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Introduce -Zembed-metadata to allow omitting full metadata from rlibs and dylibs - #137535
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(The test failure is the one situation not yet implemented, which is described in the PR description - |
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That's by design,
In #120855 the heuristics were a temporary measure to enable split metadata in bootstrap, while cargo still doesn't support it. |
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Interesting. So if I understood it correctly, you would like to remove all heuristics from rustc (so no lookup of On the other hand, it would essentially double the length of the rustc command line, because for every library that you depend on, Cargo would have to pass both |
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Yes.
My impression was that it is already the case with pipelined builds, but apparently it's indeed only |
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Some changes occurred in compiler/rustc_codegen_ssa This PR modifies cc @jieyouxu |
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Ok, rebased and removed the heuristics. I added an error message when the full metadata is not found, but I'm not sure if we should emit the error eagerly (left a comment in code). Could you please take a look at fe29fec? Thank you! |
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bjorn3
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rust-lang/compiler-team#851 was accepted, so I think that we can go forward with this. @petrochenkov What do you think about renaming the flag to |
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Seems ok to me. |
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-Zsplit-metadata to split metadata out of rlibs/dylibs-Zembed-metadata to allow omitting full metadata from rlibs and dylibs
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Ok, changed it to @rustbot ready |
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Created a tracking issue and fixed tests. |
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r=me with the metadata version updated. |
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rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead. cc @rust-lang/rust-analyzer |
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@bors r=petrochenkov rollup=never Marked as rollup=never, because this modifies metadata version. |
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☀️ Test successful - checks-actions |
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What is this?This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.Comparing ed20157 (parent) -> 8c35f4a (this PR) Test differencesShow 43 test diffsStage 1
Stage 2
Additionally, 2 doctest diffs were found. These are ignored, as they are noisy. Job group index
Job duration changes
How to interpret the job duration changes?Job durations can vary a lot, based on the actual runner instance |
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Finished benchmarking commit (8c35f4a): comparison URL. Overall result: no relevant changes - no action needed@rustbot label: -perf-regression Instruction countThis benchmark run did not return any relevant results for this metric. Max RSS (memory usage)Results (secondary -1.7%)This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
CyclesResults (secondary 3.9%)This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
Binary sizeResults (secondary 0.0%)This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
Bootstrap: 773.019s -> 774.818s (0.23%) |
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### What does this PR try to resolve? This PR adds Cargo integration for the new unstable `-Zembed-metadata` rustc flag, which was implemented in rust-lang/rust#137535 ([tracking issue](rust-lang/rust#139165)). The new behavior has to be enabled explicitly using a new unstable CLI flag `-Zno-embed-metadata`. The `-Zembed-metadata=no` rustc flag can reduce disk usage of compiled artifacts, and also the size of Rust dynamic library artifacts shipped to users. However, it is not enough to just pass this flag through `RUSTFLAGS`; it needs to be integrated within Cargo, because it interacts with how the `--emit` flag is passed to rustc, and also how `--extern` args are passed to the final linked artifact build by Cargo. Furthermore, using the flag for all crates in a crate graph compiled by Cargo would be suboptimal (this will all be described below). When you pass `-Zembed-metadata=no` to rustc, it will not store Rust metadata into the compiled artifact. This is important when compiling libs/rlibs/dylibs, since it reduces their size on disk. However, this also means that everytime we use this flag, we have to make sure that we also: - Include `metadata` in the `--emit` flag to generate a `.rmeta` file, otherwise no metadata would be generated whatsoever, which would mean that the artifact wouldn't be usable as a dependency. - Pass also `--extern <dep>=<path>.rmeta` when compiling the final linkable artifact. Before, Cargo would only pass `--extern <dep>=<path>.