Rollup merge of #156403 - SpriteOvO:type-info-refactor-variant, r=oli-obk

Add `TypeId` methods `variants` `fields` `field` for `type_info`

Tracking issue rust-lang/rust#146922

- Adds `fn TypeId::variants` returns the number of variants, for struct and union and primitive types, it's always 1.
- Adds `fn TypeId::fields` returns the number of fields.
- Adds `fn TypeId::field` returns a field representing type `FieldId`.
- Adds a new type `FieldId`, which is a wrapper of `FieldRepresentingType`'s `TypeId`.

For methods `{fields,field}`, if indexing out of bounds, a compile-time error will be raised.

Regarding the removal of `Type` items, this will be done in a later PR in one go.

r? @oli-obk

tests/ui/reflection/variant_index_out_of_bounds.rs

@@ -0,0 +1,15 @@
+#![feature(type_info)]
+
+use std::any::TypeId;
+
+fn main() {}
+
+const _: () = const {
+    TypeId::of::<Option::<()>>().fields(2);
+    //~^ ERROR indexing out of bounds: the len is 2 but the index is 2
+};
+
+const _: () = const {
+    TypeId::of::<Option::<()>>().field(0, 1);
+    //~^ ERROR indexing out of bounds: the len is 0 but the index is 1
+};

tests/ui/reflection/variant_index_out_of_bounds.stderr

@@ -0,0 +1,41 @@
+error[E0080]: indexing out of bounds: the len is 2 but the index is 2
+  --> $DIR/variant_index_out_of_bounds.rs:8:5
+   |
+LL |     TypeId::of::<Option::<()>>().fields(2);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `_::{constant#0}` failed inside this call
+   |
+note: inside `type_info::<impl TypeId>::fields`
+  --> $SRC_DIR/core/src/mem/type_info.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/variant_index_out_of_bounds.rs:7:15
+   |
+LL |   const _: () = const {
+   |  _______________^
+LL | |     TypeId::of::<Option::<()>>().fields(2);
+LL | |
+LL | | };
+   | |_^
+
+error[E0080]: indexing out of bounds: the len is 0 but the index is 1
+  --> $DIR/variant_index_out_of_bounds.rs:13:5
+   |
+LL |     TypeId::of::<Option::<()>>().field(0, 1);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `_::{constant#0}` failed inside this call
+   |
+note: inside `type_info::<impl TypeId>::field`
+  --> $SRC_DIR/core/src/mem/type_info.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/variant_index_out_of_bounds.rs:12:15
+   |
+LL |   const _: () = const {
+   |  _______________^
+LL | |     TypeId::of::<Option::<()>>().field(0, 1);
+LL | |
+LL | | };
+   | |_^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0080`.