Inline the `is_tool` check for setting `-Zforce-unstable-if-unmarked`

src/bootstrap/src/core/builder/cargo.rs

diff --git a/src/bootstrap/src/core/builder/cargo.rs b/src/bootstrap/src/core/builder/cargo.rs
index e3fa826..7f4e238 100644
--- a/src/bootstrap/src/core/builder/cargo.rs
+++ b/src/bootstrap/src/core/builder/cargo.rs
@@ -999,8 +999,18 @@ fn cargo(
             cargo.env("UPDATE_EXPECT", "1");
         }
 
-        if !mode.is_tool() {
-            cargo.env("RUSTC_FORCE_UNSTABLE", "1");
+        // Set an environment variable that tells the rustc/rustdoc wrapper
+        // binary to pass `-Zforce-unstable-if-unmarked` to the real compiler.
+        match mode {
+            // Any library crate that's part of the sysroot should be marked unstable
+            // (including third-party dependencies), unless it uses a staged_api
+            // `#![stable(..)]` attribute to explicitly mark itself stable.
+            Mode::Std | Mode::Codegen | Mode::Rustc => {
+                cargo.env("RUSTC_FORCE_UNSTABLE", "1");
+            }
+
+            // For everything else, crate stability shouldn't matter, so don't set a flag.
+            Mode::ToolBootstrap | Mode::ToolRustcPrivate | Mode::ToolStd | Mode::ToolTarget => {}
         }
 
         if let Some(x) = self.crt_static(target) {

src/bootstrap/src/lib.rs

diff --git a/src/bootstrap/src/lib.rs b/src/bootstrap/src/lib.rs
index dfa29b5..ec5c6fb 100644
--- a/src/bootstrap/src/lib.rs
+++ b/src/bootstrap/src/lib.rs
@@ -340,13 +340,6 @@ pub enum Mode {
 }
 
 impl Mode {
-    pub fn is_tool(&self) -> bool {
-        match self {
-            Mode::ToolBootstrap | Mode::ToolRustcPrivate | Mode::ToolStd | Mode::ToolTarget => true,
-            Mode::Std | Mode::Codegen | Mode::Rustc => false,
-        }
-    }
-
     pub fn must_support_dlopen(&self) -> bool {
         match self {
             Mode::Std | Mode::Codegen => true,