Provide more context on trait bounds being unmet due to imperfect derive by estebank · Pull Request #151278 · rust-lang/rust

estebank · 2026-01-18T02:31:48Z

When encountering a value that has a borrow checker error where the type was previously moved, when suggesting cloning verify that it is not already being derived. If it is, explain why the derive(Clone) doesn't apply:

note: if `TypedAddress<T>` implemented `Clone`, you could clone the value
  --> $DIR/derive-clone-implicit-bound.rs:6:1
   |
LL | #[derive(Clone, Copy)]
   |          ----- derived `Clone` adds implicit bounds on type parameters
LL | pub struct TypedAddress<T>{
   | ^^^^^^^^^^^^^^^^^^^^^^^^-^
   | |                       |
   | |                       introduces an implicit `T: Clone` bound
   | consider manually implementing `Clone` for this type
...
LL |         let old = self.return_value(offset);
   |                                     ------ you could clone this value

When encountering a bound coming from a derive macro, suggest manual impl of the trait.

Use the span for the specific param when adding bounds in builtin derive macros, so the diagnostic will point at them as well as the derive macro itself.

note: required for `Id<SomeNode>` to implement `PartialEq`
  --> $DIR/derive-implicit-bound.rs:5:10
   |
LL | #[derive(PartialEq, Eq)]
   |          ^^^^^^^^^
LL | pub struct Id<T>(PhantomData<T>);
   |               - unsatisfied trait bound introduced in this `derive` macro
   = help: consider manually implementing `PartialEq` to avoid undesired bounds

Mention that the trait could be manually implemented in E0599.

Fix #108894. Address #143714. Address ##146515 (but ideally would also suggest constraining the fn bound correctly as well).

rustbot · 2026-01-18T02:31:53Z

r? @davidtwco

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fmease · 2026-01-18T09:22:34Z

CC another related issue: #146515. This one won't be fixed by your PR IINM but we might be able to leverage & reuse some of your added logic to do so (disclaimer: I've only skimmed your PR, so I might be wrong).

There's a PR open to fix said issue, namely #150720, but your approach might be more general and could supersede it.

estebank · 2026-01-18T20:05:37Z

@fmease for that case, what do you think of output like

error[E0308]: mismatched types
 --> f30.rs:9:5
  |
8 | fn clone_me<T, K>(x: &ContainsRc<T, K>) -> ContainsRc<T, K> {
  |                                            ---------------- expected `ContainsRc<T, K>` because of return type
9 |     x.clone()
  |     -^^^^^^^^
  |     |
  |     expected `ContainsRc<T, K>`, found `&ContainsRc<T, K>`
  |     `ContainsRc<T, K>` does not implement `Clone`, so `&ContainsRc<T, K>` was cloned instead
  |
  = note: expected struct `ContainsRc<_, _>`
          found reference `&ContainsRc<_, _>`
help: `Clone` is not implemented because the following trait bounds could not be satisfied
 --> f30.rs:4:19
  |
3 | #[derive(Clone)]
  |          ----- in this derive macro expansion
4 | struct ContainsRc<T, K> {
  |                   ^  ^ derive introduces an implicit unsatisfied trait bound `K: Clone`
  |                   |
  |                   derive introduces an implicit unsatisfied trait bound `T: Clone`
  = help: consider manually implementing `Clone`

estebank · 2026-01-18T20:41:35Z

David: I can split the PR into smaller ones as needed (it's just they touch the same stderr files so it was easier to keep it as a single patchset). Each commit is logically grouped.

rustbot · 2026-01-18T20:57:33Z

HIR ty lowering was modified

cc @fmease

davidtwco

Implementation largely looks good to me, some thoughts on the new suggestions more generally

View changes since this review

tests/ui/borrowck/derive-clone-implicit-bound.stderr

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

tests/ui/consts/const-blocks/trait-error.stderr

When encountering a value that has a borrow checker error where the type was previously moved, when suggesting cloning verify that it is not already being derived. If it is, explain why the `derive(Clone)` doesn't apply: ``` note: if `TypedAddress<T>` implemented `Clone`, you could clone the value --> $DIR/derive-clone-implicit-bound.rs:6:1 | LL | #[derive(Clone, Copy)] | ----- derived `Clone` adds implicit bounds on type parameters LL | pub struct TypedAddress<T>{ | ^^^^^^^^^^^^^^^^^^^^^^^^-^ | | | | | introduces an implicit `T: Clone` bound | consider manually implementing `Clone` for this type ... LL | let old = self.return_value(offset); | ------ you could clone this value ```

