diff --git a/src/bootstrap/src/core/builder/cargo.rs b/src/bootstrap/src/core/builder/cargo.rs
index 24d7a24152a32..b76dc77683f94 100644
--- a/src/bootstrap/src/core/builder/cargo.rs
+++ b/src/bootstrap/src/core/builder/cargo.rs
@@ -14,8 +14,11 @@ use crate::{
     RemapScheme, TargetSelection, command, prepare_behaviour_dump_dir, t,
 };
 
-/// Represents flag values in `String` form with whitespace delimiter to pass it to the compiler
-/// later.
+/// Represents flag values in `String` form with a `\x1f` delimiter to pass to the compiler later.
+///
+/// Flags are emitted via `CARGO_ENCODED_RUSTFLAGS` / `CARGO_ENCODED_RUSTDOCFLAGS`,
+/// which use `\x1f` (ASCII Unit Separator) as the delimiter and therefore allow spaces
+/// within individual flag values (e.g. paths from `llvm-config --libdir`).
 ///
 /// `-Z crate-attr` flags will be applied recursively on the target code using the
 /// `rustc_parse::parser::Parser`. See `rustc_builtin_macros::cmdline_attrs::inject` for more
@@ -51,11 +54,16 @@ impl Rustflags {
     }
 
     fn arg(&mut self, arg: &str) -> &mut Self {
-        assert_eq!(arg.split(' ').count(), 1);
-        if !self.0.is_empty() {
-            self.0.push(' ');
+        assert!(
+            !arg.contains('\x1f'),
+            "rustflag must not contain the ASCII unit separator (\\x1f): {arg:?}"
+        );
+        if !arg.is_empty() {
+            if !self.0.is_empty() {
+                self.0.push('\x1f');
+            }
+            self.0.push_str(arg);
         }
-        self.0.push_str(arg);
         self
     }
 
@@ -457,14 +465,21 @@ impl From<Cargo> for BootstrapCommand {
 
         cargo.command.args(cargo.args);
 
+        // Always unset the plain RUSTFLAGS/RUSTDOCFLAGS so that downstream
+        // tools (e.g. build.rs scripts) see only the encoded form. Any flags
+        // from the caller's environment have already been folded into the
+        // Rustflags struct via `propagate_cargo_env`.
+        cargo.command.env_remove("RUSTFLAGS");
+        cargo.command.env_remove("RUSTDOCFLAGS");
+
         let rustflags = &cargo.rustflags.0;
         if !rustflags.is_empty() {
-            cargo.command.env("RUSTFLAGS", rustflags);
+            cargo.command.env("CARGO_ENCODED_RUSTFLAGS", rustflags);
         }
 
         let rustdocflags = &cargo.rustdocflags.0;
         if !rustdocflags.is_empty() {
-            cargo.command.env("RUSTDOCFLAGS", rustdocflags);
+            cargo.command.env("CARGO_ENCODED_RUSTDOCFLAGS", rustdocflags);
         }
 
         let encoded_hostflags = cargo.hostflags.encode();
@@ -1181,8 +1196,9 @@ impl Builder<'_> {
         if (mode == Mode::ToolRustcPrivate || mode == Mode::Codegen)
             && let Some(llvm_config) = self.llvm_config(target)
         {
-            let llvm_libdir =
+            let llvm_libdir_raw =
                 command(llvm_config).cached().arg("--libdir").run_capture_stdout(self).stdout();
+            let llvm_libdir = llvm_libdir_raw.trim();
             if target.is_msvc() {
                 rustflags.arg(&format!("-Clink-arg=-LIBPATH:{llvm_libdir}"));
             } else {
diff --git a/src/tools/miri/tests/ui.rs b/src/tools/miri/tests/ui.rs
index 633452f6052d7..3e46ee17c8a65 100644
--- a/src/tools/miri/tests/ui.rs
+++ b/src/tools/miri/tests/ui.rs
@@ -182,8 +182,9 @@ fn miri_config(
                         .map(Into::into)
                         .collect(),
                     envs: vec![
-                        // Reset `RUSTFLAGS` to work around <https://github.com/rust-lang/rust/pull/119574#issuecomment-1876878344>.
+                        // Reset `RUSTFLAGS`/`CARGO_ENCODED_RUSTFLAGS` to work around <https://github.com/rust-lang/rust/pull/119574#issuecomment-1876878344>.
                         ("RUSTFLAGS".into(), None),
+                        ("CARGO_ENCODED_RUSTFLAGS".into(), None),
                         // Reset `MIRIFLAGS` because it caused trouble in the past and should not be needed.
                         ("MIRIFLAGS".into(), None),
                         // Allow `cargo miri build`.