nix: trim paths in nix build by justus-camp-microsoft · Pull Request #3082 · microsoft/openvmm

justus-camp-microsoft · 2026-03-20T19:12:24Z

Our binary currently has machine-dependent paths in its output - to fix this rustc has a flag --remap-path-prefix, but unfortunately it requires a key-value mapping so we have to figure out the path at runtime to be replaced. A future rust version will have better support for trimming paths that should make this unnecessary (RFC here)

The RUSTFLAGS in our config.toml are duplicated here instead of trying to do some sort of merge/reading of the .toml file at runtime.

jstarks · 2026-03-20T19:15:15Z

I thought if we set RUSTFLAGS then basically it overwrites basically every other way of settings rust flags (config.toml, etc.). Is that right? How are we dealing with that?

Copilot

Pull request overview

Adds Nix-driven, per-target RUSTFLAGS plumbing to Flowey builds so binaries don’t embed machine-specific absolute paths (via --remap-path-prefix), while ensuring env vars survive nix-shell --pure execution.

Changes:

Define per-target RUSTFLAGS_* variables in shell.nix (mirroring .cargo/config.toml), including Cargo registry/git remaps.
Teach Flowey’s cross-build init/build steps to inject the right Nix-provided RUSTFLAGS and append a repo-path remap at build time.
Update the Nix shell command wrapper to pass env vars through nix-shell --pure using --keep, and update CI pipeline/workflows to run the new injection steps.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
`shell.nix`	Adds composed per-target `RUSTFLAGS_*` env vars (incl. remap prefixes) for Nix shells.
`flowey/flowey_lib_hvlite/src/init_cross_build.rs`	Reads per-target `RUSTFLAGS_*` from Nix shell env and injects as `RUSTFLAGS` for cargo invocations.
`flowey/flowey_lib_hvlite/src/run_cargo_build.rs`	Appends a repo checkout `--remap-path-prefix=<repo>=` to `RUSTFLAGS` at build time.
`flowey/flowey_core/src/shell.rs`	Updates `nix-shell --pure` wrapper to preserve env via `--keep` and sets env on the outer process.
`ci-flowey/openvmm-pr.yaml`	Reorders/expands Flowey steps to inject cross env before relevant build steps.
`.github/workflows/openvmm-pr-release.yaml`	Mirrors the pipeline step changes for GitHub PR release workflow runs.
`.github/workflows/openvmm-ci.yaml`	Mirrors the pipeline step changes for GitHub CI workflow runs.

Copilot · 2026-03-20T19:17:01Z

flowey/flowey_core/src/shell.rs

+                        if let Some(entry) = final_env.iter_mut().find(|(ek, _)| *ek == k) {
+                            entry.1 = v;
+                        } else {
+                            final_env.push((k, v));
+                        }
+                    }
+                    EnvChange::Remove(k) => {
+                        final_env.retain(|(ek, _)| *ek != k);


final_env.iter_mut().find(|(ek, _)| *ek == k) dereferences and moves an OsString out of a borrowed tuple (*ek), which won’t compile. Compare by reference instead (e.g., ek == &k / ek.as_os_str() == k.as_os_str()).

Suggested change

if let Some(entry) = final_env.iter_mut().find(|(ek, _)| *ek == k) {

entry.1 = v;

} else {

final_env.push((k, v));

}

}

EnvChange::Remove(k) => {

final_env.retain(|(ek, _)| *ek != k);

if let Some(entry) = final_env.iter_mut().find(|(ek, _)| ek == &k) {

entry.1 = v;

} else {

final_env.push((k, v));

}

}

EnvChange::Remove(k) => {

final_env.retain(|(ek, _)| ek != &k);

Copilot · 2026-03-20T19:17:01Z

flowey/flowey_lib_hvlite/src/init_cross_build.rs

+                        let rustflags =
+                            flowey::shell_cmd!(rt, "printenv {rustflags_env}").read()?;
+                        injected_env.insert("RUSTFLAGS".to_string(), rustflags);


nix_rustflags_env is enabled for any host platform detected as FlowPlatformLinuxDistro::Nix, but FlowPlatformLinuxDistro::Nix is also selected when $PATH contains /nix/store (e.g. NixOS outside a dev shell). In that case printenv RUSTFLAGS_X64 will fail if the shell.nix vars aren’t present, breaking cross env injection. Consider gating on IN_NIX_SHELL (or otherwise detecting the nix-shell wrapper), or making the printenv read best-effort (skip override if the env var isn’t set).

Suggested change

let rustflags =

flowey::shell_cmd!(rt, "printenv {rustflags_env}").read()?;

injected_env.insert("RUSTFLAGS".to_string(), rustflags);

match flowey::shell_cmd!(rt, "printenv {rustflags_env}").read() {

Ok(rustflags) => {

injected_env.insert("RUSTFLAGS".to_string(), rustflags);

}

Err(_) => {

// If the Nix-specific RUSTFLAGS env var is not set, skip

// overriding RUSTFLAGS instead of failing the step.

}

}

Copilot · 2026-03-20T19:17:01Z

flowey/flowey_lib_hvlite/src/run_cargo_build.rs

+                            rustflags.push_str(&format!(
+                                " --remap-path-prefix={}=",
+                                repo_path.display()


The appended --remap-path-prefix={repo_path}= is inserted into RUSTFLAGS as a whitespace-delimited string. If the checkout path contains spaces, this will be split into multiple flags and break the build / remap behavior. Consider escaping spaces (or quoting the argument in a way Cargo’s RUSTFLAGS parser accepts) before appending.

