Apply clippy::as_underscore lint

Author: taiki-e

.clippy.toml

@@ -10,4 +10,5 @@ disallowed-methods = [
     { path = "std::env::remove_var", reason = "this is not thread-safe and inherently unsafe; see <https://github.com/rust-lang/rust/issues/27970> for more" },
     { path = "std::env::set_var", reason = "this is not thread-safe and inherently unsafe; see <https://github.com/rust-lang/rust/issues/27970> for more" },
 ]
-disallowed-types = []
+disallowed-types = [
+]

Cargo.toml

@@ -125,6 +125,7 @@ unreachable_pub = "warn"
 all = "warn" # Downgrade deny-by-default lints
 pedantic = "warn"
 as_ptr_cast_mut = "warn"
+as_underscore = "warn"
 default_union_representation = "warn"
 inline_asm_x86_att_syntax = "warn"
 trailing_empty_array = "warn"