Impossible preds tests

Author: BoxyUwU

tests/ui/const-generics/backcompat/impossible_preds_repeat_expr_hack.rs

@@ -0,0 +1,39 @@
+// Check that we emit the `CONST_EVALUATABLE_UNCHECKED` FCW when a repeat expr
+// count depends on in-scope where clauses to prove an impossible bound but does
+// not otherwise depend on any in-scope generic parameters.
+
+pub trait Unimplemented<'a> {}
+
+trait Trait<T>
+where
+    for<'a> T: Unimplemented<'a>,
+{
+    const ASSOC: usize;
+}
+
+impl<T> Trait<T> for u8
+where
+    for<'a> T: Unimplemented<'a>,
+{
+    const ASSOC: usize = 1;
+}
+
+pub fn impossible_preds_repeat_expr_count()
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    let _a: [(); 1] = [(); <u8 as Trait<()>>::ASSOC];
+}
+
+struct Foo<const N: usize>;
+
+pub fn impossible_preds_normal_arg()
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    let _a: Foo<1> = Foo::<{ <u8 as Trait<()>>::ASSOC }>;
+    //~^ ERROR: the trait bound `for<'a> (): Unimplemented<'a>`
+    //~| ERROR: the trait bound `for<'a> (): Unimplemented<'a>`
+}
+
+fn main() {}

tests/ui/const-generics/backcompat/impossible_preds_repeat_expr_hack.stderr

@@ -0,0 +1,43 @@
+error[E0277]: the trait bound `for<'a> (): Unimplemented<'a>` is not satisfied
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:34:43
+   |
+LL |     let _a: Foo<1> = Foo::<{ <u8 as Trait<()>>::ASSOC }>;
+   |                                           ^^ the trait `for<'a> Unimplemented<'a>` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:5:1
+   |
+LL | pub trait Unimplemented<'a> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `Trait::ASSOC`
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:9:16
+   |
+LL |     for<'a> T: Unimplemented<'a>,
+   |                ^^^^^^^^^^^^^^^^^ required by this bound in `Trait::ASSOC`
+LL | {
+LL |     const ASSOC: usize;
+   |           ----- required by a bound in this associated constant
+
+error[E0277]: the trait bound `for<'a> (): Unimplemented<'a>` is not satisfied
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:34:31
+   |
+LL |     let _a: Foo<1> = Foo::<{ <u8 as Trait<()>>::ASSOC }>;
+   |                               ^^ the trait `for<'a> Unimplemented<'a>` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:5:1
+   |
+LL | pub trait Unimplemented<'a> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required for `u8` to implement `Trait<()>`
+  --> $DIR/impossible_preds_repeat_expr_hack.rs:14:9
+   |
+LL | impl<T> Trait<T> for u8
+   |         ^^^^^^^^     ^^
+LL | where
+LL |     for<'a> T: Unimplemented<'a>,
+   |                ----------------- unsatisfied trait bound introduced here
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/const-generics/mgca/impossible_preds_on_anon_const.rs

@@ -0,0 +1,50 @@
+#![feature(generic_const_items, min_generic_const_args)]
+#![expect(incomplete_features)]
+
+// Tests behaviour of attempting to evaluate const arguments whose wellformedness
+// depends on impossible to satisfy predicates, that are satisfied from another
+// trivially false clause in the environment.
+//
+// Additionally tests how (or if) this behaviour differs depending on whether the
+// constants where trivially false where clause is global or not.
+
+trait Unimplemented<'a> {}
+
+trait Trait {
+    const NON_GLOBAL<T>: usize
+    where
+        for<'a> T: Unimplemented<'a>;
+
+    const GLOBAL: usize
+    where
+        for<'a> (): Unimplemented<'a>;
+}
+
+impl Trait for u8 {
+    const NON_GLOBAL<T>: usize = 1
+    where
+        for<'a> T: Unimplemented<'a>;
+
+    const GLOBAL: usize = 1
+    //~^ ERROR: evaluation of constant value failed
+    where
+        for<'a> (): Unimplemented<'a>;
+}
+
+struct Foo<const N: usize>;
+
+fn non_global()
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    let _: Foo<1> = Foo::<{ <u8 as Trait>::NON_GLOBAL::<()> }>;
+}
+
+fn global()
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    let _: Foo<1> = Foo::<{ <u8 as Trait>::GLOBAL }>;
+}
+
+fn main() {}

tests/ui/const-generics/mgca/impossible_preds_on_anon_const.stderr

@@ -0,0 +1,12 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/impossible_preds_on_anon_const.rs:28:5
+   |
+LL | /     const GLOBAL: usize = 1
+LL | |
+LL | |     where
+LL | |         for<'a> (): Unimplemented<'a>;
+   | |______________________________________^ entering unreachable code
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.

tests/ui/match/impossible_preds_const_pattern.rs

@@ -0,0 +1,58 @@
+#![feature(generic_const_items)]
+#![expect(incomplete_features)]
+
+trait Unimplemented<'a> {}
+
+trait Trait<T>
+where
+    for<'a> T: Unimplemented<'a>,
+{
+    const ASSOC: usize;
+}
+
+impl<T> Trait<T> for ()
+where
+    for<'a> T: Unimplemented<'a>,
+{
+    const ASSOC: usize = 0;
+}
+
+trait Global {
+    const ASSOC: usize
+    where
+        for<'a> (): Unimplemented<'a>;
+}
+
+impl Global for () {
+    const ASSOC: usize = 0
+    //~^ ERROR: evaluation of constant value failed
+    where
+        for<'a> (): Unimplemented<'a>;
+}
+
+fn works(x: usize)
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    // In order for match exhaustiveness to determine this match is OK, CTFE must
+    // be able to evalaute `<() as Trait>::ASSOC` which depends on a trivially
+    // false bound for well formedness.
+    match x {
+        <() as Trait<()>>::ASSOC => todo!(),
+        1.. => todo!(),
+    }
+}
+
+fn errors(x: usize)
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    // This errors due to the MIR for `ASSOC` being empty due to the
+    // globally false bound.
+    match x {
+        <() as Global>::ASSOC => todo!(),
+        1.. => todo!(),
+    }
+}
+
+fn main() {}

tests/ui/match/impossible_preds_const_pattern.stderr

@@ -0,0 +1,12 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/impossible_preds_const_pattern.rs:27:5
+   |
+LL | /     const ASSOC: usize = 0
+LL | |
+LL | |     where
+LL | |         for<'a> (): Unimplemented<'a>;
+   | |______________________________________^ entering unreachable code
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.