Implement RFC 3678: Final trait methods by mu001999 · Pull Request #151783 · rust-lang/rust

mu001999 · 2026-01-28T14:54:32Z

This PR is based on #130802, with some minor changes and conflict resolution.

Futhermore, this PR excludes final methods from the vtable of a dyn Trait.

And some excerpt from the original PR description:

Implements the surface part of rust-lang/rfcs#3678.

I'm using the word "method" in the title, but in the diagnostics and the feature gate I used "associated function", since that's more accurate.

cc @joshtriplett

rustbot · 2026-01-28T14:54:37Z

Some changes occurred in compiler/rustc_builtin_macros/src/autodiff.rs

cc @ZuseZ4

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

rustbot · 2026-01-28T14:54:40Z

r? @fee1-dead

rustbot has assigned @fee1-dead.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

fmease · 2026-01-28T16:56:45Z

Could you mark errs as a coauthor in all 6 commits, I think that's only fair. Thanks! :)

Namely: Co-authored-by: Michael Goulet <michael@errs.io>

mu001999 · 2026-01-29T00:09:42Z

@fmease Done. Thanks for the reminder! :)

joshtriplett · 2026-01-29T08:50:18Z

What it doesn't implement are any optimizations that can be done on final trait methods, such as excluding them from the vtable of a dyn Trait.

Does this implement the requirement that final methods don't prevent a trait from being dyn-compatible?

mu001999 · 2026-01-30T14:31:48Z

Does this implement the requirement that final methods don't prevent a trait from being dyn-compatible?

Implemented now :). Also, updated the PR description.

Co-authored-by: Michael Goulet <michael@errs.io>

rustbot · 2026-02-12T07:13:15Z

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.

Co-authored-by: Michael Goulet <michael@errs.io>

nik-rev · 2026-02-15T18:16:24Z

compiler/rustc_ast_passes/src/ast_validation.rs

+enum AllowDefault {
+    Yes,
+    No,
+}
+
+impl AllowDefault {
+    fn when(b: bool) -> Self {
+        if b { Self::Yes } else { Self::No }
+    }
+}
+
+enum AllowFinal {
+    Yes,
+    No,
+}
+
+impl AllowFinal {
+    fn when(b: bool) -> Self {
+        if b { Self::Yes } else { Self::No }
+    }
+}


small suggestion:

struct AllowDefault(pub bool); struct AllowFinal(pub bool);

I think these two styles are not really much different. I like the enum because it is more verbose when you check: if let AllowFinal::Yes = allow_final vs if allow_final.0.

fee1-dead · 2026-02-17T01:05:13Z

compiler/rustc_ast_passes/src/ast_validation.rs

+enum AllowDefault {
+    Yes,
+    No,
+}
+
+impl AllowDefault {
+    fn when(b: bool) -> Self {
+        if b { Self::Yes } else { Self::No }
+    }
+}
+
+enum AllowFinal {
+    Yes,
+    No,
+}
+
+impl AllowFinal {
+    fn when(b: bool) -> Self {
+        if b { Self::Yes } else { Self::No }
+    }
+}


I think these two styles are not really much different. I like the enum because it is more verbose when you check: if let AllowFinal::Yes = allow_final vs if allow_final.0.

fee1-dead · 2026-02-17T01:13:51Z

LGTM, can iterate on it later
@bors r+

rust-bors · 2026-02-17T01:13:53Z

📌 Commit 8c77b6c has been approved by fee1-dead

It is now in the queue for this repository.