When encountering a bound coming from a derive macro, suggest manual impl of the trait. Use the span for the specific param when adding bounds in builtin derive macros, so the diagnostic will point at them as well as the derive macro itself. ``` error[E0277]: can't compare `SomeNode` with `SomeNode` --> f29.rs:24:15 | 24 | accept_eq(&node); | --------- ^^^^^ no implementation for `SomeNode == SomeNode` | | | required by a bound introduced by this call | = note: -Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs:279:39 = help: the trait `PartialEq` is not implemented for `SomeNode` note: required for `Id<SomeNode>` to implement `PartialEq` --> f29.rs:3:10 | 3 | #[derive(PartialEq, Eq)] | ^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro 4 | pub struct Id<T>(PhantomData<T>); | - = help: consider manually implementing `PartialEq` to avoid undesired bounds note: required by a bound in `accept_eq` --> f29.rs:15:23 | 15 | fn accept_eq(_: &impl PartialEq) { } | ^^^^^^^^^ required by this bound in `accept_eq` help: consider annotating `SomeNode` with `#[derive(PartialEq)]` | 13 + #[derive(PartialEq)] 14 | struct SomeNode(); | ```

``` note: required for `B<C>` to implement `Copy` --> $DIR/deriving-copyclone.rs:9:10 | LL | #[derive(Copy, Clone)] | ^^^^ unsatisfied trait bound introduced in this `derive` macro LL | struct B<T> { | - would need to be `Copy` ```

… for derive On type errors where the difference is expecting an owned type and getting a reference, if the expression is a `.clone()` call and the type is annotated with `#[derive(Clone)]`, we now explain implicit bounds and suggest manually implementing `Clone`. ``` error[E0308]: mismatched types --> $DIR/derive-implicit-bound-on-clone.rs:10:5 | LL | fn clone_me<T, K>(x: &ContainsRc<T, K>) -> ContainsRc<T, K> { | ---------------- expected `ContainsRc<T, K>` because of return type LL | x.clone() | ^^^^^^^^^ expected `ContainsRc<T, K>`, found `&ContainsRc<T, K>` | = note: expected struct `ContainsRc<_, _>` found reference `&ContainsRc<_, _>` note: `ContainsRc<T, K>` does not implement `Clone`, so `&ContainsRc<T, K>` was cloned instead --> $DIR/derive-implicit-bound-on-clone.rs:10:5 | LL | x.clone() | ^ help: `Clone` is not implemented because the some trait bounds could not be satisfied --> $DIR/derive-implicit-bound-on-clone.rs:5:19 | LL | #[derive(Clone)] | ----- in this derive macro expansion LL | struct ContainsRc<T, K> { | ^ ^ derive introduces an implicit unsatisfied trait bound `K: Clone` | | | derive introduces an implicit unsatisfied trait bound `T: Clone` = help: consider manually implementing `Clone` to avoid the implict type parameter bounds ```

rustbot · 2026-02-01T18:31:31Z

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.

davidtwco · 2026-02-05T16:34:48Z

@bors r+

rust-bors · 2026-02-05T16:34:52Z

📌 Commit 6756561 has been approved by davidtwco

It is now in the queue for this repository.

Provide more context on trait bounds being unmet due to imperfect derive When encountering a value that has a borrow checker error where the type was previously moved, when suggesting cloning verify that it is not already being derived. If it is, explain why the `derive(Clone)` doesn't apply: ``` note: if `TypedAddress<T>` implemented `Clone`, you could clone the value --> $DIR/derive-clone-implicit-bound.rs:6:1 | LL | #[derive(Clone, Copy)] | ----- derived `Clone` adds implicit bounds on type parameters LL | pub struct TypedAddress<T>{ | ^^^^^^^^^^^^^^^^^^^^^^^^-^ | | | | | introduces an implicit `T: Clone` bound | consider manually implementing `Clone` for this type ... LL | let old = self.return_value(offset); | ------ you could clone this value ``` When encountering a bound coming from a derive macro, suggest manual impl of the trait. Use the span for the specific param when adding bounds in builtin derive macros, so the diagnostic will point at them as well as the derive macro itself. ``` note: required for `Id<SomeNode>` to implement `PartialEq` --> $DIR/derive-implicit-bound.rs:5:10 | LL | #[derive(PartialEq, Eq)] | ^^^^^^^^^ LL | pub struct Id<T>(PhantomData<T>); | - unsatisfied trait bound introduced in this `derive` macro = help: consider manually implementing `PartialEq` to avoid undesired bounds ``` Mention that the trait could be manually implemented in E0599. Fix rust-lang#108894. Address rust-lang#143714. Address #rust-lang#146515 (but ideally would also suggest constraining the fn bound correctly as well).

