Auto merge of rust-lang#121557 - RalfJung:const-fn-call-promotion, r=<try>

restrict promotion of `const fn` calls

We only promote them in `const`/`static` initializers, but even that is still unfortunate -- we still cannot add promoteds to required_consts. But we should add them there to make sure it's always okay to evaluate every const we encounter in a MIR body.  That effort of not promoting things that can fail to evaluate is tracked in rust-lang#80619. These `const fn` calls are the last missing piece.

So I propose that we do not promote const-fn calls in const when that may fail without the entire const failing, thereby completing rust-lang#80619. Unfortunately we can't just reject promoting these functions outright due to backwards compatibility. So let's see if we can find a hack that makes crater happy...

For the record, this is the [crater analysis](rust-lang#80243 (comment)) from when I tried to entirely forbid this kind of promotion. It's a tiny amount of breakage and if we had a nice alternative for code like that, we could conceivably push it through... but sadly, inline const expressions are still blocked on t-lang concerns about post-monomorphization errors and we haven't yet figured out an implementation that can resolve those concerns. So we're forced to make progress via other means, such as terrible hacks like this.

Attempt one: only promote calls on the "safe path" at the beginning of a MIR block. This is the path that starts at the start block and continues via gotos and calls, but stops at the first branch. If we had imposed this restriction before stabilizing `if` and `match` in `const`, this would have definitely been sufficient...

EDIT: Turns out that works. :)
**Here's the t-lang [nomination comment](rust-lang#121557 (comment) And here's the [FCP comment](rust-lang#121557 (comment)).

r? `@oli-obk`

Author: bors

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -375,6 +375,11 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
 
     const PANIC_ON_ALLOC_FAIL: bool = false; // will be raised as a proper error
 
+    #[inline]
+    fn all_required_consts_are_checked(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        true
+    }
+
     #[inline(always)]
     fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
         matches!(ecx.machine.check_alignment, CheckAlignment::Error)

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -822,15 +822,13 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         self.stack_mut().push(frame);
 
         // Make sure all the constants required by this frame evaluate successfully (post-monomorphization check).
-        if M::POST_MONO_CHECKS {
-            for &const_ in &body.required_consts {
-                let c = self
-                    .instantiate_from_current_frame_and_normalize_erasing_regions(const_.const_)?;
-                c.eval(*self.tcx, self.param_env, const_.span).map_err(|err| {
-                    err.emit_note(*self.tcx);
-                    err
-                })?;
-            }
+        for &const_ in &body.required_consts {
+            let c =
+                self.instantiate_from_current_frame_and_normalize_erasing_regions(const_.const_)?;
+            c.eval(*self.tcx, self.param_env, const_.span).map_err(|err| {
+                err.emit_note(*self.tcx);
+                err
+            })?;
         }
 
         // done
@@ -1179,8 +1177,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
         M::eval_mir_constant(self, *val, span, layout, |ecx, val, span, layout| {
             let const_val = val.eval(*ecx.tcx, ecx.param_env, span).map_err(|err| {
-                // FIXME: somehow this is reachable even when POST_MONO_CHECKS is on.
-                // Are we not always populating `required_consts`?
+                if M::all_required_consts_are_checked(self)
+                    && !matches!(err, ErrorHandled::TooGeneric(..))
+                {
+                    // Looks like the const is not captued by `required_consts`, that's bad.
+                    bug!("interpret const eval failure of {val:?} which is not in required_consts");
+                }
                 err.emit_note(*ecx.tcx);
                 err
             })?;

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -140,8 +140,9 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
     /// Should the machine panic on allocation failures?
     const PANIC_ON_ALLOC_FAIL: bool;
 
-    /// Should post-monomorphization checks be run when a stack frame is pushed?
-    const POST_MONO_CHECKS: bool = true;
+    /// Determines whether `eval_mir_constant` can never fail because all required consts have
+    /// already been checked before.
+    fn all_required_consts_are_checked(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;
 
     /// Whether memory accesses should be alignment-checked.
     fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

compiler/rustc_middle/src/mir/consts.rs

@@ -238,6 +238,24 @@ impl<'tcx> Const<'tcx> {
         }
     }
 
+    /// Determines whether we need to add this const to `required_consts`. This is the case if and
+    /// only if evaluating it may error.
+    #[inline]
+    pub fn is_required_const(&self) -> bool {
+        match self {
+            Const::Ty(c) => match c.kind() {
+                ty::ConstKind::Value(_) => false, // already a value, cannot error
+                ty::ConstKind::Param(_) | ty::ConstKind::Error(_) => true, // these are errors or could be replaced by errors
+                _ => bug!(
+                    "only ConstKind::Param/Value/Error should be encountered here, got {:#?}",
+                    c
+                ),
+            },
+            Const::Unevaluated(..) => true,
+            Const::Val(..) => false, // already a value, cannot error
+        }
+    }
+
     #[inline]
     pub fn try_to_scalar(self) -> Option<Scalar> {
         match self {

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -902,6 +902,12 @@ impl<'mir, 'tcx: 'mir> rustc_const_eval::interpret::Machine<'mir, 'tcx> for Dumm
     type MemoryKind = !;
     const PANIC_ON_ALLOC_FAIL: bool = true;
 
+    #[inline(always)]
+    fn all_required_consts_are_checked(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        // We want to just eval random consts in the program, so `eval_mir_const` can fail.
+        false
+    }
+
     #[inline(always)]
     fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
         false // no reason to enforce alignment

compiler/rustc_mir_transform/src/inline.rs

@@ -706,17 +706,7 @@ impl<'tcx> Inliner<'tcx> {
         });
 
         // Copy only unevaluated constants from the callee_body into the caller_body.
-        // Although we are only pushing `ConstKind::Unevaluated` consts to
-        // `required_consts`, here we may not only have `ConstKind::Unevaluated`
-        // because we are calling `instantiate_and_normalize_erasing_regions`.
-        caller_body.required_consts.extend(callee_body.required_consts.iter().copied().filter(
-            |&ct| match ct.const_ {
-                Const::Ty(_) => {
-                    bug!("should never encounter ty::UnevaluatedConst in `required_consts`")
-                }
-                Const::Val(..) | Const::Unevaluated(..) => true,
-            },
-        ));
+        caller_body.required_consts.extend(callee_body.required_consts);
     }
 
     fn make_call_args(

compiler/rustc_mir_transform/src/lib.rs

@@ -343,6 +343,8 @@ fn mir_promoted(
         body.tainted_by_errors = Some(error_reported);
     }
 
+    // Collect `required_consts` *before* promotion, so if there are any consts being promoted
+    // we still add them to the list in the outer MIR body.
     let mut required_consts = Vec::new();
     let mut required_consts_visitor = RequiredConstsVisitor::new(&mut required_consts);
     for (bb, bb_data) in traversal::reverse_postorder(&body) {