Rollup merge of #121396 - RalfJung:mir-const-value-inspect, r=oli-obk

make it possible for outside crates to inspect a mir::ConstValue with the interpreter

For MiniRust we need to convert MIR constant values into MiniRust constant values. However, it's not currently possible to get nice high-level access to the inerts of a `ConstValue`: we can access the raw contents (the allocation / `ScalarInt`), but if it is e.g. of enum type and we want to determine which variant is encoded, we are stuck. There's only `try_destructure_mir_constant_for_user_output` which is meant for diagnostics, so it doesn't fit.

The interpreter has all the APIs to traverse such a value, so we just need a way to get such a ConstValue into an interpreter instance. This adds the public functions necessary to make that happen.

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -3,10 +3,11 @@ use either::{Left, Right};
 use rustc_hir::def::DefKind;
 use rustc_middle::mir::interpret::{AllocId, ErrorHandled, InterpErrorInfo};
 use rustc_middle::mir::{self, ConstAlloc, ConstValue};
+use rustc_middle::query::TyCtxtAt;
 use rustc_middle::traits::Reveal;
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::print::with_no_trimmed_paths;
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::def_id::LocalDefId;
 use rustc_span::Span;
 use rustc_target::abi::{self, Abi};
@@ -87,13 +88,16 @@ fn eval_body_using_ecx<'mir, 'tcx>(
 }
 
 /// The `InterpCx` is only meant to be used to do field and index projections into constants for
-/// `simd_shuffle` and const patterns in match arms. It never performs alignment checks.
+/// `simd_shuffle` and const patterns in match arms.
+///
+/// This should *not* be used to do any actual interpretation. In particular, alignment checks are
+/// turned off!
 ///
 /// The function containing the `match` that is currently being analyzed may have generic bounds
 /// that inform us about the generic bounds of the constant. E.g., using an associated constant
 /// of a function's generic parameter will require knowledge about the bounds on the generic
 /// parameter. These bounds are passed to `mk_eval_cx` via the `ParamEnv` argument.
-pub(crate) fn mk_eval_cx<'mir, 'tcx>(
+pub(crate) fn mk_eval_cx_to_read_const_val<'mir, 'tcx>(
     tcx: TyCtxt<'tcx>,
     root_span: Span,
     param_env: ty::ParamEnv<'tcx>,
@@ -108,6 +112,19 @@ pub(crate) fn mk_eval_cx<'mir, 'tcx>(
     )
 }
 
+/// Create an interpreter context to inspect the given `ConstValue`.
+/// Returns both the context and an `OpTy` that represents the constant.
+pub fn mk_eval_cx_for_const_val<'mir, 'tcx>(
+    tcx: TyCtxtAt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    val: mir::ConstValue<'tcx>,
+    ty: Ty<'tcx>,
+) -> Option<(CompileTimeEvalContext<'mir, 'tcx>, OpTy<'tcx>)> {
+    let ecx = mk_eval_cx_to_read_const_val(tcx.tcx, tcx.span, param_env, CanAccessMutGlobal::No);
+    let op = ecx.const_val_to_op(val, ty, None).ok()?;
+    Some((ecx, op))
+}
+
 /// This function converts an interpreter value into a MIR constant.
 ///
 /// The `for_diagnostics` flag turns the usual rules for returning `ConstValue::Scalar` into a
@@ -203,7 +220,7 @@ pub(crate) fn turn_into_const_value<'tcx>(
     let def_id = cid.instance.def.def_id();
     let is_static = tcx.is_static(def_id);
     // This is just accessing an already computed constant, so no need to check alignment here.
-    let ecx = mk_eval_cx(
+    let ecx = mk_eval_cx_to_read_const_val(
         tcx,
         tcx.def_span(key.value.instance.def_id()),
         key.param_env,

compiler/rustc_const_eval/src/const_eval/mod.rs

@@ -47,8 +47,7 @@ pub(crate) fn try_destructure_mir_constant_for_user_output<'tcx>(
     ty: Ty<'tcx>,
 ) -> Option<mir::DestructuredConstant<'tcx>> {
     let param_env = ty::ParamEnv::reveal_all();
-    let ecx = mk_eval_cx(tcx.tcx, tcx.span, param_env, CanAccessMutGlobal::No);
-    let op = ecx.const_val_to_op(val, ty, None).ok()?;
+    let (ecx, op) = mk_eval_cx_for_const_val(tcx, param_env, val, ty)?;
 
