diff --git a/src/bootstrap/bin/rustc.rs b/src/bootstrap/bin/rustc.rs
index 84415baa3a140..936bed482e6f5 100644
--- a/src/bootstrap/bin/rustc.rs
+++ b/src/bootstrap/bin/rustc.rs
@@ -119,19 +119,9 @@ fn main() {
         cmd.arg(format!("-Cdebuginfo={}", debuginfo_level));
     }
 
-    if env::var_os("RUSTC_EXTERNAL_TOOL").is_none() {
-        // When extending this list, add the new lints to the RUSTFLAGS of the
-        // build_bootstrap function of src/bootstrap/bootstrap.py as well as
-        // some code doesn't go through this `rustc` wrapper.
-        cmd.arg("-Wrust_2018_idioms");
-        cmd.arg("-Wunused_lifetimes");
-        if use_internal_lints(crate_name) {
-            cmd.arg("-Zunstable-options");
-            cmd.arg("-Wrustc::internal");
-        }
-        if env::var_os("RUSTC_DENY_WARNINGS").is_some() {
-            cmd.arg("-Dwarnings");
-        }
+    if use_internal_lints(crate_name) {
+        cmd.arg("-Zunstable-options");
+        cmd.arg("-Wrustc::internal");
     }
 
     if let Some(target) = target {
diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index fbae3629c4bd9..6f2bf5f397bc8 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -875,6 +875,28 @@ impl<'a> Builder<'a> {
             extra_args.push_str("-Zforce-unstable-if-unmarked");
         }
 
+        match mode {
+            Mode::ToolStd { in_tree: true } |
+            Mode::ToolRustc { in_tree: true } |
+            Mode::ToolBootstrap { in_tree: true } |
+            Mode::Std |
+            Mode::Rustc |
+            Mode::Codegen => {
+                // When extending this list, add the new lints to the RUSTFLAGS of the
+                // build_bootstrap function of src/bootstrap/bootstrap.py as well as
+                // some code doesn't go through this `rustc` wrapper.
+                extra_args.push_str(" -Wrust_2018_idioms");
+                extra_args.push_str(" -Wunused_lifetimes");
+            }
+            Mode::ToolStd { in_tree: false } |
+            Mode::ToolRustc { in_tree: false } |
+            Mode::ToolBootstrap { in_tree: false } => {}
+        }
+
+        if self.config.deny_warnings {
+            extra_args.push_str(" -Dwarnings");
+        }
+
         if !extra_args.is_empty() {
             cargo.env(
                 "RUSTFLAGS",
@@ -1038,10 +1060,6 @@ impl<'a> Builder<'a> {
 
         cargo.env("RUSTC_VERBOSE", self.verbosity.to_string());
 
-        if self.config.deny_warnings {
-            cargo.env("RUSTC_DENY_WARNINGS", "1");
-        }
-
         // Throughout the build Cargo can execute a number of build scripts
         // compiling C/C++ code and we need to pass compilers, archivers, flags, etc
         // obtained previously to those build scripts.