Rollup of 11 pull requests Successful merges: - #152700 (miri subtree update) - #152715 (`rust-analyzer` subtree update) - #151783 (Implement RFC 3678: Final trait methods) - #152512 (core: Implement feature `float_exact_integer_constants`) - #152661 (Avoid ICE in From/TryFrom diagnostic under -Znext-solver) - #152703 (Remove `rustc_query_system`) - #152206 (misc doc improvements) - #152664 (Fix mis-constructed `file_span` when generating scraped examples) - #152698 (Suppress unstable-trait notes under `-Zforce-unstable-if-unmarked`) - #152727 (`probe_op` silence ambiguity errors if tainted) - #152728 (Port #![default_lib_allocator] to the new attribute parser)

@joshtriplett

Rollup merge of #151783 - mu001999-contrib:impl/final-method, r=fee1-dead Implement RFC 3678: Final trait methods Tracking: #131179 This PR is based on #130802, with some minor changes and conflict resolution. Futhermore, this PR excludes final methods from the vtable of a dyn Trait. And some excerpt from the original PR description: > Implements the surface part of rust-lang/rfcs#3678. > > I'm using the word "method" in the title, but in the diagnostics and the feature gate I used "associated function", since that's more accurate. cc @joshtriplett

fmease · 2026-02-17T13:01:53Z

compiler/rustc_feature/src/unstable.rs

    /// Allows the use of `#[ffi_pure]` on foreign functions.
    (unstable, ffi_pure, "1.45.0", Some(58329)),
+    /// Allows marking trait functions as `final` to prevent overriding impls
+    (unstable, final_associated_functions, "CURRENT_RUSTC_VERSION", Some(1)),


Suggested change

(unstable, final_associated_functions, "CURRENT_RUSTC_VERSION", Some(1)),

(unstable, final_associated_functions, "CURRENT_RUSTC_VERSION", Some(131179)),

This was never filled out. Could sb send a patch updating this number (+ rebless the relevant tests)? Thanks in advance.

… r=fmease Update tracking issue number for final_associated_functions From rust-lang#151783 (comment)

Rollup of 11 pull requests Successful merges: - rust-lang/rust#152700 (miri subtree update) - rust-lang/rust#152715 (`rust-analyzer` subtree update) - rust-lang/rust#151783 (Implement RFC 3678: Final trait methods) - rust-lang/rust#152512 (core: Implement feature `float_exact_integer_constants`) - rust-lang/rust#152661 (Avoid ICE in From/TryFrom diagnostic under -Znext-solver) - rust-lang/rust#152703 (Remove `rustc_query_system`) - rust-lang/rust#152206 (misc doc improvements) - rust-lang/rust#152664 (Fix mis-constructed `file_span` when generating scraped examples) - rust-lang/rust#152698 (Suppress unstable-trait notes under `-Zforce-unstable-if-unmarked`) - rust-lang/rust#152727 (`probe_op` silence ambiguity errors if tainted) - rust-lang/rust#152728 (Port #![default_lib_allocator] to the new attribute parser)

… r=fmease Update tracking issue number for final_associated_functions From rust-lang#151783 (comment)

rustbot assigned fee1-dead Jan 28, 2026

mu001999 force-pushed the impl/final-method branch from fe8e71f to f0df197 Compare January 29, 2026 00:03

This comment was marked as outdated.

Sign in to view

mu001999 force-pushed the impl/final-method branch from ca5e458 to a0707ab Compare January 30, 2026 12:30

mu001999 marked this pull request as draft January 30, 2026 12:48

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 Jan 30, 2026

mu001999 force-pushed the impl/final-method branch from a0707ab to 5c978e5 Compare January 30, 2026 14:21

mu001999 marked this pull request as ready for review January 30, 2026 14:31

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 Jan 30, 2026

This comment has been minimized.

Sign in to view

mu001999 force-pushed the impl/final-method branch from 5c978e5 to 05c342d Compare January 30, 2026 15:40

This comment has been minimized.

Sign in to view

mu001999 force-pushed the impl/final-method branch from 05c342d to a630df6 Compare February 5, 2026 16:04

This comment has been minimized.

