fix(run): Override arg0 for cargo scripts

Fixes #12870

Author: epage

crates/cargo-util/src/process_builder.rs

@@ -20,6 +20,8 @@ use std::process::{Command, ExitStatus, Output, Stdio};
 pub struct ProcessBuilder {
     /// The program to execute.
     program: OsString,
+    /// Best-effort replacement for arg0
+    arg0: Option<OsString>,
     /// A list of arguments to pass to the program.
     args: Vec<OsString>,
     /// Any environment variables that should be set for the program.
@@ -75,6 +77,7 @@ impl ProcessBuilder {
     pub fn new<T: AsRef<OsStr>>(cmd: T) -> ProcessBuilder {
         ProcessBuilder {
             program: cmd.as_ref().to_os_string(),
+            arg0: None,
             args: Vec::new(),
             cwd: None,
             env: BTreeMap::new(),
@@ -92,6 +95,12 @@ impl ProcessBuilder {
         self
     }
 
+    /// (chainable) Overrides `arg0` for this program.
+    pub fn arg0<T: AsRef<OsStr>>(&mut self, arg: T) -> &mut ProcessBuilder {
+        self.arg0 = Some(arg.as_ref().to_os_string());
+        self
+    }
+
     /// (chainable) Adds `arg` to the args list.
     pub fn arg<T: AsRef<OsStr>>(&mut self, arg: T) -> &mut ProcessBuilder {
         self.args.push(arg.as_ref().to_os_string());
@@ -142,6 +151,11 @@ impl ProcessBuilder {
         self.wrappers.last().unwrap_or(&self.program)
     }
 
+    /// Gets the program arg0.
+    pub fn get_arg0(&self) -> Option<&OsStr> {
+        self.arg0.as_deref()
+    }
+
     /// Gets the program arguments.
     pub fn get_args(&self) -> impl Iterator<Item = &OsString> {
         self.wrappers
@@ -483,6 +497,11 @@ impl ProcessBuilder {
             cmd.args(iter);
             cmd
         };
+        #[cfg(unix)]
+        if let Some(arg0) = self.get_arg0() {
+            use std::os::unix::process::CommandExt as _;
+            command.arg0(arg0);
+        }
         if let Some(cwd) = self.get_cwd() {
             command.current_dir(cwd);
         }

src/cargo/ops/cargo_run.rs

@@ -3,6 +3,7 @@ use std::fmt::Write as _;
 use std::iter;
 use std::path::Path;
 
+use crate::core::MaybePackage;
 use crate::core::compiler::UnitOutput;
 use crate::core::{TargetKind, Workspace};
 use crate::ops;
@@ -105,6 +106,12 @@ pub fn run(
     let pkg = bins[0].0;
     let mut process = compile.target_process(exe, unit.kind, pkg, script_metas.as_ref())?;
 
+    if let MaybePackage::Package(pkg) = ws.root_maybe()
+        && pkg.manifest().is_embedded()
+    {
+        process.arg0(pkg.manifest_path());
+    }
+
     // Sets the working directory of the child process to the current working
     // directory of the parent process.
     // Overrides the default working directory of the `ProcessBuilder` returned

src/doc/src/reference/unstable.md

@@ -1496,6 +1496,10 @@ Differences between `cargo run --manifest-path <path>` and `cargo <path>`
 - `cargo <path>` runs with the config for `<path>` and not the current dir, more like `cargo install --path <path>`
 - `cargo <path>` is at a verbosity level below the normal default.  Pass `-v` to get normal output.
 
+When running a package with an embedded manifest,
+[`arg0`](https://doc.rust-lang.org/std/os/unix/process/trait.CommandExt.html#tymethod.arg0) will be the scripts path.
+To get the executable's path, see [`current_exe`](https://doc.rust-lang.org/std/env/fn.current_exe.html).
+
 ### Documentation Updates
 
 ## Profile `trim-paths` option

tests/testsuite/script/cargo.rs

@@ -62,7 +62,7 @@ fn arg0() {
         .masquerade_as_nightly_cargo(&["script"])
         .with_stdout_data(str![[r#"
 current_exe: [ROOT]/home/.cargo/target/[HASH]/debug/echo[EXE]
-arg0: [ROOT]/home/.cargo/target/[HASH]/debug/echo[EXE]
+arg0: [ROOT]/foo/echo.rs
 args: []
 
 "#]])