Prevent ICE on missing lang items.

[lcnr]

There are currently quite a few locations in the compiler similar to the following:

rust/src/librustc_typeck/check/mod.rs

Line 5301 in 7c34d8d

let future_trait = self.tcx.lang_items().future_trait().unwrap();

This causes an internal compiler error in case the expected lang item is not defined,
for example when using #![no_core]. (see #72099)

A better way to deal with this is to use tcx.require_lang_item(FutureTraitLangItem, opt_span) instead, which cleanly stops compilation in case the lang item is missing.

See #72170, which already did this in some places.

This issue has been assigned to @doctorn via this comment.

[doctorn]

Ah! This is my bad...

[doctorn]

@rustbot claim