[rlib|so|dll]`. Since with `-Zembed-metadata=no`, the metadata is only in the `.rmeta` file and not in the rlib/dylib, this is needed to help rustc find out where the metadata lies. - Note: this essentially doubles the cmdline length when compiling the final linked artifact. Not sure if that is a concern. The two points above is what this PR implements, and why this rustc flag needs Cargo integration. The `-Zembed-metadata` flag is only passed to libs, rlibs and dylibs. It does not seem to make sense for other crate types. The one situation where it might make sense are proc macros, but according to @bjorn3 (who initially came up with the idea for `-Zembed-metadata`, it isn't really worth it). Here is a table that summarizes the changes in passed flags and generated files on disk for rlibs and dylibs: | **Crate type** | **Flags** | **Generated files** | **Disk usage** | |--|--|--|--| | Rlib/Lib (before) | `--emit=dep-info,metadata,link` | `.rlib` (with metadata), `.rmeta` (for pipelining) | - | | Rlib/Lib (after) | `--emit=dep-info,metadata,link -Zembed-metadata=no` | `.rlib` (without metadata), `.rmeta` (for metadata/pipelining) | Reduced (metadata no longer duplicated) | | Dylib (before) | `--emit=dep-info,link` | `[.so\|.dll]` (with metadata) | - | | Dylib (after) | `--emit=dep-info,metadata,link -Zembed-metadata=no` | `[.so\|.dll]` (without metadata), `.rmeta` | Unchanged, but split between two files | Behavior for other target kinds/crate types should be unchanged. From the table above, we can see two benefits of using `-Zembed-metadata=no`: - For rlibs/dylibs, we no longer store their metadata twice in the target directory, thus reducing target directory size. - For dylibs, we store esssentially the same amount of data on disk, but the benefit is that the metadata is now in a separate .rmeta file. This means that you can ship the dylib (`.so`/`.dll`) to users without also shipping the metadata. This would slightly reduce e.g. the [size](rust-lang/rust#120855 (comment)) of the shipped rustc toolchains (note that the size reduction here is after the toolchain has been already heavily compressed). Note that if this behavior ever becomes the default, it should be possible to simplify the code quite a bit, and essentially merge the `requires_upstream_objects` and `benefits_from_split_metadata` functions. I did a very simple initial benchmark to evaluate the space savings on cargo itself and [hyperqueue](https://github.com/It4innovations/hyperqueue) (a mid-size crate from my work) using `cargo build` and `cargo build --release` with and without `-Zembed-metadata=no`:  For debug/incremental builds, the effect is smaller, as the artifact disk usage is dwarfed by incremental artifacts and debuginfo. But for (non-incremental) release builds, the disk savings (and also performed I/O operations) are significantly reduced. ### How should we test and review this PR? I wrote two basic tests. The second one tests a situation where a crate depends on a dylib dependency, which is quite rare, but the behavior of this has actually changed in this PR (see comparison table above). Testing this on various real-world projects (or even trying to enable it by default across the whole Cargo suite?) might be beneficial. ## Unresolved questions ### Is this a breaking change? With this new behavior, dylibs and rlibs will no longer contain metadata. If they are compiled with Cargo, that shouldn't matter, but other build systems might have to adapt. ### Should this become the default? I think that in terms of disk size usage and performed I/O operations, it is a pure win. It should either generate less disk data (for rlibs) or the ~same amount of data for dylibs (the data will be a bit larger, because the dylib will still contain a metadata stub header, but that's like 50 bytes and doesn't scale with the size of the dylib, so it's negligible). So I think that eventually, we should just do this by default in Cargo, unless some concerns are found. I suppose that before stabilizing we should also benchmark the effect on compilation performance.
