Auto merge of #86571 - fee1-dead:const-trait-impl-fix, r=jackh726

deny using default function in impl const Trait

Fixes #79450.

I don't know if my implementation is correct:

 - The check is in `rustc_passes::check_const`, should I put it somewhere else instead?
 - Is my approach (to checking the impl) optimal? It works for the current tests, but it might have some issues or there might be a better way of doing this.

7 files changed, 45 insertions, 0 deletions

diff --git a/src/test/ui/consts/const-float-classify.rs b/src/test/ui/consts/const-float-classify.rs
index 36fec9976be..95e7f9e9c83 100644
--- a/src/test/ui/consts/const-float-classify.rs
+++ b/src/test/ui/consts/const-float-classify.rs
@@ -53,6 +53,9 @@ impl const PartialEq<NonDet> for bool {
     fn eq(&self, _: &NonDet) -> bool {
         true
     }
+    fn ne(&self, _: &NonDet) -> bool {
+        false
+    }
 }
 
 // The result of the `is_sign` methods are not checked for correctness, since LLVM does not
diff --git a/src/test/ui/rfc-2632-const-trait-impl/call-const-trait-method-pass.rs b/src/test/ui/rfc-2632-const-trait-impl/call-const-trait-method-pass.rs
index ec6f45f956d..44814b0654e 100644
--- a/src/test/ui/rfc-2632-const-trait-impl/call-const-trait-method-pass.rs
+++ b/src/test/ui/rfc-2632-const-trait-impl/call-const-trait-method-pass.rs
@@ -17,6 +17,9 @@ impl const PartialEq for Int {
     fn eq(&self, rhs: &Self) -> bool {
         self.0 == rhs.0
     }
+    fn ne(&self, other: &Self) -> bool {
+        !self.eq(other)
+    }
 }
 
 pub trait Plus {
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
index c37990b1af3..47eed89d03d 100644
--- 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
@@ -12,6 +12,9 @@ impl const PartialEq for S {
     fn eq(&self, _: &S) -> bool {
         true
     }
+    fn ne(&self, other: &S) -> bool {
+        !self.eq(other)
+    }
 }
 
 const fn equals_self<T: PartialEq>(t: &T) -> bool {
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
index d553b2ab8ec..00a3c7f51fe 100644
--- 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
@@ -11,6 +11,9 @@ impl const PartialEq for S {
     fn eq(&self, _: &S) -> bool {
         true
     }
+    fn ne(&self, other: &S) -> bool {
+        !self.eq(other)
+    }
 }
 
 // This duplicate bound should not result in ambiguities. It should be equivalent to a single const
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
index 74b0d5fbe47..953a6511199 100644
--- 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
@@ -12,6 +12,9 @@ impl const PartialEq for S {
     fn eq(&self, _: &S) -> bool {
         true
     }
+    fn ne(&self, other: &S) -> bool {
+        !self.eq(other)
+    }
 }
 
 const fn equals_self<T: PartialEq>(t: &T) -> bool {
diff --git a/src/test/ui/rfc-2632-const-trait-impl/impl-with-default-fn.rs b/src/test/ui/rfc-2632-const-trait-impl/impl-with-default-fn.rs
new file mode 100644
index 00000000000..4ff4fa0d83b
--- /dev/null
+++ b/src/test/ui/rfc-2632-const-trait-impl/impl-with-default-fn.rs
@@ -0,0 +1,20 @@
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+trait Tr {
+    fn req(&self);
+
+    fn prov(&self) {
+        println!("lul");
+        self.req();
+    }
+}
+
+struct S;
+
+impl const Tr for S {
+    fn req(&self) {}
+}
+//~^^^ ERROR const trait implementations may not use default functions
+
+fn main() {}
diff --git a/src/test/ui/rfc-2632-const-trait-impl/impl-with-default-fn.stderr b/src/test/ui/rfc-2632-const-trait-impl/impl-with-default-fn.stderr
new file mode 100644
index 00000000000..51a7b18fa8d
--- /dev/null
+++ b/src/test/ui/rfc-2632-const-trait-impl/impl-with-default-fn.stderr
@@ -0,0 +1,10 @@
+error: const trait implementations may not use default functions
+  --> $DIR/impl-with-default-fn.rs:15:1
+   |
+LL | / impl const Tr for S {
+LL | |     fn req(&self) {}
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+