diff --git a/src/bin/cargo-miri.rs b/src/bin/cargo-miri.rs
index 8bd9947092a..37ab41b317d 100644
--- a/src/bin/cargo-miri.rs
+++ b/src/bin/cargo-miri.rs
@@ -269,7 +269,7 @@ fn ask_to_run(mut cmd: Command, ask: bool, text: &str) {
 /// `MIRI_SYSROOT`. Skipped if `MIRI_SYSROOT` is already set, in which case we expect the user has
 /// done all this already.
 fn setup(subcommand: MiriCommand) {
-    if std::env::var("MIRI_SYSROOT").is_ok() {
+    if std::env::var_os("MIRI_SYSROOT").is_some() {
         if subcommand == MiriCommand::Setup {
             println!("WARNING: MIRI_SYSROOT already set, not doing anything.")
         }
@@ -282,7 +282,7 @@ fn setup(subcommand: MiriCommand) {
 
     // First, we need xargo.
     if xargo_version().map_or(true, |v| v < XARGO_MIN_VERSION) {
-        if std::env::var("XARGO_CHECK").is_ok() {
+        if std::env::var_os("XARGO_CHECK").is_some() {
             // The user manually gave us a xargo binary; don't do anything automatically.
             show_error(format!("Your xargo is too old; please upgrade to the latest version"))
         }
@@ -292,9 +292,9 @@ fn setup(subcommand: MiriCommand) {
     }
 
     // Determine where the rust sources are located.  `XARGO_RUST_SRC` env var trumps everything.
-    let rust_src = match std::env::var("XARGO_RUST_SRC") {
-        Ok(val) => PathBuf::from(val),
-        Err(_) => {
+    let rust_src = match std::env::var_os("XARGO_RUST_SRC") {
+        Some(val) => PathBuf::from(val),
+        None => {
             // Check for `rust-src` rustup component.
             let sysroot = rustc()
                 .args(&["--print", "sysroot"])
@@ -522,7 +522,7 @@ fn is_runnable_crate() -> bool {
         is_bin || is_test
     }
 
-    let verbose = std::env::var("MIRI_VERBOSE").is_ok();
+    let verbose = std::env::var_os("MIRI_VERBOSE").is_some();
     let target_crate = is_target_crate();
 
     // Figure out which arguments we need to pass.
@@ -531,6 +531,7 @@ fn is_runnable_crate() -> bool {
     // other args for target crates - that is, crates which are ultimately
     // going to get interpreted by Miri.
     if target_crate {
+        // FIXME: breaks for non-UTF-8 sysroots (use `var_os` instead).
         let sysroot =
             std::env::var("MIRI_SYSROOT").expect("The wrapper should have set MIRI_SYSROOT");
         args.push("--sysroot".to_owned());
@@ -545,6 +546,8 @@ fn is_runnable_crate() -> bool {
         // we want to interpret under Miri. We deserialize the user-provided arguments
         // from the special environment variable "MIRI_ARGS", and feed them
         // to the 'miri' binary.
+        //
+        // `env::var` is okay here, well-formed JSON is always UTF-8.
         let magic = std::env::var("MIRI_ARGS").expect("missing MIRI_ARGS");
         let mut user_args: Vec<String> =
             serde_json::from_str(&magic).expect("failed to deserialize MIRI_ARGS");
diff --git a/src/bin/miri.rs b/src/bin/miri.rs
index 06101fe24e2..31f78aa9895 100644
--- a/src/bin/miri.rs
+++ b/src/bin/miri.rs
@@ -61,7 +61,7 @@ fn init_early_loggers() {
     // If it is not set, we avoid initializing now so that we can initialize
     // later with our custom settings, and *not* log anything for what happens before
     // `miri` gets started.
-    if env::var("RUSTC_LOG").is_ok() {
+    if env::var_os("RUSTC_LOG").is_some() {
         rustc_driver::init_rustc_env_logger();
     }
 }
@@ -69,8 +69,9 @@ fn init_early_loggers() {
 fn init_late_loggers(tcx: TyCtxt<'_>) {
     // We initialize loggers right before we start evaluation. We overwrite the `RUSTC_LOG`
     // env var if it is not set, control it based on `MIRI_LOG`.
+    // (FIXE: use `var_os`, but then we need to manually concatenate instead of `format!`.)
     if let Ok(var) = env::var("MIRI_LOG") {
-        if env::var("RUSTC_LOG").is_err() {
+        if env::var_os("RUSTC_LOG").is_none() {
             // We try to be a bit clever here: if `MIRI_LOG` is just a single level
             // used for everything, we only apply it to the parts of rustc that are
             // CTFE-related. Otherwise, we use it verbatim for `RUSTC_LOG`.
@@ -90,8 +91,8 @@ fn init_late_loggers(tcx: TyCtxt<'_>) {
 
     // If `MIRI_BACKTRACE` is set and `RUSTC_CTFE_BACKTRACE` is not, set `RUSTC_CTFE_BACKTRACE`.
     // Do this late, so we ideally only apply this to Miri's errors.
-    if let Ok(val) = env::var("MIRI_BACKTRACE") {
-        let ctfe_backtrace = match &*val {
+    if let Some(val) = env::var_os("MIRI_BACKTRACE") {
+        let ctfe_backtrace = match &*val.to_string_lossy() {
             "immediate" => CtfeBacktrace::Immediate,
             "0" => CtfeBacktrace::Disabled,
             _ => CtfeBacktrace::Capture,