Auto merge of #2472 - RalfJung:backtrace, r=RalfJung

fix RUSTC_BACKTRACE always being set

I kept wondering why Miri programs, whenever isolation is disabled, behave as if RUSTC_BACKTRACE was set. Finally I realized it's because some early rustc setup code sets that env var, and that is then propagated to the interpreted program.

So fix that by taking a copy of the environment before any rustc setup, and use that copy as the basis for what is provided to the interpreted program.

src/bin/miri.rs

@@ -1,5 +1,9 @@
 #![feature(rustc_private, stmt_expr_attributes)]
-#![allow(clippy::manual_range_contains, clippy::useless_format)]
+#![allow(
+    clippy::manual_range_contains,
+    clippy::useless_format,
+    clippy::field_reassign_with_default
+)]
 
 extern crate rustc_data_structures;
 extern crate rustc_driver;
@@ -306,6 +310,11 @@ fn parse_comma_list<T: FromStr>(input: &str) -> Result<Vec<T>, T::Err> {
 }
 
 fn main() {
+    // Snapshot a copy of the environment before `rustc` starts messing with it.
+    // (`install_ice_hook` might change `RUST_BACKTRACE`.)
+    let env_snapshot = env::vars_os().collect::<Vec<_>>();
+
+    // Earliest rustc setup.
     rustc_driver::install_ice_hook();
 
     // If the environment asks us to actually be rustc, then do that.
@@ -333,6 +342,8 @@ fn main() {
 
     // Parse our arguments and split them across `rustc` and `miri`.
     let mut miri_config = miri::MiriConfig::default();
+    miri_config.env = env_snapshot;
+
     let mut rustc_args = vec![];
     let mut after_dashdash = false;
 

src/eval.rs

@@ -1,7 +1,7 @@
 //! Main evaluator loop and setting up the initial stack frame.
 
 use std::collections::HashSet;
-use std::ffi::OsStr;
+use std::ffi::{OsStr, OsString};
 use std::iter;
 use std::panic::{self, AssertUnwindSafe};
 use std::thread;
@@ -72,6 +72,9 @@ pub enum BacktraceStyle {
 /// Configuration needed to spawn a Miri instance.
 #[derive(Clone)]
 pub struct MiriConfig {
+    /// The host environment snapshot to use as basis for what is provided to the interpreted program.
+    /// (This is still subject to isolation as well as `excluded_env_vars` and `forwarded_env_vars`.)
+    pub env: Vec<(OsString, OsString)>,
     /// Determine if validity checking is enabled.
     pub validate: bool,
     /// Determines if Stacked Borrows is enabled.
@@ -130,6 +133,7 @@ pub struct MiriConfig {
 impl Default for MiriConfig {
     fn default() -> MiriConfig {
         MiriConfig {
+            env: vec![],
             validate: true,
             stacked_borrows: true,
             check_alignment: AlignmentCheck::Int,

src/shims/env.rs

@@ -49,11 +49,11 @@ pub(crate) fn init<'mir>(
 
         // Skip the loop entirely if we don't want to forward anything.
         if ecx.machine.communicate() || !config.forwarded_env_vars.is_empty() {
-            for (name, value) in env::vars_os() {
+            for (name, value) in &config.env {
                 // Always forward what is in `forwarded_env_vars`; that list can take precedence over excluded_env_vars.
-                let forward = config.forwarded_env_vars.iter().any(|v| **v == name)
+                let forward = config.forwarded_env_vars.iter().any(|v| **v == *name)
                     || (ecx.machine.communicate()
-                        && !excluded_env_vars.iter().any(|v| **v == name));
+                        && !excluded_env_vars.iter().any(|v| **v == *name));
                 if forward {
                     let var_ptr = match target_os {
                         target if target_os_is_unix(target) =>
@@ -65,7 +65,7 @@ pub(crate) fn init<'mir>(
                                 unsupported
                             ),
                     };
-                    ecx.machine.env_vars.map.insert(name, var_ptr);
+                    ecx.machine.env_vars.map.insert(name.clone(), var_ptr);
                 }
             }
         }

tests/pass/backtrace/backtrace-global-alloc.rs

@@ -1,4 +1,5 @@
 //@compile-flags: -Zmiri-disable-isolation
+//@rustc-env: RUST_BACKTRACE=1
 
 #![feature(backtrace)]
 

tests/pass/backtrace/backtrace-std.rs

@@ -1,4 +1,5 @@
 //@compile-flags: -Zmiri-disable-isolation
+//@rustc-env: RUST_BACKTRACE=1
 
 #![feature(backtrace)]