Rollup merge of #79287 - jonas-schievink:const-trait-impl, r=oli-obk

Allow using generic trait methods in `const fn`

Next step for https://github.com/rust-lang/rust/issues/67792, this now also allows code like the following:

```rust
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);
```

This works by threading const-ness of trait predicates through trait selection, in particular through `ParamCandidate`, and exposing it in the resulting `ImplSource`.

Since this change makes two bounds `T: Trait` and `T: ?const Trait` that only differ in their const-ness be treated like different bounds, candidate winnowing has been changed to drop the `?const` candidate in favor of the const candidate, to avoid ambiguities when both a const and a non-const bound is present.

7 files changed, 144 insertions, 0 deletions

diff --git a/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-chain.rs b/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-chain.rs
new file mode 100644
index 00000000000..6a511f4ed3e
--- /dev/null
+++ b/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-chain.rs
@@ -0,0 +1,27 @@
+//! 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
+}
+
+const fn equals_self_wrapper<T: PartialEq>(t: &T) -> bool {
+    equals_self(t)
+}
+
+pub const EQ: bool = equals_self_wrapper(&S);
+
+fn main() {}
diff --git a/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-dup-bound.rs b/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-dup-bound.rs
new file mode 100644
index 00000000000..b39d27779f4
--- /dev/null
+++ b/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-dup-bound.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 const PartialEq for S {
+    fn eq(&self, _: &S) -> bool {
+        true
+    }
+}
+
+// This duplicate bound should not result in ambiguities. It should be equivalent to a single const
+// bound.
+const fn equals_self<T: PartialEq + ?const PartialEq>(t: &T) -> bool {
+    *t == *t
+}
+
+pub const EQ: bool = equals_self(&S);
+
+fn main() {}
diff --git a/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-fail.rs b/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-fail.rs
new file mode 100644
index 00000000000..6d4bfe722de
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-fail.stderr b/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-fail.stderr
new file mode 100644
index 00000000000..4b2fc56aaa7
--- /dev/null
+++ b/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`.
diff --git a/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst-opt-out.rs b/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst-opt-out.rs
new file mode 100644
index 00000000000..f0e32142221
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs b/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-nonconst.rs
new file mode 100644
index 00000000000..2c8f6354dc6
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-pass.rs b/src/test/ui/rfc-2632-const-trait-impl/call-generic-method-pass.rs
new file mode 100644
index 00000000000..e968e6ec7bb
--- /dev/null
+++ b/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() {}