const fn: allow use of trait impls from bounds

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-fail.rs

@@ -0,0 +1,11 @@
+#![feature(const_fn)]
+#![feature(const_trait_impl)]
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+pub const fn equals_self<T: ?const PartialEq>(t: &T) -> bool {
+    *t == *t
+    //~^ ERROR calls in constant functions are limited to constant functions
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-fail.stderr

@@ -0,0 +1,9 @@
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/call-generic-method-fail.rs:7:5
+   |
+LL |     *t == *t
+   |     ^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst-opt-out.rs

@@ -0,0 +1,24 @@
+// check-pass
+
+#![feature(const_fn)]
+#![feature(const_trait_impl)]
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+struct S;
+
+impl PartialEq for S {
+    fn eq(&self, _: &S) -> bool {
+        true
+    }
+}
+
+const fn equals_self<T: ?const PartialEq>(t: &T) -> bool {
+    true
+}
+
+pub const EQ: bool = equals_self(&S);
+
+// Calling `equals_self` with a type that only has a non-const impl is fine, because we opted out.
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs

@@ -0,0 +1,26 @@
+// FIXME(jschievink): this is not rejected correctly (only when the non-const impl is actually used)
+// ignore-test
+
+#![feature(const_fn)]
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+struct S;
+
+impl PartialEq for S {
+    fn eq(&self, _: &S) -> bool {
+        true
+    }
+}
+
+const fn equals_self<T: PartialEq>(t: &T) -> bool {
+    true
+}
+
+// Calling `equals_self` with something that has a non-const impl should throw an error, despite
+// it not using the impl.
+
+pub const EQ: bool = equals_self(&S);
+//~^ ERROR
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-pass.rs

@@ -0,0 +1,23 @@
+//! Basic test for calling methods on generic type parameters in `const fn`.
+
+// check-pass
+
+#![feature(const_fn)]
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+struct S;
+
+impl const PartialEq for S {
+    fn eq(&self, _: &S) -> bool {
+        true
+    }
+}
+
+const fn equals_self<T: PartialEq>(t: &T) -> bool {
+    *t == *t
+}
+
+pub const EQ: bool = equals_self(&S);
+
+fn main() {}