     // We go to `usize` as we cannot allocate anything bigger anyway.
     let (field_count, variant, down) = match ty.kind() {

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -5,7 +5,7 @@ use rustc_middle::ty::{self, ScalarInt, Ty, TyCtxt};
 use rustc_span::DUMMY_SP;
 use rustc_target::abi::{Abi, VariantIdx};
 
-use super::eval_queries::{mk_eval_cx, op_to_const};
+use super::eval_queries::{mk_eval_cx_to_read_const_val, op_to_const};
 use super::machine::CompileTimeEvalContext;
 use super::{ValTreeCreationError, ValTreeCreationResult, VALTREE_MAX_NODES};
 use crate::const_eval::CanAccessMutGlobal;
@@ -223,7 +223,7 @@ pub(crate) fn eval_to_valtree<'tcx>(
     let const_alloc = tcx.eval_to_allocation_raw(param_env.and(cid))?;
 
     // FIXME Need to provide a span to `eval_to_valtree`
-    let ecx = mk_eval_cx(
+    let ecx = mk_eval_cx_to_read_const_val(
         tcx,
         DUMMY_SP,
         param_env,
@@ -287,7 +287,8 @@ pub fn valtree_to_const_value<'tcx>(
             }
         }
         ty::Ref(_, inner_ty, _) => {
-            let mut ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, CanAccessMutGlobal::No);
+            let mut ecx =
+                mk_eval_cx_to_read_const_val(tcx, DUMMY_SP, param_env, CanAccessMutGlobal::No);
             let imm = valtree_to_ref(&mut ecx, valtree, *inner_ty);
             let imm = ImmTy::from_immediate(imm, tcx.layout_of(param_env_ty).unwrap());
             op_to_const(&ecx, &imm.into(), /* for diagnostics */ false)
@@ -314,7 +315,8 @@ pub fn valtree_to_const_value<'tcx>(
                 bug!("could not find non-ZST field during in {layout:#?}");
             }
 
-            let mut ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, CanAccessMutGlobal::No);
+            let mut ecx =
+                mk_eval_cx_to_read_const_val(tcx, DUMMY_SP, param_env, CanAccessMutGlobal::No);
 
             // Need to create a place for this valtree.
             let place = create_valtree_place(&mut ecx, layout, valtree);

compiler/rustc_const_eval/src/util/caller_location.rs

@@ -6,7 +6,7 @@ use rustc_middle::ty::layout::LayoutOf;
 use rustc_span::symbol::Symbol;
 use rustc_type_ir::Mutability;
 
-use crate::const_eval::{mk_eval_cx, CanAccessMutGlobal, CompileTimeEvalContext};
+use crate::const_eval::{mk_eval_cx_to_read_const_val, CanAccessMutGlobal, CompileTimeEvalContext};
 use crate::interpret::*;
 
 /// Allocate a `const core::panic::Location` with the provided filename and line/column numbers.
@@ -57,7 +57,12 @@ pub(crate) fn const_caller_location_provider(
     col: u32,
 ) -> mir::ConstValue<'_> {
     trace!("const_caller_location: {}:{}:{}", file, line, col);
-    let mut ecx = mk_eval_cx(tcx.tcx, tcx.span, ty::ParamEnv::reveal_all(), CanAccessMutGlobal::No);
+    let mut ecx = mk_eval_cx_to_read_const_val(
+        tcx.tcx,
+        tcx.span,
+        ty::ParamEnv::reveal_all(),
+        CanAccessMutGlobal::No,
+    );
 
     let loc_place = alloc_caller_location(&mut ecx, file, line, col);
     if intern_const_alloc_recursive(&mut ecx, InternKind::Constant, &loc_place).is_err() {