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…=petrochenkov Stabilize `-Zembed-metadata` Tracking issue: rust-lang#139165 This PR proposes to stabilize the `-Zembed-metadata` compiler flag, so that Cargo can start using `-Cembed-metadata=no` by default on the stable channel. The flag can be used to avoid duplicating the metadata of Rust crates within `.rlib` and `.rmeta` files. This can reduce the size of the `target` directory anywhere from 5% to 35% on real-world projects, depending on the chosen compilation profile ([1][kobzol-blog], [2][cargo-blog]). Additionally, `-Cembed-metadata=no` also removes the metadata out of Rust `dylib`s, and thus reduces their size. The plan is to stabilize this compiler flag so that Cargo can start using it on the stable channel. The stabilization plan thus has two parts: - Stabilize the `-Cembed-metadata` flag in `rustc` - Change Cargo to use `-Cembed-metadata=no` by default The Cargo team has prepared their side of the [stabilization report](rust-lang/cargo#17533). You can examine it to see the whole picture. ## What is being stabilized The `-Zembed-metadata` compiler flag. Note that `rustc` will still default to `-Cembed-metadata=yes`, as that is its original behavior since [Rust 1.16.0](https://blog.rust-lang.org/2017/03/16/Rust-1.16/#other-improvements). But it will now be possible to pass `-Cembed-metadata=no` on stable. ## Motivation When compiling Rust crates as libraries, Cargo uses pipelined compilation, which tells `rustc` (using `--emit=metadata`) to output an `.rmeta` file as soon as possible, so that Cargo can start follow-up `rustc` instances sooner and speed up the compilation process. This file contains the Rust-specific metadata of the compiled crate. However, after the compilation of the Rust crate finishes, the compiler will also write an `.rlib` file to disk, which contains both the compiled object code, but also the same metadata that is already contained in the `.rmeta` file. This means that after the compilation finishes, the metadata of each (non-leaf) compiled crate will be duplicated on disk. The metadata can be quite large, and it is not uncommon that it takes hundreds of megabytes for real-world projects. This duplication thus contributes to increasing the size of the `target` directory, which large size is one of the top 3 most pressing [challenges](https://blog.rust-lang.org/2026/03/02/2025-State-Of-Rust-Survey-results/#challenges-and-wishes-about-rust) that Rust users struggle with. The `-Cembed-metadata=no` flag removes this duplication (see [below](#What-does-the-flag-do) on how it works). Some size benchmark results can be found in ([1][kobzol-blog], [2][cargo-blog]). It can reach up to 35% size reduction on real-world projects in `release` mode, with the effect being smaller for `dev` builds (because incremental artifacts and debuginfo start to dominate the `target` directory size). A slight bonus is that we only write the metadata to disk once, thus resulting in less I/O. I haven't noticed any measurable effect (either positive or negative) on compile times though, at least on Linux. The main motivation is disk size, not compilation speed. ## What does the flag do When you pass `-Cembed-metadata=no`, `rustc` will not write the full metadata of the compiled crate into the emitted `.rlib` file. Instead, it will only write a ["metadata stub"](https://github.com/rust-lang/rust/blob/79ee8dc08073a0abf93d2b494ac5c1be11d4342e/compiler/rustc_metadata/src/rmeta/encoder.rs#L2552), which only contains the most basic information (name, target tuple, SVH). Note that this flag should usually be combined with `--emit=metadata` to write the metadata to a `.rmeta` file, otherwise no metadata would be generated at all. When loading crate dependencies passed via `--extern`, `rustc` will read the metadata of the passed `.rlib`. If it only contains a stub, it expects that you will also pass another `--extern` flag with a path to a `.rmeta` file with the full metadata. If you don't, you will be met with the following error: ``` error: only metadata stub found for `rlib` dependency `foo`. Please provide path to the corresponding .rmeta file with full metadata ``` Note that `rustc` will still default to the original behavior of embedding metadata, so build systems that wrap `rustc` will have to explicitly opt into using `-Cembed-metadata=no`, and at that point they will also have to start passing `--extern=<...