Get rid of a warning

library/compiler-builtins/Cargo.toml

@@ -7,7 +7,7 @@ readme = "README.md"
 repository = "https://github.com/rust-lang/compiler-builtins"
 homepage = "https://github.com/rust-lang/compiler-builtins"
 documentation = "https://docs.rs/compiler_builtins"
-edition = "2018"
+edition = "2021"
 description = """
 Compiler intrinsics used by the Rust compiler. Also available for other targets
 if necessary!

library/compiler-builtins/build.rs

@@ -219,8 +219,6 @@ fn configure_check_cfg() {
 
 #[cfg(feature = "c")]
 mod c {
-    extern crate cc;
-
     use std::collections::{BTreeMap, HashSet};
     use std::env;
     use std::fs::{self, File};

library/compiler-builtins/crates/panic-handler/Cargo.toml

@@ -2,5 +2,6 @@
 name = "panic-handler"
 version = "0.1.0"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2021"
 
 [dependencies]