From 894b42c8616f352eacf54bc830c01e33b406fb8f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomasz=20Mi=C4=85sko?= <tomasz.miasko@gmail.com>
Date: Fri, 4 Jun 2021 00:00:00 +0000
Subject: [PATCH] Disallow non-monomorphic calls to `needs_drop` in interpreter

otherwise evaluation could change after further substitutions.
---
 .../rustc_mir/src/interpret/intrinsics.rs     |  5 ++++-
 .../ui/consts/const-needs_drop-monomorphic.rs | 17 ++++++++++++++++
 .../const-needs_drop-monomorphic.stderr       | 20 +++++++++++++++++++
 3 files changed, 41 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/consts/const-needs_drop-monomorphic.rs
 create mode 100644 src/test/ui/consts/const-needs_drop-monomorphic.stderr

diff --git a/compiler/rustc_mir/src/interpret/intrinsics.rs b/compiler/rustc_mir/src/interpret/intrinsics.rs
index 99622fb310a..cdd1edbf286 100644
--- a/compiler/rustc_mir/src/interpret/intrinsics.rs
+++ b/compiler/rustc_mir/src/interpret/intrinsics.rs
@@ -56,7 +56,10 @@ crate fn eval_nullary_intrinsic<'tcx>(
             let alloc = type_name::alloc_type_name(tcx, tp_ty);
             ConstValue::Slice { data: alloc, start: 0, end: alloc.len() }
         }
-        sym::needs_drop => ConstValue::from_bool(tp_ty.needs_drop(tcx, param_env)),
+        sym::needs_drop => {
+            ensure_monomorphic_enough(tcx, tp_ty)?;
+            ConstValue::from_bool(tp_ty.needs_drop(tcx, param_env))
+        }
         sym::min_align_of | sym::pref_align_of => {
             let layout = tcx.layout_of(param_env.and(tp_ty)).map_err(|e| err_inval!(Layout(e)))?;
             let n = match name {
diff --git a/src/test/ui/consts/const-needs_drop-monomorphic.rs b/src/test/ui/consts/const-needs_drop-monomorphic.rs
new file mode 100644
index 00000000000..9f66e3cfa23
--- /dev/null
+++ b/src/test/ui/consts/const-needs_drop-monomorphic.rs
@@ -0,0 +1,17 @@
+// Check that evaluation of needs_drop<T> fails when T is not monomorphic.
+#![feature(const_generics)]
+#![allow(const_evaluatable_unchecked)]
+#![allow(incomplete_features)]
+
+struct Bool<const B: bool> {}
+impl Bool<true> {
+    fn assert() {}
+}
+fn f<T>() {
+    Bool::<{ std::mem::needs_drop::<T>() }>::assert();
+    //~^ ERROR no function or associated item named `assert` found
+    //~| ERROR constant expression depends on a generic parameter
+}
+fn main() {
+    f::<u32>();
+}
diff --git a/src/test/ui/consts/const-needs_drop-monomorphic.stderr b/src/test/ui/consts/const-needs_drop-monomorphic.stderr
new file mode 100644
index 00000000000..0770d06e15b
--- /dev/null
+++ b/src/test/ui/consts/const-needs_drop-monomorphic.stderr
@@ -0,0 +1,20 @@
+error[E0599]: no function or associated item named `assert` found for struct `Bool<{ std::mem::needs_drop::<T>() }>` in the current scope
+  --> $DIR/const-needs_drop-monomorphic.rs:11:46
+   |
+LL | struct Bool<const B: bool> {}
+   | -------------------------- function or associated item `assert` not found for this
+...
+LL |     Bool::<{ std::mem::needs_drop::<T>() }>::assert();
+   |                                              ^^^^^^ function or associated item cannot be called on `Bool<{ std::mem::needs_drop::<T>() }>` due to unsatisfied trait bounds
+
+error: constant expression depends on a generic parameter
+  --> $DIR/const-needs_drop-monomorphic.rs:11:5
+   |
+LL |     Bool::<{ std::mem::needs_drop::<T>() }>::assert();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0599`.