>.rmeta` flags to `rustc`. That is what Cargo currently does if you use `nightly` or if you use the unstable `-Zembed-metadata=no` *Cargo* flag. In theory, we could relax the requirement to pass both paths to the `.rlib` and the `.rmeta` file for each direct dependency in the future, and let `rustc --extern=foo/bar.rlib` automatically attempt to load `--extern=foo/bar.rmeta` when the `.rmeta` file path is not passed explicitly. `.rlib` and `.rmeta` files produced using `-Cembed-metadata=no` and `-Cembed-metadata=yes` can be freely combined, and a downstream `rustc` invocation that consumes artifacts produced by `=no` does not need to itself receive the `=no` flag. ## Ecosystem impact The `-Zembed-metadata` flag has been used for the standard library that we ship since January 2026 (rust-lang#145343) and for the compiler itself and codegen backends (cg_clif and cg_gcc) since August 2026 (rust-lang#151061). The flag has also been used by Cargo by default on the `nightly` channel since the second half of August 2026. A blog post with a call for testing can be found [here](https://blog.rust-lang.org/inside-rust/2026/08/18/reducing-target-dir-size-on-nightly/). I'm not aware of any regressions or issues reported in relation to this, not is the Cargo team (AFAIK). There are some unresolved questions about backwards compability if Cargo starts passing `-Cembed-metadata=no` by default, but those are related strictly to Cargo, and not the compiler flag itself (because the compiler flag will remain opt-in). ## History The issue of duplicated metadata was originally noted back in 2015 (rust-lang#23366, rust-lang#29511), back then because of the metadata being duplicated in Rust `dylib`s. Since Cargo started using pipelined compilation by default, it became more acute, because the metadata of all compiled `.rlib` crate dependencies started being duplicated in the `target` directory. @bjorn3 proposed to add a compiler flag to split out the metadata into a separate file in rust-lang#57076. The implementation was attempted in rust-lang#93945 and later in rust-lang#120855, but there was non-trivial complexity due to Cargo integration. In 2025, I opened an [MCP](rust-lang/compiler-team#851) with the plan to implement this flag in two stages (first in rustc, and only then in Cargo), to reduce the implementation complexity, which was accepted. The implementation in `rustc` then happened in rust-lang#137535. Implementation history: - rust-lang#137535 (April 2025): Introduction of `-Zembed-metadata`, reviewed by @petrochenkov - rust-lang/cargo#15378 (May 2025): Initial integration in Cargo, reviewed by @epage - rust-lang#145343 (January 2026): Dogfood the flag for stdlib, reviewed by @bjorn3 - rust-lang#151061 (August 2026): Dogfood the flag for rustc and codegen backends, authored by @bjorn3 - rust-lang/cargo#17267 (August 2026): Enabled the `-Cembed-metadata=no` flag by default by Cargo on the `nightly` channel ## Unresolved questions/concerns ### Command-line length limit This is more related to the Cargo side of things, but I also wanted to mention it here, because there is a possible improvement we could do on the compiler side in the future. As noted [above](#What-does-the-flag-do), usage of `-Cembed-metadata=no` requires the build system/user to pass two `--extern` flags for each direct dependency of a given `rustc` invocation. This can increase the size of the executed command-line arguments, which can in edge cases start running into operating system limits. Recently, we saw something similar in action with the Cargo's new build dir layout stabilization, where Cargo suddenly started passing a lot more `-L` flags to `rustc`, which caused issues around CLI length limits and `PATH` env. var. length limits. Cargo can work around the CLI length limit by using response files (`rustc @args`), but not all downstream tools (such as `sccache`) support those fully (yet). Based on the experience with the new build dir layout, some work was done to improve this support in the ecosystem though, and to optimize the number of flags that Cargo passes to `rustc`. If this became a problem in practice, we could teach `rustc` to start implicitly assuming `--extern=foo.rmeta` when `--extern=foo.rlib` was passed. This would reduce the number of required `--extern` flags passed to the same count as with `-Cembed-metadata=yes` (or without using that flag at all). I think that we can always do this retroactively if needed without breaking backwards compatibility, so it does not have to block this stabilization. ### Landing this change Given that this compiler flag is expected to have tight integration with Cargo, and Cargo already uses it on the nightly channel by default, we have to be a bit careful about how we actually land the `-Zembed-metadata` => `-Cembed-metadata` change. Problem A: if we simply switched it from `-Z` to `-C` in `rust-lang/rust`, then nightly would break from the following day, because Cargo would be still attempting to pass the `-Z` flag, while `rustc` would already be expecting the `-C` variant of the flag. Problem B: furthermore, bootstrap currently unconditionally passes `-Zembed-metadata=no`, both to in-tree Cargo and to beta Cargo, so that also requires some attention. There are several options of how we could resolve this: 1. Conclude the nightly Cargo experiment, so that it will not pass `-Zembed-metadata=no` to `rustc` by default anymore. Then sync Cargo to `rust-lang/rust`, and only then merge this PR. This solves A. 2. Move Cargo from a git submodule to a (Josh) subtree, so that we can atomically change both Cargo and `rustc` to use `-C` instead of `-Z` (inside this PR), to avoid breaking nightly users. This solves A. - Even though I would like to do this migration anyway, I don't think that we need to tie it to this stabilization, as it can take some time before we resolve all concerns about the migration. 