…uwer Rollup of 9 pull requests Successful merges: - #151278 (Provide more context on trait bounds being unmet due to imperfect derive) - #151955 (escape symbol names in global asm) - #149329 (Mark match arms in try and for as being from desugarings.) - #151474 (Minor structural improvements) - #151744 (fix refining_impl_trait suggestion with return_type_notation) - #152107 (Convert to inline diagnostics in `rustc_borrowck`) - #152117 (Convert to inline diagnostics in `rustc_trait_selection`) - #152136 (Consolidate type const checks on `tcx.is_type_const`) - #152170 (Port `rustc_effective_visibility` to the new attribute parser)

rust-bors · 2026-02-05T20:02:19Z

⌛ Testing commit 6756561 with merge 4ea8b57...

Workflow: https://github.com/rust-lang/rust/actions/runs/21726517594

Provide more context on trait bounds being unmet due to imperfect derive When encountering a value that has a borrow checker error where the type was previously moved, when suggesting cloning verify that it is not already being derived. If it is, explain why the `derive(Clone)` doesn't apply: ``` note: if `TypedAddress<T>` implemented `Clone`, you could clone the value --> $DIR/derive-clone-implicit-bound.rs:6:1 | LL | #[derive(Clone, Copy)] | ----- derived `Clone` adds implicit bounds on type parameters LL | pub struct TypedAddress<T>{ | ^^^^^^^^^^^^^^^^^^^^^^^^-^ | | | | | introduces an implicit `T: Clone` bound | consider manually implementing `Clone` for this type ... LL | let old = self.return_value(offset); | ------ you could clone this value ``` When encountering a bound coming from a derive macro, suggest manual impl of the trait. Use the span for the specific param when adding bounds in builtin derive macros, so the diagnostic will point at them as well as the derive macro itself. ``` note: required for `Id<SomeNode>` to implement `PartialEq` --> $DIR/derive-implicit-bound.rs:5:10 | LL | #[derive(PartialEq, Eq)] | ^^^^^^^^^ LL | pub struct Id<T>(PhantomData<T>); | - unsatisfied trait bound introduced in this `derive` macro = help: consider manually implementing `PartialEq` to avoid undesired bounds ``` Mention that the trait could be manually implemented in E0599. Fix #108894. Address #143714. Address ##146515 (but ideally would also suggest constraining the fn bound correctly as well).

JonathanBrouwer · 2026-02-05T20:08:04Z

@bors yield
Yielding to enclosing rollup

rust-bors · 2026-02-05T20:08:08Z

Auto build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/21726517594

The next pull request likely to be tested is #152181.

…uwer Rollup of 9 pull requests Successful merges: - #151278 (Provide more context on trait bounds being unmet due to imperfect derive) - #151955 (escape symbol names in global asm) - #149329 (Mark match arms in try and for as being from desugarings.) - #151474 (Minor structural improvements) - #151744 (fix refining_impl_trait suggestion with return_type_notation) - #152107 (Convert to inline diagnostics in `rustc_borrowck`) - #152117 (Convert to inline diagnostics in `rustc_trait_selection`) - #152136 (Consolidate type const checks on `tcx.is_type_const`) - #152170 (Port `rustc_effective_visibility` to the new attribute parser)

Provide more context on trait bounds being unmet due to imperfect derive When encountering a value that has a borrow checker error where the type was previously moved, when suggesting cloning verify that it is not already being derived. If it is, explain why the `derive(Clone)` doesn't apply: ``` note: if `TypedAddress<T>` implemented `Clone`, you could clone the value --> $DIR/derive-clone-implicit-bound.rs:6:1 | LL | #[derive(Clone, Copy)] | ----- derived `Clone` adds implicit bounds on type parameters LL | pub struct TypedAddress<T>{ | ^^^^^^^^^^^^^^^^^^^^^^^^-^ | | | | | introduces an implicit `T: Clone` bound | consider manually implementing `Clone` for this type ... LL | let old = self.return_value(offset); | ------ you could clone this value ``` When encountering a bound coming from a derive macro, suggest manual impl of the trait. Use the span for the specific param when adding bounds in builtin derive macros, so the diagnostic will point at them as well as the derive macro itself. ``` note: required for `Id<SomeNode>` to implement `PartialEq` --> $DIR/derive-implicit-bound.rs:5:10 | LL | #[derive(PartialEq, Eq)] | ^^^^^^^^^ LL | pub struct Id<T>(PhantomData<T>); | - unsatisfied trait bound introduced in this `derive` macro = help: consider manually implementing `PartialEq` to avoid undesired bounds ``` Mention that the trait could be manually implemented in E0599. Fix rust-lang#108894. Address rust-lang#143714. Address #rust-lang#146515 (but ideally would also suggest constraining the fn bound correctly as well).

