Y
+where
+    [(); size_of::<T>()]: Sized,
+{
+    fn f(self) {
+        self.g();
+    }
+    fn g(self) {}
+}
+
+fn main() {}

Diff for: tests/ui/const-generics/generic_const_exprs/const_kind_expr/relate_ty_with_infer_2.rs

@@ -0,0 +1,151 @@
+// check-pass
+#![feature(inline_const, generic_const_exprs)]
+#![allow(incomplete_features)]
+use std::marker::PhantomData;
+
+pub struct Equal<const T: usize, const R: usize>();
+pub trait True {}
+impl<const T: usize> True for Equal<T, T> {}
+
+// replacement for generativity
+pub struct Id<'id>(PhantomData<fn(&'id ()) -> &'id ()>);
+pub struct Guard<'id>(Id<'id>);
+fn make_guard<'id>(i: &'id Id<'id>) -> Guard<'id> {
+    Guard(Id(PhantomData))
+}
+
+impl<'id> Into<Id<'id>> for Guard<'id> {
+    fn into(self) -> Id<'id> {
+        self.0
+    }
+}
+
+pub struct Arena<'life> {
+    bytes: *mut [u8],
+    //bitmap: RefCell<RoaringBitmap>,
+    _token: PhantomData<Id<'life>>,
+}
+
+#[repr(transparent)]
+pub struct Item<'life, T> {
+    data: T,
+    _phantom: PhantomData<Id<'life>>,
+}
+
+#[repr(transparent)]
+pub struct Token<'life, 'borrow, 'compact, 'reborrow, T>
+where
+    'life: 'reborrow,
+    T: Tokenize<'life, 'borrow, 'compact, 'reborrow>,
+{
+    //ptr: *mut <T as Tokenize>::Tokenized,
+    ptr: core::ptr::NonNull<T::Tokenized>,
+    _phantom: PhantomData<Id<'life>>,
+    _compact: PhantomData<&'borrow Guard<'compact>>,
+    _result: PhantomData<&'reborrow T::Untokenized>,
+}
+
+impl<'life> Arena<'life> {
+    pub fn tokenize<'before, 'compact, 'borrow, 'reborrow, T, U>(
+        &self,
+        guard: &'borrow Guard<'compact>,
+        item: Item<'life, &'before mut T>,
+    ) -> Token<'life, 'borrow, 'compact, 'reborrow, U>
+    where
+        T: Tokenize<'life, 'borrow, 'compact, 'reborrow, Untokenized = U>,
+        T::Untokenized: Tokenize<'life, 'borrow, 'compact, 'reborrow>,
+        Equal<{ core::mem::size_of::<T>() }, { core::mem::size_of::<U>() }>: True,
+        'compact: 'borrow,
+        'life: 'reborrow,
+        'life: 'compact,
+        'life: 'borrow,
+        // 'borrow: 'before ??
+    {
+        let dst = item.data as *mut T as *mut T::Tokenized;
+        Token {
+            ptr: core::ptr::NonNull::new(dst as *mut _).unwrap(),
+            _phantom: PhantomData,
+            _compact: PhantomData,
+            _result: PhantomData,
+        }
+    }
+}
+
+pub trait Tokenize<'life, 'borrow, 'compact, 'reborrow>
+where
+    'compact: 'borrow,
+    'life: 'reborrow,
+    'life: 'borrow,
+    'life: 'compact,
+{
+    type Tokenized;
+    type Untokenized;
+    const TO: fn(&Arena<'life>, &'borrow Guard<'compact>, Self) -> Self::Tokenized;
+    const FROM: fn(&'reborrow Arena<'life>, Self::Tokenized) -> Self::Untokenized;
+}
+
+macro_rules! tokenize {
+    ($to:expr, $from:expr) => {
+        const TO: fn(&Arena<'life>, &'borrow Guard<'compact>, Self) -> Self::Tokenized = $to;
+        const FROM: fn(&'reborrow Arena<'life>, Self::Tokenized) -> Self::Untokenized = $from;
+    };
+}
+
+struct Foo<'life, 'borrow>(Option<Item<'life, &'borrow mut Bar>>);
+struct TokenFoo<'life, 'borrow, 'compact, 'reborrow>(
+    Option<Token<'life, 'borrow, 'compact, 'reborrow, Bar>>,
+);
+struct Bar(u8);
+
+impl<'life, 'before, 'borrow, 'compact, 'reborrow> Tokenize<'life, 'borrow, 'compact, 'reborrow>
+    for Foo<'life, 'before>
+where
+    'compact: 'borrow,
+    'life: 'reborrow,
+    'life: 'borrow,
+    'life: 'compact,
+{
+    type Tokenized = TokenFoo<'life, 'borrow, 'compact, 'reborrow>;
+    type Untokenized = Foo<'life, 'reborrow>;
+    tokenize!(foo_to, foo_from);
+}
+
+impl<'life, 'borrow, 'compact, 'reborrow> Tokenize<'life, 'borrow, 'compact, 'reborrow> for Bar
+where
+    'compact: 'borrow,
+    'life: 'reborrow,
+    'life: 'borrow,
+    'life: 'compact,
+{
+    type Tokenized = Bar;
+    type Untokenized = Bar;
+    tokenize!(bar_to, bar_from);
+}
+
+fn bar_to<'life, 'borrow, 'compact>(
+    arena: &Arena<'life>,
+    guard: &'borrow Guard<'compact>,
+    s: Bar,
+) -> Bar {
+    s
+}
+fn bar_from<'life, 'reborrow>(arena: &'reborrow Arena<'life>, s: Bar) -> Bar {
+    s
+}
+
+fn foo_to<'life, 'borrow, 'compact, 'reborrow, 'before>(
+    arena: &'before Arena<'life>,
+    guard: &'borrow Guard<'compact>,
+    s: Foo<'life, 'before>,
+) -> TokenFoo<'life, 'borrow, 'compact, 'reborrow> {
+    let Foo(bar) = s;
+    TokenFoo(bar.map(|bar| arena.tokenize(guard, bar)))
+}
+fn foo_from<'life, 'borrow, 'compact, 'reborrow>(
+    arena: &'reborrow Arena<'life>,
+    s: TokenFoo<'life, 'borrow, 'compact, 'reborrow>,
+) -> Foo<'life, 'reborrow> {
+    Foo(s.0.map(|bar| panic!()))
+}
+
+fn main() {}