Rollup merge of #74159 - lcnr:const-generic-ty-decl, r=varkor

forbid generic params in the type of const params

implements and closes #74152

fixes #74101, closes #71169, fixes #73491, closes #62878

@eddyb and I talked [on zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/type.20of.20const.20parameters/near/203405696) about this and we probably want to also forbid generic consts in the default
type of a parameter, e.g. `struct Foo<T, U = [u8; std::mem::size_of::<T>()]>`, this is currently still allowed
and I will probably fix that in a followup PR.

r? @varkor @eddyb

2 files changed, 16 insertions, 1 deletions

diff --git a/src/librustc_error_codes/error_codes/E0671.md b/src/librustc_error_codes/error_codes/E0671.md
index 449fb8ffc89..a993ce826a7 100644
--- a/src/librustc_error_codes/error_codes/E0671.md
+++ b/src/librustc_error_codes/error_codes/E0671.md
@@ -3,7 +3,7 @@
 Const parameters cannot depend on type parameters.
 The following is therefore invalid:
 
-```compile_fail,E0741
+```compile_fail,E0770
 #![feature(const_generics)]
 
 fn const_id<T, const N: T>() -> T { // error
diff --git a/src/librustc_error_codes/error_codes/E0770.md b/src/librustc_error_codes/error_codes/E0770.md
new file mode 100644
index 00000000000..278bf9b907b
--- /dev/null
+++ b/src/librustc_error_codes/error_codes/E0770.md
@@ -0,0 +1,15 @@
+The type of a const parameter references other generic parameters.
+
+Erroneous code example:
+
+```compile_fail,E0770
+#![feature(const_generics)]
+fn foo<T, const N: T>() {} // error!
+```
+
+To fix this error, use a concrete type for the const parameter:
+
+```
+#![feature(const_generics)]
+fn foo<T, const N: usize>() {}
+```