Suggested change

rustflags.push_str(&format!(

" --remap-path-prefix={}=",

repo_path.display()

// Quote the path so that spaces do not split this into multiple flags

let mut path_str = repo_path.display().to_string();

path_str = path_str.replace('"', "\\\"");

rustflags.push_str(&format!(

" --remap-path-prefix=\"{}\"=",

path_str

Copilot · 2026-03-20T19:17:02Z

flowey/flowey_core/src/shell.rs

+                        if let Some(entry) = final_env.iter_mut().find(|(ek, _)| *ek == k) {
+                            entry.1 = v;
+                        } else {
+                            final_env.push((k, v));
+                        }
+                    }
+                    EnvChange::Remove(k) => {
+                        final_env.retain(|(ek, _)| *ek != k);


final_env.retain(|(ek, _)| *ek != k) has the same issue as above: *ek attempts to move an OsString out of a borrow and won’t compile. Use a reference comparison (e.g., ek != &k) instead.

Suggested change

if let Some(entry) = final_env.iter_mut().find(|(ek, _)| *ek == k) {

entry.1 = v;

} else {

final_env.push((k, v));

}

}

EnvChange::Remove(k) => {

final_env.retain(|(ek, _)| *ek != k);

if let Some(entry) = final_env.iter_mut().find(|(ek, _)| ek == &k) {

entry.1 = v;

} else {

final_env.push((k, v));

}

}

EnvChange::Remove(k) => {

final_env.retain(|(ek, _)| ek != &k);

smalis-msft · 2026-03-21T17:55:09Z

Yeah we definitely don't want to just lose rustflags from other sources, that'll break all sorts of local one-off flows people might need. Is there any sort of current timeline on the rustc PR for stabilization? Or is it at least implemented unstably (behind a -Z flag) that we could use with RUSTC_BOOTSTRAP?

justus-camp-microsoft · 2026-03-23T22:04:02Z

Yeah we definitely don't want to just lose rustflags from other sources, that'll break all sorts of local one-off flows people might need. Is there any sort of current timeline on the rustc PR for stabilization? Or is it at least implemented unstably (behind a -Z flag) that we could use with RUSTC_BOOTSTRAP?

I had no idea RUSTC_BOOTSTRAP existed - that certainly makes this change way easier. I updated this PR to set the required environment variables that trigger path stripping for the Nix build. Thanks for the direction 😄

smalis-msft · 2026-03-24T15:49:56Z

Yeah, using RUSTC_BOOTSTRAP isn't "good", but we do it elsewhere already, and it's easier than dealing with the potential flakiness of nightly.

Copilot

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

smalis-msft · 2026-03-24T15:58:39Z

shell.nix


  RUST_BACKTRACE = 1;
  SOURCE_DATE_EPOCH = 12345;
+  # Need unstable features on the stable toolchain


Let's list exactly which unstable features, so that hopefully someday we can remove bootstrap once they're stable

justus-camp-microsoft requested review from a team as code owners March 20, 2026 19:12

Copilot AI review requested due to automatic review settings March 20, 2026 19:12

Copilot started reviewing on behalf of justus-camp-microsoft March 20, 2026 19:13 View session

Copilot AI reviewed Mar 20, 2026

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justus-camp-microsoft force-pushed the nix_rustflags branch from 578e27a to 64e8115 Compare March 23, 2026 22:02

justus-camp-microsoft changed the title ~~flowey: support for nix-defined RUSTFLAGS~~ nix: trim paths in nix build Mar 23, 2026

Copilot AI review requested due to automatic review settings March 24, 2026 20:03

justus-camp-microsoft force-pushed the nix_rustflags branch from 64e8115 to 04c9c15 Compare March 24, 2026 20:03

Copilot started reviewing on behalf of justus-camp-microsoft March 24, 2026 20:04 View session

Copilot AI reviewed Mar 24, 2026

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smalis-msft reviewed Mar 25, 2026

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nix: use unstable trim paths option for nix build

f5af141

justus-camp-microsoft force-pushed the nix_rustflags branch from 04c9c15 to f5af141 Compare March 25, 2026 21:33

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nix: trim paths in nix build#3082

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-                        let rustflags =
-                            flowey::shell_cmd!(rt, "printenv {rustflags_env}").read()?;
-                        injected_env.insert("RUSTFLAGS".to_string(), rustflags);
+                        match flowey::shell_cmd!(rt, "printenv {rustflags_env}").read() {
+                            Ok(rustflags) => {
+                                injected_env.insert("RUSTFLAGS".to_string(), rustflags);
+                            }
+                            Err(_) => {
+                                // If the Nix-specific RUSTFLAGS env var is not set, skip
+                                // overriding RUSTFLAGS instead of failing the step.
+                            }
+                        }

-                            rustflags.push_str(&format!(
-                                " --remap-path-prefix={}=",
-                                repo_path.display()
+                            // Quote the path so that spaces do not split this into multiple flags
+                            let mut path_str = repo_path.display().to_string();
+                            path_str = path_str.replace('"', "\\\"");
+                            rustflags.push_str(&format!(
+                                " --remap-path-prefix=\"{}\"=",
+                                path_str

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