Create stable metric to measure long computation in Const Eval

This patch adds a `MirPass` that tracks the number of back-edges and
function calls in the CFG, adds a new MIR instruction to increment a
counter every time they are encountered during Const Eval, and emit a
warning if a configured limit is breached.

5 files changed, 15 insertions, 3 deletions

diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 4da893e4c07..dae7e84e415 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -1461,6 +1461,7 @@ impl Debug for Statement<'_> {
             }
             Coverage(box ref coverage) => write!(fmt, "Coverage::{:?}", coverage.kind),
             Intrinsic(box ref intrinsic) => write!(fmt, "{intrinsic}"),
+            ConstEvalCounter => write!(fmt, "ConstEvalCounter"),
             Nop => write!(fmt, "nop"),
         }
     }
diff --git a/compiler/rustc_middle/src/mir/query.rs b/compiler/rustc_middle/src/mir/query.rs
index a8a4532223c..c281fe00591 100644
--- a/compiler/rustc_middle/src/mir/query.rs
+++ b/compiler/rustc_middle/src/mir/query.rs
@@ -441,10 +441,14 @@ impl<'tcx> TyCtxt<'tcx> {
 
     #[inline]
     pub fn mir_for_ctfe_opt_const_arg(self, def: ty::WithOptConstParam<DefId>) -> &'tcx Body<'tcx> {
-        if let Some((did, param_did)) = def.as_const_arg() {
+        let res = if let Some((did, param_did)) = def.as_const_arg() {
+            info!("calling mir_for_ctfe_of_const_arg for DedId {did:?}");
             self.mir_for_ctfe_of_const_arg((did, param_did))
         } else {
+            info!("calling mir_for_ctfe for DefId {:?}", def.did);
             self.mir_for_ctfe(def.did)
-        }
+        };
+        //info!("RES OF CALLING MIR_FOR_CTFE_OPT_CONST_ARG: {:#?}", res);
+        res
     }
 }
diff --git a/compiler/rustc_middle/src/mir/spanview.rs b/compiler/rustc_middle/src/mir/spanview.rs
index 887ee571575..7efe1fde093 100644
--- a/compiler/rustc_middle/src/mir/spanview.rs
+++ b/compiler/rustc_middle/src/mir/spanview.rs
@@ -250,6 +250,7 @@ pub fn statement_kind_name(statement: &Statement<'_>) -> &'static str {
         AscribeUserType(..) => "AscribeUserType",
         Coverage(..) => "Coverage",
         Intrinsic(..) => "Intrinsic",
+        ConstEvalCounter => "ConstEvalCounter",
         Nop => "Nop",
     }
 }
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index 52c2b10cbbe..faf903a5949 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -286,7 +286,10 @@ pub enum StatementKind<'tcx> {
     /// This is permitted for both generators and ADTs. This does not necessarily write to the
     /// entire place; instead, it writes to the minimum set of bytes as required by the layout for
     /// the type.
-    SetDiscriminant { place: Box<Place<'tcx>>, variant_index: VariantIdx },
+    SetDiscriminant {
+        place: Box<Place<'tcx>>,
+        variant_index: VariantIdx,
+    },
 
     /// Deinitializes the place.
     ///
@@ -355,6 +358,8 @@ pub enum StatementKind<'tcx> {
     /// This avoids adding a new block and a terminator for simple intrinsics.
     Intrinsic(Box<NonDivergingIntrinsic<'tcx>>),
 
+    ConstEvalCounter,
+
     /// No-op. Useful for deleting instructions without affecting statement indices.
     Nop,
 }
diff --git a/compiler/rustc_middle/src/mir/visit.rs b/compiler/rustc_middle/src/mir/visit.rs
index 1a264d2d5af..3ddac5e11fb 100644
--- a/compiler/rustc_middle/src/mir/visit.rs
+++ b/compiler/rustc_middle/src/mir/visit.rs
@@ -427,6 +427,7 @@ macro_rules! make_mir_visitor {
                             }
                         }
                     }
+                    StatementKind::ConstEvalCounter => {}
                     StatementKind::Nop => {}
                 }
             }