MGCA: Check that const arguments in struct exprs are correctly typed

@camelid

Currently the wellformedness requirements of ty::Const does not require that ty::Const as the fields of ADTs have the type of the ADT's field. See this test

rust/tests/ui/const-generics/mgca/adt_expr_fields_type_check.rs

Lines 4 to 15 in 5497a36

    
           #![feature(min_generic_const_args, adt_const_params)] 
        
           #![expect(incomplete_features)] 
        
           #[derive(Eq, PartialEq, std::marker::ConstParamTy)] 
        
           struct Foo<T> { field: T } 
        
           fn accepts<const N: Foo<u8>>() {} 
        
           fn bar<const N: bool>() { 
        
               // `N` is not of type `u8` but we don't actually check this anywhere yet 
        
               accepts::<{ Foo::<u8> { field: N }}>(); 
        
           }

To fix this we should update wf::obligations for ty::Const to emit ConstArgHasType goals for ty::Consts in ValTreeKind::Branch according to the type of their parent ValTree node.

cc @camelid I've assigned you as we've talked about you implementing this on zulip

Rollup merge of rust-lang#150713 - mgca-typeck-struct-fields, r=BoxyUwU

Unrolled build for #150713

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MGCA: Check that const arguments in struct exprs are correctly typed #150623

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	#![feature(min_generic_const_args, adt_const_params)]
	#![expect(incomplete_features)]

	#[derive(Eq, PartialEq, std::marker::ConstParamTy)]
	struct Foo<T> { field: T }

	fn accepts<const N: Foo<u8>>() {}

	fn bar<const N: bool>() {
	// `N` is not of type `u8` but we don't actually check this anywhere yet
	accepts::<{ Foo::<u8> { field: N }}>();
	}

MGCA: Check that const arguments in struct exprs are correctly typed #150623

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