Auto merge of #74595 - lcnr:ConstEvaluatable-fut-compat, r=oli-obk

make `ConstEvaluatable` more strict

relevant zulip discussion: https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/.60ConstEvaluatable.60.20generic.20functions/near/204125452

Let's see how much this impacts. Depending on how this goes this should probably be a future compat warning.

Short explanation: we currently forbid anonymous constants which depend on generic types, e.g. `[0; std::mem::size_of::<T>]` currently errors.

We previously checked this by evaluating the constant and returned an error if that failed. This however allows things like
```rust
const fn foo<T>() -> usize {
    if std::mem::size_of::<*mut T>() < 8 { // size of *mut T does not depend on T
        std::mem::size_of::<T>()
    } else {
        8
    }
}

fn test<T>() {
    let _ = [0; foo::<T>()];
}
```
which is a backwards compatibility hazard. This also has worrying interactions with mir optimizations (https://github.com/rust-lang/rust/pull/74491#issuecomment-661890421) and intrinsics (#74538).

r? `@oli-obk` `@eddyb`

compiler/rustc_middle/src/mir/mod.rs

@@ -186,6 +186,23 @@ pub struct Body<'tcx> {
     /// FIXME(oli-obk): rewrite the promoted during promotion to eliminate the cell components.
     pub ignore_interior_mut_in_const_validation: bool,
 
+    /// Does this body use generic parameters. This is used for the `ConstEvaluatable` check.
+    ///
+    /// Note that this does not actually mean that this body is not computable right now.
+    /// The repeat count in the following example is polymorphic, but can still be evaluated
+    /// without knowing anything about the type parameter `T`.
+    ///
+    /// ```rust
+    /// fn test<T>() {
+    ///     let _ = [0; std::mem::size_of::<*mut T>()];
+    /// }
+    /// ```
+    ///
+    /// **WARNING**: Do not change this flags after the MIR was originally created, even if an optimization
+    /// removed the last mention of all generic params. We do not want to rely on optimizations and
+    /// potentially allow things like `[u8; std::mem::size_of::<T>() * 0]` due to this.
+    pub is_polymorphic: bool,
+
     predecessor_cache: PredecessorCache,
 }
 
@@ -208,7 +225,7 @@ impl<'tcx> Body<'tcx> {
             local_decls.len()
         );
 
-        Body {
+        let mut body = Body {
             phase: MirPhase::Build,
             basic_blocks,
             source_scopes,
@@ -224,8 +241,11 @@ impl<'tcx> Body<'tcx> {
             span,
             required_consts: Vec::new(),
             ignore_interior_mut_in_const_validation: false,
+            is_polymorphic: false,
             predecessor_cache: PredecessorCache::new(),
-        }
+        };
+        body.is_polymorphic = body.has_param_types_or_consts();
+        body
     }
 
     /// Returns a partially initialized MIR body containing only a list of basic blocks.
@@ -234,7 +254,7 @@ impl<'tcx> Body<'tcx> {
     /// is only useful for testing but cannot be `#[cfg(test)]` because it is used in a different
     /// crate.
     pub fn new_cfg_only(basic_blocks: IndexVec<BasicBlock, BasicBlockData<'tcx>>) -> Self {
-        Body {
+        let mut body = Body {
             phase: MirPhase::Build,
             basic_blocks,
             source_scopes: IndexVec::new(),
@@ -250,8 +270,11 @@ impl<'tcx> Body<'tcx> {
             generator_kind: None,
             var_debug_info: Vec::new(),
             ignore_interior_mut_in_const_validation: false,
+            is_polymorphic: false,
             predecessor_cache: PredecessorCache::new(),
-        }
+        };
+        body.is_polymorphic = body.has_param_types_or_consts();
+        body
     }
 
     #[inline]

compiler/rustc_session/src/lint/builtin.rs

@@ -539,6 +539,16 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    pub CONST_EVALUATABLE_UNCHECKED,
+    Warn,
+    "detects a generic constant is used in a type without a emitting a warning",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #76200 <https://github.com/rust-lang/rust/issues/76200>",
+        edition: None,
+    };
+}
+
 declare_lint_pass! {
     /// Does nothing as a lint pass, but registers some `Lint`s
     /// that are used by other parts of the compiler.
@@ -612,6 +622,7 @@ declare_lint_pass! {
         UNSAFE_OP_IN_UNSAFE_FN,
         INCOMPLETE_INCLUDE,
         CENUM_IMPL_DROP_CAST,
+        CONST_EVALUATABLE_UNCHECKED,
     ]
 }
 

