add test for #122549

Fixes #122549

tests/ui/const-generics/ice-unexpected-inference-var-122549.rs

@@ -0,0 +1,29 @@
+// Regression test for https://github.com/rust-lang/rust/issues/122549
+// was fixed by https://github.com/rust-lang/rust/pull/122749
+
+trait ConstChunksExactTrait<T> {
+    fn const_chunks_exact<const N: usize>(&self) -> ConstChunksExact<'a, T, { N }>;
+    //~^ ERROR undeclared lifetime
+}
+
+impl<T> ConstChunksExactTrait<T> for [T] {}
+//~^ ERROR not all trait items implemented, missing: `const_chunks_exact`
+struct ConstChunksExact<'rem, T: 'a, const N: usize> {}
+//~^ ERROR use of undeclared lifetime name `'a`
+//~^^ ERROR lifetime parameter
+//~^^^ ERROR type parameter
+impl<'a, T, const N: usize> Iterator for ConstChunksExact<'a, T, {}> {
+//~^ ERROR the const parameter `N` is not constrained by the impl trait, self type, or predicates
+//~^^ ERROR mismatched types
+    type Item = &'a [T; N];
+}
+
+fn main() {
+    let slice = &[1i32, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+
+    let mut iter = [[1, 2, 3], [4, 5, 6], [7, 8, 9]].iter();
+
+    for a in slice.const_chunks_exact::<3>() {
+        assert_eq!(a, iter.next().unwrap());
+    }
+}

tests/ui/const-generics/ice-unexpected-inference-var-122549.stderr

@@ -0,0 +1,67 @@
+error[E0261]: use of undeclared lifetime name `'a`
+  --> $DIR/ice-unexpected-inference-var-122549.rs:5:70
+   |
+LL |     fn const_chunks_exact<const N: usize>(&self) -> ConstChunksExact<'a, T, { N }>;
+   |                                                                      ^^ undeclared lifetime
+   |
+help: consider introducing lifetime `'a` here
+   |
+LL |     fn const_chunks_exact<'a, const N: usize>(&self) -> ConstChunksExact<'a, T, { N }>;
+   |                           +++
+help: consider introducing lifetime `'a` here
+   |
+LL | trait ConstChunksExactTrait<'a, T> {
+   |                             +++
+
+error[E0261]: use of undeclared lifetime name `'a`
+  --> $DIR/ice-unexpected-inference-var-122549.rs:11:34
+   |
+LL | struct ConstChunksExact<'rem, T: 'a, const N: usize> {}
+   |                         -        ^^ undeclared lifetime
+   |                         |
+   |                         help: consider introducing lifetime `'a` here: `'a,`
+
+error[E0046]: not all trait items implemented, missing: `const_chunks_exact`
+  --> $DIR/ice-unexpected-inference-var-122549.rs:9:1
+   |
+LL |     fn const_chunks_exact<const N: usize>(&self) -> ConstChunksExact<'a, T, { N }>;
+   |     ------------------------------------------------------------------------------- `const_chunks_exact` from trait
+...
+LL | impl<T> ConstChunksExactTrait<T> for [T] {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `const_chunks_exact` in implementation
+
+error[E0392]: lifetime parameter `'rem` is never used
+  --> $DIR/ice-unexpected-inference-var-122549.rs:11:25
+   |
+LL | struct ConstChunksExact<'rem, T: 'a, const N: usize> {}
+   |                         ^^^^ unused lifetime parameter
+   |
+   = help: consider removing `'rem`, referring to it in a field, or using a marker such as `PhantomData`
+
+error[E0392]: type parameter `T` is never used
+  --> $DIR/ice-unexpected-inference-var-122549.rs:11:31
+   |
+LL | struct ConstChunksExact<'rem, T: 'a, const N: usize> {}
+   |                               ^ unused type parameter
+   |
+   = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
+
+error[E0207]: the const parameter `N` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/ice-unexpected-inference-var-122549.rs:15:13
+   |
+LL | impl<'a, T, const N: usize> Iterator for ConstChunksExact<'a, T, {}> {
+   |             ^^^^^^^^^^^^^^ unconstrained const parameter
+   |
+   = note: expressions using a const parameter must map each value to a distinct output value
+   = note: proving the result of expressions other than the parameter are unique is not supported
+
+error[E0308]: mismatched types
+  --> $DIR/ice-unexpected-inference-var-122549.rs:15:66
+   |
+LL | impl<'a, T, const N: usize> Iterator for ConstChunksExact<'a, T, {}> {
+   |                                                                  ^^ expected `usize`, found `()`
+
+error: aborting due to 7 previous errors
+
+Some errors have detailed explanations: E0046, E0207, E0261, E0308, E0392.
+For more information about an error, try `rustc --explain E0046`.