feat: Stablize CARGO_RUSTC_CURRENT_DIR

This provides what cargo sets as the `current_dir` for the `rustc`
process.
While `std::file!` is unspecified in what it is relative to,
it is relatively safe, it is generally relative to `rustc`s
`current_dir`.

This can be useful for snapshot testing.
For example, `snapbox` has been using this macro on nightly since
assert-rs/snapbox#247, falling back to finding a parent of
`CARGO_MANIFEST_DIR`, if present.
This has been in use in Cargo since rust-lang#13441.

This was added in rust-lang#12996.
Relevant points discussed in that issue:
- This diverged from the original proposal from the Cargo team of having
  a `CARGO_WORKSPACE_DIR` that is the "workspace" of the package being
  built (ie registry packages would map to `CARGO_MANIFEST_DIR`).
  In looking at the `std::file!` use case, `CARGO_MANIFEST_DIR`, no
  matter how we defined it, would only sort of work because no sane
  definition of that maps to `rustc`'s `current_dir`.a
  This instead focuses on the mechanism currently being used.
- Using "current dir" in the name is meant to be consistent with
  `std::env::current_dir`.
- I can go either way on `CARGO_RUSTC` vs `RUSTC`.  Existing related
  variables:
  - `RUSTC`
  - `RUSTC_WRAPPER`
  - `RUSTC_WORKSPACE_WRAPPER`
  - `RUSTFLAGS` (no `C`)
  - `CARGO_CACHE_RUSTC_INFO`

Note that rust-lang#3946 was overly broad and covered many use cases.
One of those was for packages to look up information on their
dependents.
Issue rust-lang#13484 is being left open to track that.

Fixes rust-lang#3946

src/cargo/core/compiler/mod.rs

@@ -696,16 +696,14 @@ fn prepare_rustc(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> CargoResult
         let tmp = build_runner.files().layout(unit.kind).prepare_tmp()?;
         base.env("CARGO_TARGET_TMPDIR", tmp.display().to_string());
     }
-    if build_runner.bcx.gctx.nightly_features_allowed {
-        // This must come after `build_base_args` (which calls `add_path_args`) so that the `cwd`
-        // is set correctly.
-        base.env(
-            "CARGO_RUSTC_CURRENT_DIR",
-            base.get_cwd()
-                .map(|c| c.display().to_string())
-                .unwrap_or(String::new()),
-        );
-    }
+    // This must come after `build_base_args` (which calls `add_path_args`) so that the `cwd`
+    // is set correctly.
+    base.env(
+        "CARGO_RUSTC_CURRENT_DIR",
+        base.get_cwd()
+            .map(|c| c.display().to_string())
+            .unwrap_or(String::new()),
+    );
 
     Ok(base)
 }

src/doc/src/reference/environment-variables.md

@@ -265,7 +265,7 @@ corresponding environment variable is set to the empty string, `""`.
   where integration tests or benchmarks are free to put any data needed by
   the tests/benches. Cargo initially creates this directory but doesn't
   manage its content in any way, this is the responsibility of the test code.
-* `CARGO_RUSTC_CURRENT_DIR` --- This is a path that `rustc` is invoked from **(nightly only)**.
+* `CARGO_RUSTC_CURRENT_DIR` --- This is a path that `rustc` is invoked from
 
 [Cargo target]: cargo-targets.md
 [binaries]: cargo-targets.md#binaries

tests/testsuite/build.rs

@@ -1705,30 +1705,22 @@ fn crate_env_vars() {
     };
 
     println!("build");
-    p.cargo("build -v")
-        .masquerade_as_nightly_cargo(&["CARGO_RUSTC_CURRENT_DIR"])
-        .run();
+    p.cargo("build -v").run();
 
     println!("bin");
     p.process(&p.bin("foo-bar"))
         .with_stdout("0-5-1 @ alpha.1 in [CWD]")
         .run();
 
     println!("example");
-    p.cargo("run --example ex-env-vars -v")
-        .masquerade_as_nightly_cargo(&["CARGO_RUSTC_CURRENT_DIR"])
-        .run();
+    p.cargo("run --example ex-env-vars -v").run();
 
     println!("test");
-    p.cargo("test -v")
-        .masquerade_as_nightly_cargo(&["CARGO_RUSTC_CURRENT_DIR"])
-        .run();
+    p.cargo("test -v").run();
 
     if is_nightly() {
         println!("bench");
-        p.cargo("bench -v")
-            .masquerade_as_nightly_cargo(&["CARGO_RUSTC_CURRENT_DIR"])
-            .run();
+        p.cargo("bench -v").run();
     }
 }
 
@@ -1814,11 +1806,9 @@ fn cargo_rustc_current_dir_foreign_workspace_dep() {
 
     // Verify it works from a different workspace
     foo.cargo("test -p baz")
-        .masquerade_as_nightly_cargo(&["CARGO_RUSTC_CURRENT_DIR"])
         .with_stdout_contains("running 1 test\ntest baz_env ... ok")
         .run();
     foo.cargo("test -p baz_member")
-        .masquerade_as_nightly_cargo(&["CARGO_RUSTC_CURRENT_DIR"])
         .with_stdout_contains("running 1 test\ntest baz_member_env ... ok")
         .run();
 }
@@ -1863,33 +1853,10 @@ fn cargo_rustc_current_dir_non_local_dep() {
         .build();
 
     p.cargo("test -p bar")
-        .masquerade_as_nightly_cargo(&["CARGO_RUSTC_CURRENT_DIR"])
         .with_stdout_contains("running 1 test\ntest bar_env ... ok")
         .run();
 }
 
-#[cargo_test]
-fn cargo_rustc_current_dir_is_not_stable() {
-    if is_nightly() {
-        return;
-    }
-    let p = project()
-        .file(
-            "tests/env.rs",
-            r#"
-                use std::path::Path;
-
-                #[test]
-                fn env() {
-                    assert_eq!(option_env!("CARGO_RUSTC_CURRENT_DIR"), None);
-                }
-            "#,
-        )
-        .build();
-
-    p.cargo("test").run();
-}
-
 #[cargo_test]
 fn crate_authors_env_vars() {
     let p = project()