Rollup merge of rust-lang#88782 - asquared31415:issue-79559, r=cjgillot

Fix ICE when `start` lang item has wrong generics

In my previous pr rust-lang#87875 I missed the requirements on the `start` lang item due to its relative difficulty to test and opting for more conservative estimates.  This fixes that by updating the requirement to be exactly one generic type.

The `start` lang item should have exactly one generic type for the return type of the `main` fn ptr passed to it.  I believe having zero would previously *sometimes* compile (often with the use of `fn() -> ()` as the fn ptr but it was likely UB to call if the return type of `main` was not `()` as far as I know) however it also sometimes would not for various errors including ICEs and LLVM errors depending on exact situations.  Having more than 1 generic has always failed with an ICE because only the one generic type is expected and provided.

Fixes rust-lang#79559, fixes rust-lang#73584, fixes rust-lang#83117 (all duplicates)
Relevant to rust-lang#9307

r? ````@cjgillot````

Author: Manishearth

tests/ui/def_id_nocore.rs

@@ -15,11 +15,12 @@ pub trait Copy {}
 pub unsafe trait Freeze {}
 
 #[lang = "start"]
-#[start]
-fn start(_argc: isize, _argv: *const *const u8) -> isize {
+fn start<T>(_main: fn() -> T, _argc: isize, _argv: *const *const u8) -> isize {
     0
 }
 
+fn main() {}
+
 struct A;
 
 impl A {

tests/ui/def_id_nocore.stderr

@@ -1,5 +1,5 @@
 error: methods called `as_*` usually take `self` by reference or `self` by mutable reference
-  --> $DIR/def_id_nocore.rs:26:19
+  --> $DIR/def_id_nocore.rs:27:19
    |
 LL |     pub fn as_ref(self) -> &'static str {
    |                   ^^^^