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -0,0 +1,61 @@
+use rustc_hir::def::DefKind;
+use rustc_infer::infer::InferCtxt;
+use rustc_middle::mir::interpret::ErrorHandled;
+use rustc_middle::ty::subst::SubstsRef;
+use rustc_middle::ty::{self, TypeFoldable};
+use rustc_session::lint;
+use rustc_span::def_id::DefId;
+use rustc_span::Span;
+
+pub fn is_const_evaluatable<'cx, 'tcx>(
+    infcx: &InferCtxt<'cx, 'tcx>,
+    def: ty::WithOptConstParam<DefId>,
+    substs: SubstsRef<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    span: Span,
+) -> Result<(), ErrorHandled> {
+    let future_compat_lint = || {
+        if let Some(local_def_id) = def.did.as_local() {
+            infcx.tcx.struct_span_lint_hir(
+                lint::builtin::CONST_EVALUATABLE_UNCHECKED,
+                infcx.tcx.hir().local_def_id_to_hir_id(local_def_id),
+                span,
+                |err| {
+                    err.build("cannot use constants which depend on generic parameters in types")
+                        .emit();
+                },
+            );
+        }
+    };
+
+    // FIXME: We should only try to evaluate a given constant here if it is fully concrete
+    // as we don't want to allow things like `[u8; std::mem::size_of::<*mut T>()]`.
+    //
+    // We previously did not check this, so we only emit a future compat warning if
+    // const evaluation succeeds and the given constant is still polymorphic for now
+    // and hopefully soon change this to an error.
+    //
+    // See #74595 for more details about this.
+    let concrete = infcx.const_eval_resolve(param_env, def, substs, None, Some(span));
+
+    let def_kind = infcx.tcx.def_kind(def.did);
+    match def_kind {
+        DefKind::AnonConst => {
+            let mir_body = if let Some(def) = def.as_const_arg() {
+                infcx.tcx.optimized_mir_of_const_arg(def)
+            } else {
+                infcx.tcx.optimized_mir(def.did)
+            };
+            if mir_body.is_polymorphic && concrete.is_ok() {
+                future_compat_lint();
+            }
+        }
+        _ => {
+            if substs.has_param_types_or_consts() && concrete.is_ok() {
+                future_compat_lint();
+            }
+        }
+    }
+
+    concrete.map(drop)
+}

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -10,6 +10,7 @@ use rustc_middle::ty::ToPredicate;
 use rustc_middle::ty::{self, Binder, Const, Ty, TypeFoldable};
 use std::marker::PhantomData;
 
+use super::const_evaluatable;
 use super::project;
 use super::select::SelectionContext;
 use super::wf;
@@ -458,15 +459,15 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                 }
 
                 ty::PredicateAtom::ConstEvaluatable(def_id, substs) => {
-                    match self.selcx.infcx().const_eval_resolve(
-                        obligation.param_env,
+                    match const_evaluatable::is_const_evaluatable(
+                        self.selcx.infcx(),
                         def_id,
                         substs,
-                        None,
-                        Some(obligation.cause.span),
+                        obligation.param_env,
+                        obligation.cause.span,
                     ) {
-                        Ok(_) => ProcessResult::Changed(vec![]),
-                        Err(err) => ProcessResult::Error(CodeSelectionError(ConstEvalFailure(err))),
+                        Ok(()) => ProcessResult::Changed(vec![]),
+                        Err(e) => ProcessResult::Error(CodeSelectionError(ConstEvalFailure(e))),
                     }
                 }
 

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -7,6 +7,7 @@ pub mod auto_trait;
 mod chalk_fulfill;
 pub mod codegen;
 mod coherence;
+mod const_evaluatable;
 mod engine;
 pub mod error_reporting;
 mod fulfill;

