do not handle MIRI_SYSROOT in the driver at all, rely fully on the --sysroot flag

README.md

@@ -250,9 +250,12 @@ Several `-Z` flags are relevant for Miri:
 
 Moreover, Miri recognizes some environment variables:
 
-* `MIRI_SYSROOT` (recognized by `miri`, `cargo miri` and the test suite)
-  indicates the sysroot to use.
-* `MIRI_TARGET` (recognized by the test suite) indicates which target
+* `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during
+  Miri executions, also [see above][testing-miri].
+* `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite)
+  indicates the sysroot to use.  To do the same thing with `miri`
+  directly, use the `--sysroot` flag.
+* `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
   architecture to test against.  `miri` and `cargo miri` accept the `--target`
   flag for the same purpose.
 

miri

@@ -140,7 +140,7 @@ run|run-debug)
     cargo build $CARGO_BUILD_FLAGS
     find_sysroot
     # Then run the actual command.
-    exec cargo run $CARGO_BUILD_FLAGS "$@"
+    exec cargo run $CARGO_BUILD_FLAGS -- --sysroot "$MIRI_SYSROOT" "$@"
     ;;
 *)
     echo "Unknown command: $COMMAND"

src/bin/cargo-miri.rs

@@ -125,7 +125,6 @@ fn list_targets() -> impl Iterator<Item=cargo_metadata::Target> {
 fn test_sysroot_consistency() {
     fn get_sysroot(mut cmd: Command) -> PathBuf {
         let out = cmd.arg("--print").arg("sysroot")
-            .env_remove("MIRI_SYSROOT") // We want to test their "native" sysroot, not the manually set one
             .output().expect("Failed to run rustc to get sysroot info");
         assert!(out.status.success(), "Bad statuc code when getting sysroot info");
         let sysroot = out.stdout.lines().nth(0)
@@ -298,7 +297,7 @@ path = "lib.rs"
         Some(target) => target == rustc_version::version_meta().unwrap().host,
     };
     let sysroot = if is_host { dir.join("HOST") } else { PathBuf::from(dir) };
-    std::env::set_var("MIRI_SYSROOT", &sysroot);
+    std::env::set_var("MIRI_SYSROOT", &sysroot); // pass the env var to the processes we spawn, which will turn it into "--sysroot" flags
     if print_env {
         println!("MIRI_SYSROOT={}", sysroot.display());
     } else if !ask_user {
@@ -425,9 +424,9 @@ fn inside_cargo_rustc() {
 
     let rustc_args = std::env::args().skip(2); // skip `cargo rustc`
     let mut args: Vec<String> = rustc_args
-            .chain(Some("--sysroot".to_owned()))
-            .chain(Some(sysroot))
-            .collect();
+        .chain(Some("--sysroot".to_owned()))
+        .chain(Some(sysroot))
+        .collect();
     args.splice(0..0, miri::miri_default_args().iter().map(ToString::to_string));
 
     // See if we can find the `cargo-miri` markers. Those only get added to the binary we want to
@@ -458,7 +457,6 @@ fn inside_cargo_rustc() {
     } else {
         Command::new("rustc")
     };
-    command.env_remove("MIRI_SYSROOT"); // we already set the --sysroot flag
     command.args(&args);
     if has_arg_flag("-v") {
         eprintln!("+ {:?}", command);

src/bin/miri.rs

@@ -165,12 +165,7 @@ fn main() {
 
     // Determine sysroot.
     let sysroot_flag = "--sysroot".to_string();
-    if let Ok(sysroot) = std::env::var("MIRI_SYSROOT") {
-        // MIRI_SYSROOT takes priority. rustc will ensure for us that this errors if there
-        // already is a "--sysroot" flag (because now there would be two).
-        rustc_args.push(sysroot_flag);
-        rustc_args.push(sysroot);
-    } else if !rustc_args.contains(&sysroot_flag) {
+    if !rustc_args.contains(&sysroot_flag) {
         // We need to *always* set a --sysroot, as the "default" rustc uses is
         // somewhere in the directory miri was built in.
         // If neither MIRI_SYSROOT nor --sysroot are given, fall back to env

tests/compiletest.rs

@@ -25,7 +25,15 @@ fn rustc_lib_path() -> PathBuf {
     option_env!("RUSTC_LIB_PATH").unwrap().into()
 }
 
-fn mk_config(mode: &str) -> compiletest::common::ConfigWithTemp {
+fn run_tests(mode: &str, path: &str, target: &str, mut flags: Vec<String>) {
+    // Some flags we always want.
+    flags.push("-Dwarnings -Dunused".to_owned()); // overwrite the -Aunused in compiletest-rs
+    flags.push("--edition 2018".to_owned());
+    if let Ok(sysroot) = std::env::var("MIRI_SYSROOT") {
+        flags.push(format!("--sysroot {}", sysroot));
+    }
+
+    // The rest of the configuration.
     let mut config = compiletest::Config::default().tempdir();
     config.mode = mode.parse().expect("Invalid mode");
     config.rustc_path = miri_path();
@@ -35,7 +43,10 @@ fn mk_config(mode: &str) -> compiletest::common::ConfigWithTemp {
     }
     config.filter = env::args().nth(1);
     config.host = get_host();
-    config
+    config.src_base = PathBuf::from(path);
+    config.target = target.to_owned();
+    config.target_rustcflags = Some(flags.join(" "));
+    compiletest::run_tests(&config);
 }
 
 fn compile_fail(path: &str, target: &str, opt: bool) {
@@ -48,8 +59,6 @@ fn compile_fail(path: &str, target: &str, opt: bool) {
     ).green().bold());
 
     let mut flags = Vec::new();
-    flags.push("-Dwarnings -Dunused".to_owned()); // overwrite the -Aunused in compiletest-rs
-    flags.push("--edition 2018".to_owned());
     if opt {
         // Optimizing too aggressivley makes UB detection harder, but test at least
         // the default value.
@@ -57,11 +66,7 @@ fn compile_fail(path: &str, target: &str, opt: bool) {
         flags.push("-Zmir-opt-level=1".to_owned());
     }
 
-    let mut config = mk_config("compile-fail");
-    config.src_base = PathBuf::from(path);
-    config.target = target.to_owned();
-    config.target_rustcflags = Some(flags.join(" "));
-    compiletest::run_tests(&config);
+    run_tests("compile-fail", path, target, flags);
 }
 
 fn miri_pass(path: &str, target: &str, opt: bool) {
@@ -74,17 +79,11 @@ fn miri_pass(path: &str, target: &str, opt: bool) {
     ).green().bold());
 
     let mut flags = Vec::new();
-    flags.push("-Dwarnings -Dunused".to_owned()); // overwrite the -Aunused in compiletest-rs
-    flags.push("--edition 2018".to_owned());
     if opt {
         flags.push("-Zmir-opt-level=3".to_owned());
     }
 
-    let mut config = mk_config("ui");
-    config.src_base = PathBuf::from(path);
-    config.target = target.to_owned();
-    config.target_rustcflags = Some(flags.join(" "));
-    compiletest::run_tests(&config);
+    run_tests("ui", path, target, flags);
 }
 
 fn get_host() -> String {