Relink don't rebuild: add a baseline, sound implementation that can be incrementally improved by susitsm · Pull Request #155871 · rust-lang/rust

susitsm · 2026-04-27T10:33:56Z

This PR implemenets a sound but not too useful implementation of relink don't rebuild with the unstable -Z public-api-hash flag. It currently uses the stable hash of all items in the metadata.

The goal is to give a base implementation that can be used for experimentation. It can be incrementally improved over time by removing or stable sorting items. The PR also adds new test attributes rustc_public_hash_changed and rustc_public_hash_unchanged and an example test using them.

What are non-goals for this PR: a useful, optimized implementation of RDR and public api hashing. That has many more challenges which will each require careful review.

A non exhaustive list of the challenges for the feature I ran into while trying to make the hash useful (this should probably go in a tracking issue, but I'm not aware of one)

Name resolution diagnostics use private items. Importing the private my_crate::func prints private function as the error
Stripped cfg items are included in rmeta for improved diagnostics.
Which types to include? A simple is this public is not enough. RPIT returned types and the private types used in the included mir for generic/inline functions are probably needed.
It might make sense to have multiple hashes. Rmeta generation (cargo check), could use a much simpler hash than codegen, it does not need private types from mir (or any mir at all?)

rustbot · 2026-04-27T10:33:59Z

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

rustbot · 2026-04-27T10:34:03Z

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jdonszelmann · 2026-04-28T10:24:39Z

This is a large change, might take me multiple review passes. I'll see if I can do the first today or tomorrow :)

jdonszelmann · 2026-04-28T13:34:09Z

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rustbot · 2026-04-28T13:34:15Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

susitsm · 2026-04-29T10:13:23Z

@rustbot review

susitsm · 2026-04-29T12:31:31Z

Just realized that there is already a soundness hole: the crate_hash query itself. It won't be updated downstream if only the public hash changes. I'll have to review what exactly it is used for, maybe it is as simple as replacing it with the public hash. Except for proc macros, that might need some work

jdonszelmann · 2026-04-29T13:49:02Z

+    //    should be added here as
+    //    ```
+    //    // FIXME do we need this                      // or a comment about why we need this
+    //    let _ = my_new_field;


this part is outdated now, about the let _ =

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jdonszelmann · 2026-04-29T13:49:59Z

+    //    `hash_crate_root_public_api`" into `encode_my_new_field`
+    // 3. Only remove/change what is hashed in a separate PR. Removing items just from the hash
+    //    should be done with extreme scrutiny. A better way might be to sort the query result
+    //    in its provider, or filter which values we encode. That also helps with rmeta size.


this should maybe be a fixme at the end

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jdonszelmann · 2026-04-29T13:54:16Z

        }
    }

+    // When changed, make sure to update the hashing in `hash_crate_root_public_api`


ideally, we of course made it so the hashing of that and the encoding here share some of the same source. Maybe by implementing a trait, or calling a method here that adds to the RDR hash, or returning a closure, idk. That way you don't have to think about all these comments and nonlocal subtleties

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that, or even a rustc lint at some point that automatically prompts you if you've made a change here and didn't update the other file. not 100% sure yet what the logic there would be, but this, though better, still seems brittle

Reworked this part in a way that the compiler can help a bit more with not forgetting it and doing it correctly. Introduced a new HashableCrateRoot struct that derives StableHash. The public api hash is just the stable hash of this struct. When hashing is done, CrateRoot is constructed from the calculated public and private hashes and the values moved from HashableCrateRoot.

jdonszelmann · 2026-04-29T13:56:54Z

@rustbot author

cezarbbb

Hello, I have read your code and I think it is very well written, but I have some questions I would like to confirm.

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rustbot · 2026-05-08T13:33:30Z

These commits modify compiler targets.
(See the Target Tier Policy.)

rustbot · 2026-05-08T13:33:33Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

susitsm · 2026-05-08T13:46:23Z

The last commit also fixes the issue I described with the resolver in #155871 (comment). It looks up dependencies based on their public api hash, but the resolver considers both public and private hashes when looking for duplicate candidates. It will return an error if there are two crates with (public, private1), (public, private2) hashes

…ll encoded data

… rmeta without parents)

…end on public_api_hash instead of crate_hash

…tributes for testing

…stc_public_hash_unchanged attributes

…lic hashes in the resolver When the resolver resolved transitive dependecies, it used the `crate_hash` of dependecies saved inside the rmeta of downstream crates to locate them. With public api hashing enabled, it is not sound to save that hash into downstream crates. Only the public hash can be saved. This modifies the locator to find all crates with the given public hash, but look for conflicing crates using the (public, private) hash pair. So if there are multiple crates with the same public hash, but different private hash (which should only happen if there was some kind of hash collision while making the public hashes or the StableCrateId which is included in it), it will be reported as having multiple candidates for the crate.

susitsm · 2026-05-09T06:53:05Z

After some discussion with the author of #154724, it might be good idea to do a perf run. The rmeta encoding branches a lot on the hash_public_api unstable option, and might need some optimizations. Other than that, this is ready for @rustbot review

rust-bors · 2026-05-09T17:27:38Z

☔ The latest upstream changes (presumably #156361) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot added A-attributes Area: Attributes (`#[…]`, `#![…]`) S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 27, 2026

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susitsm force-pushed the relink-dont-rebuild-base branch from 0e63248 to 4b676bd Compare April 27, 2026 10:47

jdonszelmann reviewed Apr 28, 2026

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Comment thread compiler/rustc_attr_parsing/src/attributes/rustc_internal.rs Outdated

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susitsm force-pushed the relink-dont-rebuild-base branch from ab0ab17 to 981dee4 Compare April 30, 2026 18:04

implement StableHash for TargetTuple

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susitsm force-pushed the relink-dont-rebuild-base branch from 981dee4 to f07198b Compare May 8, 2026 13:33

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susitsm force-pushed the relink-dont-rebuild-base branch from f07198b to cd53915 Compare May 8, 2026 14:44

susitsm added 13 commits May 9, 2026 08:39

tidy: move TargetTuple to a new file

c46e842

rdr: implement rmeta public api hash as the stable hash of (almost) a…

346cfd4

…ll encoded data

Add public api hash debug logs

04c1362

rdr: elaborate on how EII-s should be included in the public api hash

cfdfcc5

rdr: document the process of hashing new fields added to CrateRoot

5989a84

rdr: document hashed_lazy_array

f8e21df

rdr: hash spans as if they had no parents (spans are encoded into the…

504f859

… rmeta without parents)

rdr: add the public_api_hash query. Queries provided by rmeta now dep…

294e8a3

…end on public_api_hash instead of crate_hash

rdr: add rustc_public_hash_unchanged and rustc_public_hash_changed at…

f3a45e4

…tributes for testing

rdr: add incremental test exercising rustc_public_hash_changed and ru…

53ced96

…stc_public_hash_unchanged attributes

rdr: use public_api_hash when hashing dependencies

949b4d4

rdr: use public_api_hash in the rmeta headers

bdf7e0b

susitsm force-pushed the relink-dont-rebuild-base branch from cd53915 to b9829db Compare May 9, 2026 06:44

rdr: add hash FIXME comments to LazyTables fields

d292f0b

susitsm force-pushed the relink-dont-rebuild-base branch from b9829db to ff5f988 Compare May 9, 2026 06:51

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susitsm force-pushed the relink-dont-rebuild-base branch from ff5f988 to d292f0b Compare May 9, 2026 08:10

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A non exhaustive list of the challenges for the feature I ran into while trying to make the hash useful (this should probably go in a tracking issue, but I'm not aware of one)

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