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -6,6 +6,7 @@ use self::EvaluationResult::*;
 use self::SelectionCandidate::*;
 
 use super::coherence::{self, Conflict};
+use super::const_evaluatable;
 use super::project;
 use super::project::normalize_with_depth_to;
 use super::util;
@@ -542,14 +543,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             ty::PredicateAtom::ConstEvaluatable(def_id, substs) => {
-                match self.tcx().const_eval_resolve(
-                    obligation.param_env,
+                match const_evaluatable::is_const_evaluatable(
+                    self.infcx,
                     def_id,
                     substs,
-                    None,
-                    None,
+                    obligation.param_env,
+                    obligation.cause.span,
                 ) {
-                    Ok(_) => Ok(EvaluatedToOk),
+                    Ok(()) => Ok(EvaluatedToOk),
                     Err(ErrorHandled::TooGeneric) => Ok(EvaluatedToAmbig),
                     Err(_) => Ok(EvaluatedToErr),
                 }

src/test/ui/const_evaluatable/associated-const.rs

@@ -0,0 +1,11 @@
+// check-pass
+struct Foo<T>(T);
+impl<T> Foo<T> {
+    const VALUE: usize = std::mem::size_of::<T>();
+}
+
+fn test<T>() {
+    let _ = [0; Foo::<u8>::VALUE];
+}
+
+fn main() {}

src/test/ui/const_evaluatable/function-call.rs

@@ -0,0 +1,19 @@
+// check-pass
+
+const fn foo<T>() -> usize {
+    // We might instead branch on `std::mem::size_of::<*mut T>() < 8` here,
+    // which would cause this function to fail on 32 bit systems.
+    if false {
+        std::mem::size_of::<T>()
+    } else {
+        8
+    }
+}
+
+fn test<T>() {
+    let _ = [0; foo::<T>()];
+    //~^ WARN cannot use constants which depend on generic parameters in types
+    //~| WARN this was previously accepted by the compiler but is being phased out
+}
+
+fn main() {}

src/test/ui/const_evaluatable/function-call.stderr

@@ -0,0 +1,12 @@
+warning: cannot use constants which depend on generic parameters in types
+  --> $DIR/function-call.rs:14:17
+   |
+LL |     let _ = [0; foo::<T>()];
+   |                 ^^^^^^^^^^
+   |
+   = note: `#[warn(const_evaluatable_unchecked)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #76200 <https://github.com/rust-lang/rust/issues/76200>
+
+warning: 1 warning emitted
+

src/test/ui/enum-discriminant/issue-70453-polymorphic-ctfe.rs

@@ -1,5 +1,4 @@
 // run-pass
-
 #![feature(arbitrary_enum_discriminant, core_intrinsics)]
 
 extern crate core;
@@ -9,6 +8,8 @@ use core::intrinsics::discriminant_value;
 enum MyWeirdOption<T> {
     None = 0,
     Some(T) = core::mem::size_of::<*mut T>(),
+    //~^ WARN cannot use constants which depend on generic parameters in types
+    //~| WARN this was previously accepted by the compiler but is being phased out
 }
 
 fn main() {

src/test/ui/enum-discriminant/issue-70453-polymorphic-ctfe.stderr

@@ -0,0 +1,12 @@
+warning: cannot use constants which depend on generic parameters in types
+  --> $DIR/issue-70453-polymorphic-ctfe.rs:10:15
+   |
+LL |     Some(T) = core::mem::size_of::<*mut T>(),
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(const_evaluatable_unchecked)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #76200 <https://github.com/rust-lang/rust/issues/76200>
+
+warning: 1 warning emitted
+

src/test/ui/impl-trait/issue-56445.rs

@@ -5,8 +5,7 @@
 
 use std::marker::PhantomData;
 
-pub struct S<'a>
-{
+pub struct S<'a> {
     pub m1: PhantomData<&'a u8>,
     pub m2: [u8; S::size()],
 }

src/test/ui/lazy_normalization_consts/issue-73980.rs

@@ -10,5 +10,7 @@ impl<T: ?Sized> L<T> {
 }
 
 impl<T> X<T, [u8; L::<T>::S]> {}
+//~^ WARN cannot use constants which depend on generic parameters
+//~| WARN this was previously accepted by the compiler but is being phased out
 
 fn main() {}

src/test/ui/lazy_normalization_consts/issue-73980.stderr

@@ -0,0 +1,12 @@
+warning: cannot use constants which depend on generic parameters in types
+  --> $DIR/issue-73980.rs:12:9
+   |
+LL | impl<T> X<T, [u8; L::<T>::S]> {}
+   |         ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(const_evaluatable_unchecked)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #76200 <https://github.com/rust-lang/rust/issues/76200>
+
+warning: 1 warning emitted
+