From ef50938afe4cf6f96db5cb67521e637b48819f2b Mon Sep 17 00:00:00 2001 From: Ed Page Date: Tue, 17 Dec 2024 15:42:39 -0600 Subject: [PATCH] Specify the behavior of `file!` This takes the current behavior of `file!` and documents it so it is safe to make assumptions about. For example, Cargo could provide a `CARGO_RUSTC_CURRENT_DIR` as a base path for `file!`. Example use cases - Being able to look up test assets relative to the current file ([example](https://github.com/rust-lang/cargo/blob/b9026bf654d7fac283465e58b8b76742244ef07d/tests/testsuite/cargo_add/add_basic/mod.rs#L34)) - Inline snapshotting libraries being able to update Rust source code ([example](https://github.com/rust-lang/cargo/blob/b9026bf654d7fac283465e58b8b76742244ef07d/tests/testsuite/alt_registry.rs#L36-L45)) See rust-lang/cargo#3946 for more context. T-libs-api discussed two solutions in rust-lang/libs-team#478 - `file_absolute!`: - Has less meaning in other build tools like buck2 - Bakes in the assumption that a full path is available (e.g. with trim-paths) - Specifying `file!`s behavior (this PR): - Leaves it to the user to deal with trim-paths - Even though `file!` is currently unspecified, changing it would likely have too large of an impact on the ecosystem at this time. A future possibility is that rustc could have a flag that controls modifies the base path used for `file!`. That seems purely additive with specifying the behavior and we do not want to block on it. It would also likely be too disruptive for Cargo users (as mentioned). However, we tried to keep this in mind when specifying the behavior. --- library/core/src/macros/mod.rs | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/library/core/src/macros/mod.rs b/library/core/src/macros/mod.rs index 771c2d31b60e..27b26ac65104 100644 --- a/library/core/src/macros/mod.rs +++ b/library/core/src/macros/mod.rs @@ -1295,6 +1295,14 @@ pub(crate) mod builtin { /// first macro invocation leading up to the invocation of the `file!` /// macro. /// + /// The file name is derived from the root module's source path passed to the Rust compiler + /// (e.g. `./src/lib.rs`) + /// end the sequence the compiler takes to get from root module to the module containing `file!`. + /// (e.g. inside `mod foo;` in `./src/lib.rs`, `file!` would be `./src/foo.rs`), + /// modified by any flags passed to the Rust compiler (e.g. `--remap-path-prefix`). + /// If the source path is relative, + /// the initial base directory will be the working directory of the Rust compiler. + /// /// # Examples /// /// ```