Support pretty printing of invalid constants

Make it possible to pretty print invalid constants by introducing a
fallible variant of `destructure_const` and falling back to debug
formatting when it fails.

compiler/rustc_const_eval/src/const_eval/mod.rs

@@ -11,7 +11,8 @@ use rustc_middle::{
 use rustc_span::{source_map::DUMMY_SP, symbol::Symbol};
 
 use crate::interpret::{
-    intern_const_alloc_recursive, ConstValue, InternKind, InterpCx, MPlaceTy, MemPlaceMeta, Scalar,
+    intern_const_alloc_recursive, ConstValue, InternKind, InterpCx, InterpResult, MPlaceTy,
+    MemPlaceMeta, Scalar,
 };
 
 mod error;
@@ -132,42 +133,39 @@ fn const_to_valtree_inner<'tcx>(
     }
 }
 
-/// This function uses `unwrap` copiously, because an already validated constant
-/// must have valid fields and can thus never fail outside of compiler bugs. However, it is
-/// invoked from the pretty printer, where it can receive enums with no variants and e.g.
-/// `read_discriminant` needs to be able to handle that.
-pub(crate) fn destructure_const<'tcx>(
+/// This function should never fail for validated constants. However, it is also invoked from the
+/// pretty printer which might attempt to format invalid constants and in that case it might fail.
+pub(crate) fn try_destructure_const<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     val: ty::Const<'tcx>,
-) -> mir::DestructuredConst<'tcx> {
+) -> InterpResult<'tcx, mir::DestructuredConst<'tcx>> {
     trace!("destructure_const: {:?}", val);
     let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, false);
-    let op = ecx.const_to_op(val, None).unwrap();
+    let op = ecx.const_to_op(val, None)?;
 
     // We go to `usize` as we cannot allocate anything bigger anyway.
     let (field_count, variant, down) = match val.ty().kind() {
         ty::Array(_, len) => (usize::try_from(len.eval_usize(tcx, param_env)).unwrap(), None, op),
-        ty::Adt(def, _) if def.variants.is_empty() => {
-            return mir::DestructuredConst { variant: None, fields: &[] };
-        }
         ty::Adt(def, _) => {
-            let variant = ecx.read_discriminant(&op).unwrap().1;
-            let down = ecx.operand_downcast(&op, variant).unwrap();
+            let variant = ecx.read_discriminant(&op)?.1;
+            let down = ecx.operand_downcast(&op, variant)?;
             (def.variants[variant].fields.len(), Some(variant), down)
         }
         ty::Tuple(substs) => (substs.len(), None, op),
         _ => bug!("cannot destructure constant {:?}", val),
     };
 
-    let fields_iter = (0..field_count).map(|i| {
-        let field_op = ecx.operand_field(&down, i).unwrap();
-        let val = op_to_const(&ecx, &field_op);
-        ty::Const::from_value(tcx, val, field_op.layout.ty)
-    });
-    let fields = tcx.arena.alloc_from_iter(fields_iter);
+    let fields = (0..field_count)
+        .map(|i| {
+            let field_op = ecx.operand_field(&down, i)?;
+            let val = op_to_const(&ecx, &field_op);
+            Ok(ty::Const::from_value(tcx, val, field_op.layout.ty))
+        })
+        .collect::<InterpResult<'tcx, Vec<_>>>()?;
+    let fields = tcx.arena.alloc_from_iter(fields);
 
-    mir::DestructuredConst { variant, fields }
+    Ok(mir::DestructuredConst { variant, fields })
 }
 
 pub(crate) fn deref_const<'tcx>(

compiler/rustc_const_eval/src/lib.rs

@@ -41,9 +41,9 @@ pub fn provide(providers: &mut Providers) {
     providers.eval_to_const_value_raw = const_eval::eval_to_const_value_raw_provider;
     providers.eval_to_allocation_raw = const_eval::eval_to_allocation_raw_provider;
     providers.const_caller_location = const_eval::const_caller_location;
-    providers.destructure_const = |tcx, param_env_and_value| {
+    providers.try_destructure_const = |tcx, param_env_and_value| {
         let (param_env, value) = param_env_and_value.into_parts();
-        const_eval::destructure_const(tcx, param_env, value)
+        const_eval::try_destructure_const(tcx, param_env, value).ok()
     };
     providers.const_to_valtree = |tcx, param_env_and_value| {
         let (param_env, raw) = param_env_and_value.into_parts();

compiler/rustc_middle/src/mir/interpret/queries.rs

@@ -98,4 +98,12 @@ impl<'tcx> TyCtxt<'tcx> {
         let raw_const = self.eval_to_allocation_raw(param_env.and(gid))?;
         Ok(self.global_alloc(raw_const.alloc_id).unwrap_memory())
     }
+
+    /// Destructure a constant ADT or array into its variant index and its field values.
+    pub fn destructure_const(
+        self,
+        param_env_and_val: ty::ParamEnvAnd<'tcx, ty::Const<'tcx>>,
+    ) -> mir::DestructuredConst<'tcx> {
+        self.try_destructure_const(param_env_and_val).unwrap()
+    }
 }

compiler/rustc_middle/src/query/mod.rs

@@ -924,10 +924,12 @@ rustc_queries! {
     }
 
     /// Destructure a constant ADT or array into its variant index and its
-    /// field values.
-    query destructure_const(
+    /// field values or return `None` if constant is invalid.
+    ///
+    /// Use infallible `TyCtxt::destructure_const` when you know that constant is valid.
+    query try_destructure_const(
         key: ty::ParamEnvAnd<'tcx, ty::Const<'tcx>>
-    ) -> mir::DestructuredConst<'tcx> {
+    ) -> Option<mir::DestructuredConst<'tcx>> {
         desc { "destructure constant" }
         remap_env_constness
     }

