Fix rust-lang#15099

Overwrite `$CARGO` when the current exe is detected to be a cargo
binary.

See: rust-lang#15099 (comment)

Author: smoelius

src/cargo/util/context/mod.rs

@@ -493,9 +493,25 @@ impl GlobalContext {
                     paths::resolve_executable(&argv0)
                 }
 
+                // Determines whether `path` is a cargo binary.
+                // See: https://github.com/rust-lang/cargo/issues/15099#issuecomment-2666737150
+                fn is_cargo(path: &Path) -> bool {
+                    path.file_stem() == Some(OsStr::new("cargo"))
+                }
+
+                let from_current_exe = from_current_exe();
+                if from_current_exe.as_deref().is_ok_and(is_cargo) {
+                    return from_current_exe;
+                }
+
+                let from_argv = from_argv();
+                if from_argv.as_deref().is_ok_and(is_cargo) {
+                    return from_argv;
+                }
+
                 let exe = from_env()
-                    .or_else(|_| from_current_exe())
-                    .or_else(|_| from_argv())
+                    .or(from_current_exe)
+                    .or(from_argv)
                     .context("couldn't get the path to cargo executable")?;
                 Ok(exe)
             })

tests/testsuite/cargo_command.rs

@@ -391,10 +391,13 @@ fn cargo_subcommand_env() {
         .canonicalize()
         .unwrap();
     let envtest_bin = envtest_bin.to_str().unwrap();
+    // Previously, `$CARGO` would be left at `envtest_bin`. However, with the
+    // fix for #15099, `$CARGO` is now overwritten with the path to the current
+    // exe when it is detected to be a cargo binary.
     cargo_process("envtest")
         .env("PATH", &path)
         .env(cargo::CARGO_ENV, &envtest_bin)
-        .with_stdout_data(format!("{}\n", envtest_bin).raw().raw())
+        .with_stdout_data(format!("{}\n", cargo.display()).raw())
         .run();
 }
 

tests/testsuite/freshness.rs

@@ -1,5 +1,6 @@
 //! Tests for fingerprinting (rebuild detection).
 
+use std::env::consts::EXE_SUFFIX;
 use std::fs::{self, OpenOptions};
 use std::io;
 use std::io::prelude::*;
@@ -3182,3 +3183,75 @@ fn use_mtime_cache_in_cargo_home() {
 "#]])
         .run();
 }
+
+#[cargo_test]
+fn overwrite_cargo_environment_variable() {
+    // If passed arguments `arg1 arg2 ...`, this program runs them as a command.
+    // If passed no arguments, this program simply prints `$CARGO`.
+    let p = project()
+        .file("Cargo.toml", &basic_manifest("foo", "1.0.0"))
+        .file(
+            "src/main.rs",
+            r#"
+                fn main() {
+                    let mut args = std::env::args().skip(1);
+                    if let Some(arg1) = args.next() {
+                        let status = std::process::Command::new(arg1)
+                            .args(args)
+                            .status()
+                            .unwrap();
+                        assert!(status.success());
+                    } else {
+                        println!("{}", std::env::var("CARGO").unwrap());
+                    }
+                }
+            "#,
+        )
+        .build();
+
+    // Create two other cargo binaries in the project root, one with the wrong
+    // name and one with the right name.
+    let cargo_exe = cargo_test_support::cargo_exe();
+    let wrong_name_path = p.root().join(format!("wrong_name{EXE_SUFFIX}"));
+    let other_cargo_path = p.root().join(cargo_exe.file_name().unwrap());
+    std::fs::hard_link(&cargo_exe, &wrong_name_path).unwrap();
+    std::fs::hard_link(&cargo_exe, &other_cargo_path).unwrap();
+
+    // The output of each of the following commands should be `path-to-cargo`:
+    // ```
+    // cargo run
+    // cargo run -- cargo run
+    // cargo run -- wrong_name run
+    // ```
+
+    let cargo = cargo_exe.display().to_string();
+    let wrong_name = wrong_name_path.display().to_string();
+    let stdout_cargo = format!("{}[EXE]\n", cargo_exe.with_extension("").display());
+
+    for cmd in [
+        "run",
+        &format!("run -- {cargo} run"),
+        &format!("run -- {wrong_name} run"),
+    ] {
+        p.cargo(cmd).with_stdout_data(&stdout_cargo).run();
+    }
+
+    // The output of the following command should be `path-to-other-cargo`:
+    // ```
+    // cargo run -- other_cargo run
+    // ```
+
+    let other_cargo = other_cargo_path.display().to_string();
+    let stdout_other_cargo = format!(
+        "{}[EXE]\n",
+        other_cargo_path
+            .with_extension("")
+            .to_str()
+            .unwrap()
+            .replace(p.root().parent().unwrap().to_str().unwrap(), "[ROOT]")
+    );
+
+    p.cargo(&format!("run -- {other_cargo} run"))
+        .with_stdout_data(stdout_other_cargo)
+        .run();
+}

tests/testsuite/test.rs

@@ -3909,22 +3909,20 @@ test env_test ... ok
         .run();
 
     // Check that `cargo test` propagates the environment's $CARGO
-    let rustc = cargo_util::paths::resolve_executable("rustc".as_ref())
-        .unwrap()
-        .canonicalize()
-        .unwrap();
-    let stderr_rustc = format!(
+    let cargo_exe = cargo_test_support::cargo_exe();
+    let other_cargo_path = p.root().join(cargo_exe.file_name().unwrap());
+    std::fs::hard_link(&cargo_exe, &other_cargo_path).unwrap();
+    let stderr_other_cargo = format!(
         "{}[EXE]",
-        rustc
+        other_cargo_path
             .with_extension("")
             .to_str()
             .unwrap()
-            .replace(rustc_host, "[HOST_TARGET]")
+            .replace(p.root().parent().unwrap().to_str().unwrap(), "[ROOT]")
     );
-    p.cargo("test --lib -- --nocapture")
-        // we use rustc since $CARGO is only used if it points to a path that exists
-        .env(cargo::CARGO_ENV, rustc)
-        .with_stderr_contains(stderr_rustc)
+    p.process(other_cargo_path)
+        .args(&["test", "--lib", "--", "--nocapture"])
+        .with_stderr_contains(stderr_other_cargo)
         .with_stdout_data(str![[r#"
 ...
 test env_test ... ok