Auto merge of #70825 - eddyb:enum-discr-correct-generics-parent, r=nikomatsakis

typeck: always expose explicit enum discriminant `AnonConst`s' parent in `generics_of`.

This is similar to #70452 but for explicit `enum` discriminant constant expressions.
However, unlike #70452, this PR should have no effect on stable code, as while it alleviates #43408 errors, there is no way to actually compile an `enum` with generic parameters *and* explicit discriminants, without `#![feature(arbitrary_enum_discriminant)]`, as explicit discriminant expression don't count as uses of parameters (if they did, they would count as invariant uses).

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There's also 2 other commits here, both related to #70453:
* "ty: use `delay_span_bug` in `ty::AdtDef::eval_explicit_discr`." - hides the ICEs demonstrated on #70453, when there are other errors (which the next commit adds)
* "typeck/wfcheck: require that explicit enum discriminants const-evaluate succesfully." - closes #70453 by picking alternative "2", i.e. erroring when a discriminant doesn't fully const-evaluate from the perspective of the `enum` definition

In the future, it might be possible to allow `enum` discriminants to actually depend on parameters, but that will likely require #68436 + some way to restrict the values so no two variants can end up with overlapping discriminants.

As this PR would close #70453, it shouldn't be merged until a decision is reached there.

r? @nikomatsakis

5 files changed, 79 insertions, 0 deletions

diff --git a/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice-2.rs b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice-2.rs
new file mode 100644
index 00000000000..0cfb93d4668
--- /dev/null
+++ b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice-2.rs
@@ -0,0 +1,16 @@
+#![feature(arbitrary_enum_discriminant, core_intrinsics)]
+
+extern crate core;
+use core::intrinsics::discriminant_value;
+
+#[repr(usize)]
+enum MyWeirdOption<T> {
+    None = 0,
+    Some(T) = std::mem::size_of::<T>(),
+    //~^ ERROR constant expression depends on a generic parameter
+}
+
+fn main() {
+    assert_eq!(discriminant_value(&MyWeirdOption::<u8>::None), 0);
+    assert_eq!(discriminant_value(&MyWeirdOption::Some(0u8)), 1);
+}
diff --git a/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice-2.stderr b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice-2.stderr
new file mode 100644
index 00000000000..91d488a07cc
--- /dev/null
+++ b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice-2.stderr
@@ -0,0 +1,10 @@
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-70453-generics-in-discr-ice-2.rs:9:15
+   |
+LL |     Some(T) = std::mem::size_of::<T>(),
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice.rs b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice.rs
new file mode 100644
index 00000000000..676f1115dde
--- /dev/null
+++ b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice.rs
@@ -0,0 +1,17 @@
+#![feature(core_intrinsics)]
+
+extern crate core;
+use core::intrinsics::discriminant_value;
+
+#[repr(usize)]
+enum MyWeirdOption<T> {
+//~^ ERROR parameter `T` is never used
+    None = 0,
+    Some = std::mem::size_of::<T>(),
+    //~^ ERROR constant expression depends on a generic parameter
+}
+
+fn main() {
+    assert_eq!(discriminant_value(&MyWeirdOption::<u8>::None), 0);
+    assert_eq!(discriminant_value(&MyWeirdOption::<u8>::Some), 1);
+}
diff --git a/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr
new file mode 100644
index 00000000000..52e58aa4c6d
--- /dev/null
+++ b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr
@@ -0,0 +1,19 @@
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-70453-generics-in-discr-ice.rs:10:12
+   |
+LL |     Some = std::mem::size_of::<T>(),
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error[E0392]: parameter `T` is never used
+  --> $DIR/issue-70453-generics-in-discr-ice.rs:7:20
+   |
+LL | enum MyWeirdOption<T> {
+   |                    ^ unused parameter
+   |
+   = help: consider removing `T`, referring to it in a field, or using a marker such as `std::marker::PhantomData`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0392`.
diff --git a/src/test/ui/enum-discriminant/issue-70453-polymorphic-ctfe.rs b/src/test/ui/enum-discriminant/issue-70453-polymorphic-ctfe.rs
new file mode 100644
index 00000000000..05911a9a303
--- /dev/null
+++ b/src/test/ui/enum-discriminant/issue-70453-polymorphic-ctfe.rs
@@ -0,0 +1,17 @@
+// run-pass
+
+#![feature(arbitrary_enum_discriminant, core_intrinsics)]
+
+extern crate core;
+use core::intrinsics::discriminant_value;
+
+#[repr(usize)]
+enum MyWeirdOption<T> {
+    None = 0,
+    Some(T) = core::mem::size_of::<*mut T>(),
+}
+
+fn main() {
+    assert_eq!(discriminant_value(&MyWeirdOption::<()>::None), 0);
+    assert_eq!(discriminant_value(&MyWeirdOption::Some(())), core::mem::size_of::<usize>() as u64);
+}