diff --git a/compiler/rustc_target/src/spec/avr_gnu_base.rs b/compiler/rustc_target/src/spec/avr_gnu_base.rs
index 9cc10032c71..67a7684da2c 100644
--- a/compiler/rustc_target/src/spec/avr_gnu_base.rs
+++ b/compiler/rustc_target/src/spec/avr_gnu_base.rs
@@ -11,7 +11,6 @@ pub fn target(target_cpu: String) -> Target {
         pointer_width: 16,
         options: TargetOptions {
             c_int_width: "16".to_string(),
-            os: "unknown".to_string(),
             cpu: target_cpu.clone(),
             exe_suffix: ".elf".to_string(),
 
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 2a4ae786fb7..dc8c41ecedc 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -713,6 +713,9 @@ pub struct TargetOptions {
     /// Width of c_int type. Defaults to "32".
     pub c_int_width: String,
     /// OS name to use for conditional compilation. Defaults to "none".
+    /// "none" implies a bare metal target without `std` library.
+    /// A couple of targets having `std` also use "unknown" as an `os` value,
+    /// but they are exceptions.
     pub os: String,
     /// Environment name to use for conditional compilation. Defaults to "".
     pub env: String,