Rollup merge of #77303 - lcnr:const-evaluatable-TooGeneric, r=oli-obk,varkor

const evaluatable: improve `TooGeneric` handling

Instead of emitting an error in `fulfill`, we now correctly stall on inference variables.

As `const_eval_resolve` returns `ErrorHandled::TooGeneric` when encountering generic parameters on which
we actually do want to error, we check for inference variables and eagerly emit an error if they don't exist, returning `ErrorHandled::Reported` instead.

Also contains a small bugfix for `ConstEquate` where we previously only stalled on type variables. This is probably a leftover from
when we did not yet support stalling on const inference variables.

r? @oli-obk cc @varkor @eddyb

5 files changed, 49 insertions, 41 deletions

diff --git a/src/test/ui/const-generics/const_evaluatable_checked/cross_crate_predicate.rs b/src/test/ui/const-generics/const_evaluatable_checked/cross_crate_predicate.rs
index 52b89cfa045..e3a4d9a96aa 100644
--- a/src/test/ui/const-generics/const_evaluatable_checked/cross_crate_predicate.rs
+++ b/src/test/ui/const-generics/const_evaluatable_checked/cross_crate_predicate.rs
@@ -5,10 +5,10 @@ extern crate const_evaluatable_lib;
 
 fn user<T>() {
     let _ = const_evaluatable_lib::test1::<T>();
-    //~^ ERROR constant expression depends
-    //~| ERROR constant expression depends
-    //~| ERROR constant expression depends
-    //~| ERROR constant expression depends
+    //~^ ERROR unconstrained generic constant
+    //~| ERROR unconstrained generic constant
+    //~| ERROR unconstrained generic constant
+    //~| ERROR unconstrained generic constant
 }
 
 fn main() {}
diff --git a/src/test/ui/const-generics/const_evaluatable_checked/cross_crate_predicate.stderr b/src/test/ui/const-generics/const_evaluatable_checked/cross_crate_predicate.stderr
index 4af68118be3..8a298b47fff 100644
--- a/src/test/ui/const-generics/const_evaluatable_checked/cross_crate_predicate.stderr
+++ b/src/test/ui/const-generics/const_evaluatable_checked/cross_crate_predicate.stderr
@@ -1,54 +1,50 @@
-error: constant expression depends on a generic parameter
+error: unconstrained generic constant
   --> $DIR/cross_crate_predicate.rs:7:13
    |
 LL |     let _ = const_evaluatable_lib::test1::<T>();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   | 
-  ::: $DIR/auxiliary/const_evaluatable_lib.rs:6:10
    |
-LL |     [u8; std::mem::size_of::<T>() - 1]: Sized,
-   |          ---------------------------- required by this bound in `test1`
+help: consider adding a `where` bound for this expression
+  --> $DIR/auxiliary/const_evaluatable_lib.rs:6:10
    |
-   = note: this may fail depending on what value the parameter takes
+LL |     [u8; std::mem::size_of::<T>() - 1]: Sized,
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: constant expression depends on a generic parameter
+error: unconstrained generic constant
   --> $DIR/cross_crate_predicate.rs:7:13
    |
 LL |     let _ = const_evaluatable_lib::test1::<T>();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   | 
-  ::: $DIR/auxiliary/const_evaluatable_lib.rs:4:27
    |
-LL | pub fn test1<T>() -> [u8; std::mem::size_of::<T>() - 1]
-   |                           ---------------------------- required by this bound in `test1`
+help: consider adding a `where` bound for this expression
+  --> $DIR/auxiliary/const_evaluatable_lib.rs:4:27
    |
-   = note: this may fail depending on what value the parameter takes
+LL | pub fn test1<T>() -> [u8; std::mem::size_of::<T>() - 1]
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: constant expression depends on a generic parameter
+error: unconstrained generic constant
   --> $DIR/cross_crate_predicate.rs:7:13
    |
 LL |     let _ = const_evaluatable_lib::test1::<T>();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   | 
-  ::: $DIR/auxiliary/const_evaluatable_lib.rs:6:10
    |
-LL |     [u8; std::mem::size_of::<T>() - 1]: Sized,
-   |          ---------------------------- required by this bound in `test1`
+help: consider adding a `where` bound for this expression
+  --> $DIR/auxiliary/const_evaluatable_lib.rs:6:10
    |
-   = note: this may fail depending on what value the parameter takes
+LL |     [u8; std::mem::size_of::<T>() - 1]: Sized,
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: constant expression depends on a generic parameter
+error: unconstrained generic constant
   --> $DIR/cross_crate_predicate.rs:7:13
    |
 LL |     let _ = const_evaluatable_lib::test1::<T>();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   | 
-  ::: $DIR/auxiliary/const_evaluatable_lib.rs:4:27
    |
-LL | pub fn test1<T>() -> [u8; std::mem::size_of::<T>() - 1]
-   |                           ---------------------------- required by this bound in `test1`
+help: consider adding a `where` bound for this expression
+  --> $DIR/auxiliary/const_evaluatable_lib.rs:4:27
    |
-   = note: this may fail depending on what value the parameter takes
+LL | pub fn test1<T>() -> [u8; std::mem::size_of::<T>() - 1]
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 4 previous errors
 