Sign in to view

Parse and lower final for methods

7077797

Co-authored-by: Michael Goulet <michael@errs.io>

mu001999 force-pushed the impl/final-method branch from 897ebf0 to 526d9ed Compare February 12, 2026 07:13

mu001999 and others added 6 commits February 12, 2026 15:18

Validate final usages in AST

460cda8

Co-authored-by: Michael Goulet <michael@errs.io>

Validate no override impl definitions

3572d48

Co-authored-by: Michael Goulet <michael@errs.io>

Fix rustfmt

f0e1c8f

Co-authored-by: Michael Goulet <michael@errs.io>

Render final associated functions correctly in rustdoc

f0a019b

Co-authored-by: Michael Goulet <michael@errs.io>

Fix clippy ast utils

2e0ff1f

Co-authored-by: Michael Goulet <michael@errs.io>

Exclude final methods from dyn-compat check and vtable

8c77b6c

mu001999 force-pushed the impl/final-method branch from 526d9ed to 8c77b6c Compare February 12, 2026 07:18

nik-rev reviewed Feb 15, 2026

View reviewed changes

fee1-dead approved these changes Feb 17, 2026

View reviewed changes

rust-bors bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 17, 2026

Zalathar mentioned this pull request Feb 17, 2026

Rollup of 11 pull requests #152739

Merged

rust-bors bot merged commit 1367126 into rust-lang:main Feb 17, 2026
11 checks passed

rustbot added this to the 1.95.0 milestone Feb 17, 2026

fmease reviewed Feb 17, 2026

View reviewed changes

fmease mentioned this pull request Feb 17, 2026

Tracking issue for RFC 3678: Trait method impl restrictions (final methods) #131179

Open

7 tasks

mu001999 mentioned this pull request Feb 18, 2026

Update tracking issue number for final_associated_functions #152778

Open

jhpratt added a commit to jhpratt/rust that referenced this pull request Feb 18, 2026

Rollup merge of rust-lang#152778 - mu001999-contrib:fix/final-method,…

44d9f42

… r=fmease Update tracking issue number for final_associated_functions From rust-lang#151783 (comment)

jhpratt added a commit to jhpratt/rust that referenced this pull request Feb 18, 2026

Rollup merge of rust-lang#152778 - mu001999-contrib:fix/final-method,…

a3bb32f

… r=fmease Update tracking issue number for final_associated_functions From rust-lang#151783 (comment)

Zalathar added a commit to Zalathar/rust that referenced this pull request Feb 18, 2026

Rollup merge of rust-lang#152778 - mu001999-contrib:fix/final-method,…

8a2c6ea

… r=fmease Update tracking issue number for final_associated_functions From rust-lang#151783 (comment)

	(unstable, final_associated_functions, "CURRENT_RUSTC_VERSION", Some(1)),
	(unstable, final_associated_functions, "CURRENT_RUSTC_VERSION", Some(131179)),

Uh oh!

Conversation

mu001999 commented Jan 28, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Jan 28, 2026

Uh oh!

rustbot commented Jan 28, 2026

Uh oh!

fmease commented Jan 28, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

mu001999 commented Jan 29, 2026

Uh oh!

joshtriplett commented Jan 29, 2026

Uh oh!

This comment was marked as outdated.

mu001999 commented Jan 30, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

This comment has been minimized.

This comment has been minimized.

This comment has been minimized.

This comment has been minimized.

This comment has been minimized.

This comment has been minimized.

rustbot commented Feb 12, 2026

Uh oh!

nik-rev Feb 15, 2026

Choose a reason for hiding this comment

Uh oh!

fee1-dead Feb 17, 2026

Choose a reason for hiding this comment

Uh oh!

fee1-dead Feb 17, 2026

Choose a reason for hiding this comment

Uh oh!

fee1-dead commented Feb 17, 2026

Uh oh!

rust-bors bot commented Feb 17, 2026

Uh oh!

Uh oh!

fmease Feb 17, 2026

Choose a reason for hiding this comment

Uh oh!

mu001999 Feb 18, 2026

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

7 participants

mu001999 commented Jan 28, 2026 •

edited

Loading

fmease commented Jan 28, 2026 •

edited

Loading

mu001999 commented Jan 30, 2026 •

edited

Loading