3. Do what 2. proposes, but without a subtree. That means that we have to: - Change to `-C` in Cargo - Change to `-C` in `rust-lang/rust` - Sync the Cargo change back to `rust-lang/rust` in the same day that we land the `rustc` change While this solves A, it might run into some staging issues in bootstrap, and it looks very complicated to pull off. 4. Teach `rustc` to accept both the `-Z` and the `-C` variant of the flag for some time. This solves both A and B, and also gives people who use `-Zembed-metadata` manually on `nightly` some breathing room to upgrade to the `-C` variant of the flag (but GitHub code search didn't find many usages of this flag, and this is normally an expected breakage for users of unstable compiler flags). I decided to go with 4, because the integration with Cargo is tight, and modifying bootstrap back and forth to support `-Z`/no flag /`-C` would be busy work. This solves both A and B and requires little implementation effort. ## Acknowledgments - @bjorn3 for coming up with the idea, providing a reference implementation and mentoring me to finish up the implementation - @petrochenkov for reviewing the compiler side of things - @epage and @weihanglo for reviewing the Cargo side of things [kobzol-blog]: https://kobzol.github.io/rust/rustc/2025/06/02/reduce-cargo-target-dir-size-with-z-no-embed-metadata.html#cargo-integration [cargo-blog]: https://blog.rust-lang.org/inside-rust/2026/08/18/reducing-target-dir-size-on-nightly/#how-much-does-it-help CC @bjorn3 @petrochenkov @weihanglo @epage r? petrochenkov @rustbot label +needs-fcp +T-compiler
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Rollup merge of #163436 - Kobzol:stabilize-embed-metadata, r=petrochenkov Stabilize `-Zembed-metadata` Tracking issue: #139165 This PR proposes to stabilize the `-Zembed-metadata` compiler flag, so that Cargo can start using `-Cembed-metadata=no` by default on the stable channel. The flag can be used to avoid duplicating the metadata of Rust crates within `.rlib` and `.rmeta` files. This can reduce the size of the `target` directory anywhere from 5% to 35% on real-world projects, depending on the chosen compilation profile ([1][kobzol-blog], [2][cargo-blog]). Additionally, `-Cembed-metadata=no` also removes the metadata out of Rust `dylib`s, and thus reduces their size. The plan is to stabilize this compiler flag so that Cargo can start using it on the stable channel. The stabilization plan thus has two parts: - Stabilize the `-Cembed-metadata` flag in `rustc` - Change Cargo to use `-Cembed-metadata=no` by default The Cargo team has prepared their side of the [stabilization report](rust-lang/cargo#17533). You can examine it to see the whole picture. ## What is being stabilized The `-Zembed-metadata` compiler flag. Note that `rustc` will still default to `-Cembed-metadata=yes`, as that is its original behavior since [Rust 1.16.0](https://blog.rust-lang.org/2017/03/16/Rust-1.16/#other-improvements). But it will now be possible to pass `-Cembed-metadata=no` on stable. ## Motivation When compiling Rust crates as libraries, Cargo uses pipelined compilation, which tells `rustc` (using `--emit=metadata`) to output an `.rmeta` file as soon as possible, so that Cargo can start follow-up `rustc` instances sooner and speed up the compilation process. This file contains the Rust-specific metadata of the compiled crate. However, after the compilation of the Rust crate finishes, the compiler will also write an `.rlib` file to disk, which contains both the compiled object code, but also the same metadata that is already contained in the `.rmeta` file. This means that after the compilation finishes, the metadata of each (non-leaf) compiled crate will be duplicated on disk. The metadata can be quite large, and it is not uncommon that it takes hundreds of megabytes for real-world projects. This duplication thus contributes to increasing the size of the `target` directory, which large size is one of the top 3 most pressing [challenges](https://blog.rust-lang.org/2026/03/02/2025-State-Of-Rust-Survey-results/#challenges-and-wishes-about-rust) that Rust users struggle with. The `-Cembed-metadata=no` flag removes this duplication (see [below](#What-does-the-flag-do) on how it works). Some size benchmark results can be found in ([1][kobzol-blog], [2][cargo-blog]). It can reach up to 35% size reduction on real-world projects in `release` mode, with the effect being smaller for `dev` builds (because incremental artifacts and debuginfo start to dominate the `target` directory size). A slight bonus is that we only write the metadata to disk once, thus resulting in less I/O. I haven't noticed any measurable effect (either positive or negative) on compile times though, at least on Linux. The main motivation is disk size, not compilation speed. ## What does the flag do When you pass `-Cembed-metadata=no`, `rustc` will not write the full metadata of the compiled crate into the emitted `.rlib` file. Instead, it will only write a ["metadata stub"](https://github.com/rust-lang/rust/blob/79ee8dc08073a0abf93d2b494ac5c1be11d4342e/compiler/rustc_metadata/src/rmeta/encoder.rs#L2552), which only contains the most basic information (name, target tuple, SVH). Note that this flag should usually be combined with `--emit=metadata` to write the metadata to a `.rmeta` file, otherwise no metadata would be generated at all. When loading crate dependencies passed via `--extern`, `rustc` will read the metadata of the passed `.rlib`. If it only contains a stub, it expects that you will also pass another `--extern` flag with a path to a `.rmeta` file with the full metadata. If you don't, you will be met with the following error: ``` error: only metadata stub found for `rlib` dependency `foo`. Please provide path to the corresponding .rmeta file with full metadata ``` Note that `rustc` will still default to the original behavior of embedding metadata, so build systems that wrap `rustc` will have to explicitly opt into using `-Cembed-metadata=no`, and at that point they will also have to start passing `--extern=<...>.rmeta` flags to `rustc`. That is what Cargo currently does if you use `nightly` or if you use the unstable `-Zembed-metadata=no` *Cargo* flag. In theory, we could relax the requirement to pass both paths to the `.rlib` and the `.rmeta` file for each direct dependency in the future, and let `rustc --extern=foo/bar.rlib` automatically attempt to load `--extern=foo/bar.rmeta` when the `.rmeta` file path is not passed explicitly. `.rlib` and `.rmeta` files produced using `-Cembed-metadata=no` and `-Cembed-metadata=yes` can be freely combined, and a downstream `rustc` invocation that consumes artifacts produced by `=no` does not need to itself receive the `=no` flag. ## Ecosystem impact The `-Zembed-metadata` flag has been used for the standard library that we ship since January 2026 (#145343) and for the compiler itself and codegen backends (cg_clif and cg_gcc) since August 2026 (#151061). The flag has also been used by Cargo by default on the `nightly` channel since the second half of August 2026. A blog post with a call for testing can be found [here](https://blog.rust-lang.org/inside-rust/2026/08/18/reducing-target-dir-size-on-nightly/). I'm not aware of any regressions or issues reported in relation to this, not is the Cargo team (AFAIK). There are some unresolved questions about backwards compability if Cargo starts passing `-Cembed-metadata=no` by default, but those are related strictly to Cargo, and not the compiler flag itself (because the compiler flag will remain opt-in). ## History The issue of duplicated metadata was originally noted back in 2015 (#23366, #29511), back then because of the metadata being duplicated in Rust `dylib`s. Since Cargo started using pipelined compilation by default, it became more acute, because the metadata of all compiled `.rlib` crate dependencies started being duplicated in the `target` directory. @bjorn3 proposed to add a compiler flag to split out the metadata into a separate file in #57076. The implementation was attempted in #93945 and later in #120855, but there was non-trivial complexity due to Cargo integration. In 2025, I opened an [MCP](rust-lang/compiler-team#851) with the plan to implement this flag in two stages (first in rustc, and only then in Cargo), to reduce the implementation complexity, which was accepted. The implementation in `rustc` then happened in #137535. Implementation history: - #137535 (April 2025): Introduction of `-Zembed-metadata`, reviewed by @petrochenkov - rust-lang/cargo#15378 (May 2025): Initial integration in Cargo, reviewed by @epage - #145343 (January 2026): Dogfood the flag for stdlib, reviewed by @bjorn3 - #151061 (August 2026): Dogfood the flag for rustc and codegen backends, authored by @bjorn3 - rust-lang/cargo#17267 (August 2026): Enabled the `-Cembed-metadata=no` flag by default by Cargo on the `nightly` channel ## Unresolved questions/concerns ### Command-line length limit This is more related to the Cargo side of things, but I also wanted to mention it here, because there is a possible improvement we could do on the compiler side in the future. As noted [above](#What-does-the-flag-do), usage of `-Cembed-metadata=no` requires the build system/user to pass two `--extern` flags for each direct dependency of a given `rustc` invocation. This can increase the size of the executed command-line arguments, which can in edge cases start running into operating system limits. Recently, we saw something similar in action with the Cargo's new build dir layout stabilization, where Cargo suddenly started passing a lot more `-L` flags to `rustc`, which caused issues around CLI