diff --git a/cargo-miri/bin.rs b/cargo-miri/bin.rs
index f514f4e3e5c3..d35940897191 100644
--- a/cargo-miri/bin.rs
+++ b/cargo-miri/bin.rs
@@ -612,7 +612,9 @@ fn phase_cargo_runner(binary: &Path, binary_args: env::Args) {
     let info: CrateRunInfo = serde_json::from_reader(file)
         .unwrap_or_else(|_| show_error(format!("File {:?} does not contain valid JSON", binary)));
 
-    // Set missing env vars.
+    // Set missing env vars. Looks like `build.rs` vars are still set at run-time, but
+    // `CARGO_BIN_EXE_*` are not. This means we can give the run-time environment precedence,
+    // to rather do too little than too much.
     for (name, val) in info.env {
         if env::var_os(&name).is_none() {
             env::set_var(name, val);
diff --git a/test-cargo-miri/build.rs b/test-cargo-miri/build.rs
index b1f5fc172620..9851ccf39f3f 100644
--- a/test-cargo-miri/build.rs
+++ b/test-cargo-miri/build.rs
@@ -12,4 +12,6 @@ fn not_in_miri() -> i32 {
 fn main() {
     not_in_miri();
     println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-env-changed=MIRITESTVAR");
+    println!("cargo:rustc-env=MIRITESTVAR=testval");
 }
diff --git a/test-cargo-miri/run-test.py b/test-cargo-miri/run-test.py
index 665e9834e8e5..2250835c9998 100755
--- a/test-cargo-miri/run-test.py
+++ b/test-cargo-miri/run-test.py
@@ -50,11 +50,14 @@ def test(name, cmd, stdout_ref, stderr_ref, stdin=b'', env={}):
         fail("stderr does not match reference")
 
 def test_cargo_miri_run():
-    test("`cargo miri run`",
+    test("`cargo miri run` (without isolation)",
         cargo_miri("run"),
         "stdout.ref1", "stderr.ref1",
         stdin=b'12\n21\n',
-        env={'MIRIFLAGS': "-Zmiri-disable-isolation"},
+        env={
+            'MIRIFLAGS': "-Zmiri-disable-isolation",
+            'MIRITESTVAR': "wrongval", # make sure the build.rs value takes precedence
+        },
     )
     test("`cargo miri run` (with arguments and target)",
         cargo_miri("run") + ["--bin", "cargo-miri-test", "--", "hello world", '"hello world"'],
diff --git a/test-cargo-miri/src/main.rs b/test-cargo-miri/src/main.rs
index 17808f57061e..b36b0c725f85 100644
--- a/test-cargo-miri/src/main.rs
+++ b/test-cargo-miri/src/main.rs
@@ -3,6 +3,9 @@ use byteorder::{BigEndian, ByteOrder};
 use std::io::{self, BufRead};
 
 fn main() {
+    // Check env var set by `build.rs`.
+    assert_eq!(env!("MIRITESTVAR"), "testval");
+
     // Exercise external crate, printing to stdout.
     let buf = &[1,2,3,4];
     let n = <BigEndian as ByteOrder>::read_u32(buf);