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.

bjorn3 · 2026-04-28T14:05:28Z

+        let mut public_api_hasher = PublicApiHasher::default();
        let tcx = self.tcx;
        let mut stats: Vec<(&'static str, usize)> = Vec::with_capacity(32);



The encode_crate_deps below should record the public api hash rather than crate hash when doing rdr, right?

View changes since the review

Short answer: you are right, that will improve it while keeping it sound. I will add that to this PR.

Long answer: one of the main goals of RDR is to enable early cutoff, including the public hash of all dependencies goes against that. We should only include public hashes of dependencies we reexport in some way, or better, only include the hash of the part we reexport. Where reexport here can mean pub use, inlinable/generic/const eval mir reachable through local inlinable/generic/const eval mir and some more stuff we are not aware of yet. Doing this correctly and maintainably is likely the single most technically challenging part of getting RDR right.

changed it to use public_api_hash, added some comments. 044529d

…ll encoded data

… rmeta without parents)

…end on public_api_hash instead of crate_hash

…tributes for testing

…stc_public_hash_unchanged attributes

…when changing some rmeta encoder functions

rustbot · 2026-04-29T10:12:26Z

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 · 2026-04-29T10:13:23Z

@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 _ =

View changes since the review

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

View changes since the review

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

View changes since the review

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

jdonszelmann · 2026-04-29T13:56:54Z

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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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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 28, 2026

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susitsm force-pushed the relink-dont-rebuild-base branch from 4b676bd to 1682f38 Compare April 29, 2026 09:55

susitsm added 13 commits April 29, 2026 12:04

rdr: add RDRHashes field to CrateRoot

f877fb4

rdr: add public_api_hash unstable option

27034fc

derive HashStable for TargetModifier

5612408

derive HashStable for DeniedPartialMitigation

9db9131

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

9132e4b

…ll encoded data

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

1ce1fe3

… rmeta without parents)

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

bc16057

…end on public_api_hash instead of crate_hash

rdr: remove crate_hash from public api hash

280b75e

rdr: add public api hash debug logs

68bf2d4

rdr: add rustc_public_hash_unchanged and rustc_public_hash_changed at…

c26b9e4

…tributes for testing

rdr: add incremental test exercising rustc_public_hash_changed and ru…

e330cd4

…stc_public_hash_unchanged attributes

rdr: use public_api_hash when hashing dependencies

17f9bd3

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

041a81f

susitsm added 2 commits April 29, 2026 12:11

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

8adbe4c

rdr: add comments indicating that public api hashing must be updated …

ab0ab17

…when changing some rmeta encoder functions

susitsm force-pushed the relink-dont-rebuild-base branch from 1682f38 to ab0ab17 Compare April 29, 2026 10:12

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 29, 2026

jdonszelmann reviewed Apr 29, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 29, 2026

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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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