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1459,10 +1459,18 @@ pub trait PrettyPrinter<'tcx>:
             // FIXME(eddyb) for `--emit=mir`/`-Z dump-mir`, we should provide the
             // correct `ty::ParamEnv` to allow printing *all* constant values.
             (_, ty::Array(..) | ty::Tuple(..) | ty::Adt(..)) if !ty.has_param_types_or_consts() => {
-                let contents =
-                    self.tcx().destructure_const(ty::ParamEnv::reveal_all().and(
-                        self.tcx().mk_const(ty::ConstS { val: ty::ConstKind::Value(ct), ty }),
-                    ));
+                let Some(contents) = self.tcx().try_destructure_const(
+                    ty::ParamEnv::reveal_all()
+                        .and(self.tcx().mk_const(ty::ConstS { val: ty::ConstKind::Value(ct), ty })),
+                ) else {
+                    // Fall back to debug pretty printing for invalid constants.
+                    p!(write("{:?}", ct));
+                    if print_ty {
+                        p!(": ", print(ty));
+                    }
+                    return Ok(self);
+                };
+
                 let fields = contents.fields.iter().copied();
 
                 match *ty.kind() {

src/test/mir-opt/const_prop/invalid_constant.main.ConstProp.diff

@@ -0,0 +1,55 @@
+- // MIR for `main` before ConstProp
++ // MIR for `main` after ConstProp
+
+  fn main() -> () {
+      let mut _0: ();                      // return place in scope 0 at $DIR/invalid_constant.rs:15:11: 15:11
+      let _1: std::option::Option<()>;     // in scope 0 at $DIR/invalid_constant.rs:16:5: 16:12
+      let mut _2: std::option::Option<std::option::Option<()>>; // in scope 0 at $DIR/invalid_constant.rs:16:7: 16:11
+      scope 1 (inlined f) {                // at $DIR/invalid_constant.rs:16:5: 16:12
+          debug x => _2;                   // in scope 1 at $DIR/invalid_constant.rs:16:5: 16:12
+          let mut _3: isize;               // in scope 1 at $DIR/invalid_constant.rs:16:5: 16:12
+          let _4: std::option::Option<()>; // in scope 1 at $DIR/invalid_constant.rs:16:5: 16:12
+          scope 2 {
+              debug y => _4;               // in scope 2 at $DIR/invalid_constant.rs:16:5: 16:12
+          }
+      }
+
+      bb0: {
+          discriminant(_2) = 0;            // scope 0 at $DIR/invalid_constant.rs:16:7: 16:11
+-         _3 = discriminant(_2);           // scope 1 at $DIR/invalid_constant.rs:16:5: 16:12
+-         switchInt(move _3) -> [0_isize: bb3, otherwise: bb2]; // scope 1 at $DIR/invalid_constant.rs:16:5: 16:12
++         _3 = const 0_isize;              // scope 1 at $DIR/invalid_constant.rs:16:5: 16:12
++         switchInt(const 0_isize) -> [0_isize: bb3, otherwise: bb2]; // scope 1 at $DIR/invalid_constant.rs:16:5: 16:12
+      }
+
+      bb1: {
+          nop;                             // scope 0 at $DIR/invalid_constant.rs:15:11: 17:2
+          return;                          // scope 0 at $DIR/invalid_constant.rs:17:2: 17:2
+      }
+
+      bb2: {
+-         _4 = ((_2 as Some).0: std::option::Option<()>); // scope 1 at $DIR/invalid_constant.rs:16:5: 16:12
+-         _1 = _4;                         // scope 2 at $DIR/invalid_constant.rs:16:5: 16:12
++         _4 = const Scalar(0x02): Option::<()>; // scope 1 at $DIR/invalid_constant.rs:16:5: 16:12
++                                          // ty::Const
++                                          // + ty: std::option::Option<()>
++                                          // + val: Value(Scalar(0x02))
++                                          // mir::Constant
++                                          // + span: $DIR/invalid_constant.rs:16:5: 16:12
++                                          // + literal: Const { ty: std::option::Option<()>, val: Value(Scalar(0x02)) }
++         _1 = const Scalar(0x02): Option::<()>; // scope 2 at $DIR/invalid_constant.rs:16:5: 16:12
++                                          // ty::Const
++                                          // + ty: std::option::Option<()>
++                                          // + val: Value(Scalar(0x02))
++                                          // mir::Constant
++                                          // + span: $DIR/invalid_constant.rs:16:5: 16:12
++                                          // + literal: Const { ty: std::option::Option<()>, val: Value(Scalar(0x02)) }
+          goto -> bb1;                     // scope 0 at $DIR/invalid_constant.rs:10:20: 10:21
+      }
+
+      bb3: {
+          discriminant(_1) = 0;            // scope 1 at $DIR/invalid_constant.rs:16:5: 16:12
+          goto -> bb1;                     // scope 0 at $DIR/invalid_constant.rs:9:17: 9:21
+      }
+  }
+

src/test/mir-opt/const_prop/invalid_constant.rs

@@ -0,0 +1,17 @@
+// Verify that we can pretty print invalid constant introduced
+// by constant propagation. Regression test for issue #93688.
+//
+// compile-flags: -Copt-level=0 -Zinline-mir
+
+#[inline(always)]
+pub fn f(x: Option<Option<()>>) -> Option<()> {
+    match x {
+        None => None,
+        Some(y) => y,
+    }
+}
+
+// EMIT_MIR invalid_constant.main.ConstProp.diff
+fn main() {
+    f(None);
+}