From 3b9453bfe23a861ac1622f3562df7f1f980a4b34 Mon Sep 17 00:00:00 2001
From: Deadbeef <ent3rm4n@gmail.com>
Date: Wed, 30 Jun 2021 23:56:43 +0800
Subject: [PATCH 1/2] use is_const_fn_raw when encoding constness

this properly encodes cross-crate constness data.
---
 compiler/rustc_metadata/src/rmeta/encoder.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 7600739800041..4231db620b012 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -1223,7 +1223,12 @@ impl EncodeContext<'a, 'tcx> {
                 let fn_data = if let hir::ImplItemKind::Fn(ref sig, body) = ast_item.kind {
                     FnData {
                         asyncness: sig.header.asyncness,
-                        constness: sig.header.constness,
+                        // Can be inside `impl const Trait`, so using sig.header.constness is not reliable
+                        constness: if self.tcx.is_const_fn_raw(def_id) {
+                            hir::Constness::Const
+                        } else {
+                            hir::Constness::NotConst
+                        },
                         param_names: self.encode_fn_param_names_for_body(body),
                     }
                 } else {

From c424510746c3e890157e3c30ad7fcf2051b9f769 Mon Sep 17 00:00:00 2001
From: Deadbeef <ent3rm4n@gmail.com>
Date: Wed, 30 Jun 2021 23:57:17 +0800
Subject: [PATCH 2/2] Add tests for cross-crate usage of `impl const`

---
 .../auxiliary/cross-crate.rs                  | 22 +++++++++++++++++++
 .../cross-crate-feature-disabled.rs           | 18 +++++++++++++++
 .../cross-crate-feature-disabled.stderr       | 15 +++++++++++++
 .../cross-crate-feature-enabled.rs            | 20 +++++++++++++++++
 .../cross-crate-feature-enabled.stderr        |  9 ++++++++
 5 files changed, 84 insertions(+)
 create mode 100644 src/test/ui/rfc-2632-const-trait-impl/auxiliary/cross-crate.rs
 create mode 100644 src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-disabled.rs
 create mode 100644 src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-disabled.stderr
 create mode 100644 src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-enabled.rs
 create mode 100644 src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-enabled.stderr

diff --git a/src/test/ui/rfc-2632-const-trait-impl/auxiliary/cross-crate.rs b/src/test/ui/rfc-2632-const-trait-impl/auxiliary/cross-crate.rs
new file mode 100644
index 0000000000000..4285eaf18aaad
--- /dev/null
+++ b/src/test/ui/rfc-2632-const-trait-impl/auxiliary/cross-crate.rs
@@ -0,0 +1,22 @@
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+pub trait MyTrait {
+    fn func(self);
+}
+
+pub struct NonConst;
+
+impl MyTrait for NonConst {
+    fn func(self) {
+
+    }
+}
+
+pub struct Const;
+
+impl const MyTrait for Const {
+    fn func(self) {
+
+    }
+}
diff --git a/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-disabled.rs b/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-disabled.rs
new file mode 100644
index 0000000000000..abd11d8b0e923
--- /dev/null
+++ b/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-disabled.rs
@@ -0,0 +1,18 @@
+// aux-build: cross-crate.rs
+extern crate cross_crate;
+
+use cross_crate::*;
+
+fn non_const_context() {
+    NonConst.func();
+    Const.func();
+}
+
+const fn const_context() {
+    NonConst.func();
+    //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+    Const.func();
+    //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+}
+
+fn main() {}
diff --git a/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-disabled.stderr b/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-disabled.stderr
new file mode 100644
index 0000000000000..b86583b9e079f
--- /dev/null
+++ b/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-disabled.stderr
@@ -0,0 +1,15 @@
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/cross-crate-feature-disabled.rs:12:5
+   |
+LL |     NonConst.func();
+   |     ^^^^^^^^^^^^^^^
+
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/cross-crate-feature-disabled.rs:14:5
+   |
+LL |     Const.func();
+   |     ^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0015`.
diff --git a/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-enabled.rs b/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-enabled.rs
new file mode 100644
index 0000000000000..b79ccc7712ff2
--- /dev/null
+++ b/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-enabled.rs
@@ -0,0 +1,20 @@
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+// aux-build: cross-crate.rs
+extern crate cross_crate;
+
+use cross_crate::*;
+
+fn non_const_context() {
+    NonConst.func();
+    Const.func();
+}
+
+const fn const_context() {
+    NonConst.func();
+    //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+    Const.func();
+}
+
+fn main() {}
diff --git a/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-enabled.stderr b/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-enabled.stderr
new file mode 100644
index 0000000000000..a544c0dd285fc
--- /dev/null
+++ b/src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-enabled.stderr
@@ -0,0 +1,9 @@
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/cross-crate-feature-enabled.rs:15:5
+   |
+LL |     NonConst.func();
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0015`.