diff --git a/src/test/ui/const-generics/const_evaluatable_checked/infer-too-generic.rs b/src/test/ui/const-generics/const_evaluatable_checked/infer-too-generic.rs
new file mode 100644
index 00000000000..cad06ea4004
--- /dev/null
+++ b/src/test/ui/const-generics/const_evaluatable_checked/infer-too-generic.rs
@@ -0,0 +1,24 @@
+// run-pass
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+
+use std::{mem, ptr};
+
+fn split_first<T, const N: usize>(arr: [T; N]) -> (T, [T; N - 1])
+where
+    [T; N - 1]: Sized,
+{
+    let arr = mem::ManuallyDrop::new(arr);
+    unsafe {
+        let head = ptr::read(&arr[0]);
+        let tail = ptr::read(&arr[1..] as *const [T] as *const [T; N - 1]);
+        (head, tail)
+    }
+}
+
+fn main() {
+    let arr = [0, 1, 2, 3, 4];
+    let (head, tail) = split_first(arr);
+    assert_eq!(head, 0);
+    assert_eq!(tail, [1, 2, 3, 4]);
+}
diff --git a/src/test/ui/const-generics/issues/issue-76595.rs b/src/test/ui/const-generics/issues/issue-76595.rs
index 0a16ca181f5..9fdbbff66e9 100644
--- a/src/test/ui/const-generics/issues/issue-76595.rs
+++ b/src/test/ui/const-generics/issues/issue-76595.rs
@@ -14,5 +14,4 @@ fn test<T, const P: usize>() where Bool<{core::mem::size_of::<T>() > 4}>: True {
 fn main() {
     test::<2>();
     //~^ ERROR wrong number of type
-    //~| ERROR constant expression depends
 }
diff --git a/src/test/ui/const-generics/issues/issue-76595.stderr b/src/test/ui/const-generics/issues/issue-76595.stderr
index bbc81693fc0..f258d297718 100644
--- a/src/test/ui/const-generics/issues/issue-76595.stderr
+++ b/src/test/ui/const-generics/issues/issue-76595.stderr
@@ -4,17 +4,6 @@ error[E0107]: wrong number of type arguments: expected 1, found 0
 LL |     test::<2>();
    |     ^^^^^^^^^ expected 1 type argument
 
-error: constant expression depends on a generic parameter
-  --> $DIR/issue-76595.rs:15:5
-   |
-LL | fn test<T, const P: usize>() where Bool<{core::mem::size_of::<T>() > 4}>: True {
-   |                                         ------------------------------- required by this bound in `test`
-...
-LL |     test::<2>();
-   |     ^^^^^^^^^
-   |
-   = note: this may fail depending on what value the parameter takes
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0107`.