Resolve external preludes for modules inside blocks

[roife]

The original issue was closed by the bot 😅, but we need to resolve this problem.

          Looks like we fail to resolve the extern prelude in local modules in block modules, it works fine if the `mod cs` is at the top-level (not nested inside a function or anything)

fn main() {
    mod f {
        use core as _;
    }
}

          here `core` does not resolve

Originally posted in #17032 (comment)

[roife]

Sorry... I open the issue with reference in new issue, and am unable to add the bug label to this issue. 😳

---

I added self.inject_prelude(); in

rust-analyzer/crates/hir-def/src/nameres/collector.rs

Lines 367 to 384 in 07ae540

	fn seed_with_inner(&mut self, tree_id: TreeId) {
	let item_tree = tree_id.item_tree(self.db);
	let is_cfg_enabled = item_tree
	.top_level_attrs(self.db, self.def_map.krate)
	.cfg()
	.map_or(true, |cfg| self.cfg_options.check(&cfg) != Some(false));
	if is_cfg_enabled {
	ModCollector {
	def_collector: self,
	macro_depth: 0,
	module_id: DefMap::ROOT,
	tree_id,
	item_tree: &item_tree,
	mod_dir: ModDir::root(),
	}
	.collect_in_top_module(item_tree.top_level_items());
	}
	}

, removed

rust-analyzer/crates/hir-def/src/nameres/path_resolution.rs

Lines 473 to 477 in 07ae540

	if self.block.is_some() {
	// Don't resolve extern prelude in block `DefMap`s, defer it to the crate def map so
	// that blocks can properly shadow them
	return PerNs::none();
	}

and

rust-analyzer/crates/hir-def/src/nameres/path_resolution.rs

Lines 518 to 521 in 07ae540

	if self.block.is_some() {
	// Don't resolve extern prelude in block `DefMap`s.
	return PerNs::none();
	}

This resolves the issue, but I don't understand what "so that blocks can properly shadow them" means.

[Veykril]

The shadowing is about stuff like

(assuming a core entry in the extern prelude)

fn f() {
   mod core {
       pub fn foo () {}
   }
   core::foo();
}

This should look into the local module, not the prelude.

[roife]

It seems that it can be resolved correctly with the modifications above:

[Veykril]

The problem with that approach might be that it fails for nsted block def maps

fn foo() {
    mod core { mod mem { fn drop<T>(_: T) {} } }
    {
        use self as _; // forces a block def map here
        core::mem::drop(0); // should resolve to the local item, not the prelude one
    }
}