Auto merge of rust-lang#104507 - WaffleLapkin:asderefsyou, r=wesleywiser

Use `as_deref` in compiler (but only where it makes sense)

This simplifies some code :3

(there are some changes that are not exacly `as_deref`, but more like "clever `Option`/`Result` method use")

Author: bors

build_system/utils.rs

@@ -104,5 +104,5 @@ pub(crate) fn copy_dir_recursively(from: &Path, to: &Path) {
 }
 
 pub(crate) fn is_ci() -> bool {
-    env::var("CI").as_ref().map(|val| &**val) == Ok("true")
+    env::var("CI").as_deref() == Ok("true")
 }

src/config.rs

@@ -2,7 +2,7 @@ use std::env;
 use std::str::FromStr;
 
 fn bool_env_var(key: &str) -> bool {
-    env::var(key).as_ref().map(|val| &**val) == Ok("1")
+    env::var(key).as_deref() == Ok("1")
 }
 
 /// The mode to use for compilation.