length limits and `PATH` env. var. length limits. Cargo can work around the CLI length limit by using response files (`rustc @args`), but not all downstream tools (such as `sccache`) support those fully (yet). Based on the experience with the new build dir layout, some work was done to improve this support in the ecosystem though, and to optimize the number of flags that Cargo passes to `rustc`. If this became a problem in practice, we could teach `rustc` to start implicitly assuming `--extern=foo.rmeta` when `--extern=foo.rlib` was passed. This would reduce the number of required `--extern` flags passed to the same count as with `-Cembed-metadata=yes` (or without using that flag at all). I think that we can always do this retroactively if needed without breaking backwards compatibility, so it does not have to block this stabilization. ### Landing this change Given that this compiler flag is expected to have tight integration with Cargo, and Cargo already uses it on the nightly channel by default, we have to be a bit careful about how we actually land the `-Zembed-metadata` => `-Cembed-metadata` change. Problem A: if we simply switched it from `-Z` to `-C` in `rust-lang/rust`, then nightly would break from the following day, because Cargo would be still attempting to pass the `-Z` flag, while `rustc` would already be expecting the `-C` variant of the flag. Problem B: furthermore, bootstrap currently unconditionally passes `-Zembed-metadata=no`, both to in-tree Cargo and to beta Cargo, so that also requires some attention. There are several options of how we could resolve this: 1. Conclude the nightly Cargo experiment, so that it will not pass `-Zembed-metadata=no` to `rustc` by default anymore. Then sync Cargo to `rust-lang/rust`, and only then merge this PR. This solves A. 2. Move Cargo from a git submodule to a (Josh) subtree, so that we can atomically change both Cargo and `rustc` to use `-C` instead of `-Z` (inside this PR), to avoid breaking nightly users. This solves A. - Even though I would like to do this migration anyway, I don't think that we need to tie it to this stabilization, as it can take some time before we resolve all concerns about the migration. 3. Do what 2. proposes, but without a subtree. That means that we have to: - Change to `-C` in Cargo - Change to `-C` in `rust-lang/rust` - Sync the Cargo change back to `rust-lang/rust` in the same day that we land the `rustc` change While this solves A, it might run into some staging issues in bootstrap, and it looks very complicated to pull off. 4. Teach `rustc` to accept both the `-Z` and the `-C` variant of the flag for some time. This solves both A and B, and also gives people who use `-Zembed-metadata` manually on `nightly` some breathing room to upgrade to the `-C` variant of the flag (but GitHub code search didn't find many usages of this flag, and this is normally an expected breakage for users of unstable compiler flags). I decided to go with 4, because the integration with Cargo is tight, and modifying bootstrap back and forth to support `-Z`/no flag /`-C` would be busy work. This solves both A and B and requires little implementation effort. ## Acknowledgments - @bjorn3 for coming up with the idea, providing a reference implementation and mentoring me to finish up the implementation - @petrochenkov for reviewing the compiler side of things - @epage and @weihanglo for reviewing the Cargo side of things [kobzol-blog]: https://kobzol.github.io/rust/rustc/2025/06/02/reduce-cargo-target-dir-size-with-z-no-embed-metadata.html#cargo-integration [cargo-blog]: https://blog.rust-lang.org/inside-rust/2026/08/18/reducing-target-dir-size-on-nightly/#how-much-does-it-help CC @bjorn3 @petrochenkov @weihanglo @epage r? petrochenkov @rustbot label +needs-fcp +T-compiler
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This is a continuation of #120855 (I was mentored by @bjorn3 to move it forward). Most of the original code was written by bjorn3, I tried to clean it up a bit and add some documentation and tests.
This PR introduces a new unstable compiler flag called
-Zembed-metadata=[no|yes], with the default beingyes(see #57076 for context). When set tono, rustc will only store a small metadata stub inside rlibs/dylibs instead of the full metadata, to keep their size smaller. It should be used in combination with--emit=metadata, so that the users of such a compiled library can still read the metadata from the corresponding.rmetafile. This comment shows an example of binary/artifact size wins that can be achieved using this approach.Contrary to #120855, this PR only introduces the new flag, along with a couple of run-make tests and documentation, but does not yet use it in bootstrap to actually compile rustc. I plan to do that as a follow-up step (along with integration in Cargo, which should ideally just always pass this flag to reduce the size of target directories).
Fixes #23366
Closes #29511
Fixes #57076
Another attempt of #93945 and #120855.
r? @petrochenkov