…uwer Rollup of 11 pull requests Successful merges: - #152174 (stdarch subtree update) - #151278 (Provide more context on trait bounds being unmet due to imperfect derive) - #151955 (escape symbol names in global asm) - #149329 (Mark match arms in try and for as being from desugarings.) - #151474 (Minor structural improvements) - #151744 (fix refining_impl_trait suggestion with return_type_notation) - #152107 (Convert to inline diagnostics in `rustc_borrowck`) - #152117 (Convert to inline diagnostics in `rustc_trait_selection`) - #152136 (Consolidate type const checks on `tcx.is_type_const`) - #152170 (Port `rustc_effective_visibility` to the new attribute parser) - #152184 (Port rustc_abi to the attribute parser)

Rollup of 4 pull requests Successful merges: - #152174 (stdarch subtree update) - #151278 (Provide more context on trait bounds being unmet due to imperfect derive) - #151955 (escape symbol names in global asm) - #152194 (Remove the 4 failing tests from rustdoc-gui) Failed merges: - #152191 (Convert to inline diagnostics in `rustc_hir_analysis`)

Rollup merge of #151278 - estebank:issue-108894, r=davidtwco Provide more context on trait bounds being unmet due to imperfect derive When encountering a value that has a borrow checker error where the type was previously moved, when suggesting cloning verify that it is not already being derived. If it is, explain why the `derive(Clone)` doesn't apply: ``` note: if `TypedAddress<T>` implemented `Clone`, you could clone the value --> $DIR/derive-clone-implicit-bound.rs:6:1 | LL | #[derive(Clone, Copy)] | ----- derived `Clone` adds implicit bounds on type parameters LL | pub struct TypedAddress<T>{ | ^^^^^^^^^^^^^^^^^^^^^^^^-^ | | | | | introduces an implicit `T: Clone` bound | consider manually implementing `Clone` for this type ... LL | let old = self.return_value(offset); | ------ you could clone this value ``` When encountering a bound coming from a derive macro, suggest manual impl of the trait. Use the span for the specific param when adding bounds in builtin derive macros, so the diagnostic will point at them as well as the derive macro itself. ``` note: required for `Id<SomeNode>` to implement `PartialEq` --> $DIR/derive-implicit-bound.rs:5:10 | LL | #[derive(PartialEq, Eq)] | ^^^^^^^^^ LL | pub struct Id<T>(PhantomData<T>); | - unsatisfied trait bound introduced in this `derive` macro = help: consider manually implementing `PartialEq` to avoid undesired bounds ``` Mention that the trait could be manually implemented in E0599. Fix #108894. Address #143714. Address ##146515 (but ideally would also suggest constraining the fn bound correctly as well).

rustbot assigned davidtwco Jan 18, 2026

rustbot added 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 Jan 18, 2026

estebank force-pushed the issue-108894 branch from 023f134 to 167ae23 Compare January 18, 2026 02:44

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fmease mentioned this pull request Jan 18, 2026

Do not suggest derive if there is already an impl #150720

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rustbot added the WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) label Jan 18, 2026

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davidtwco reviewed Jan 19, 2026

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estebank commented Jan 20, 2026

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tests/ui/consts/const-blocks/trait-error.stderr Outdated Show resolved Hide resolved

estebank added 10 commits February 1, 2026 18:20

Mention that the trait could be manually impld in E0599

b88fb8a

Keep span of type in builtin derive macros expansions

aebf1fd

Fix incorrect suggestion caused by change in derive expansion Span

b9e5cf7

Tweak span labels

dd81483

Point at destructor when denying deriving Copy

20000ca

fix typo

23b766c

estebank added 6 commits February 1, 2026 18:20

reword span label

6c8eee8

Change note to help

879633f

Tweak span in E0599

8543404

Tweak help to unify formatting and wording

dffec20

Do not suggest manual impl Copy

c95210f

fix test after rebase

6756561

estebank force-pushed the issue-108894 branch from 8a7ea29 to 6756561 Compare February 1, 2026 18:31

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 5, 2026

JonathanBrouwer mentioned this pull request Feb 5, 2026

Rollup of 9 pull requests #152181

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JonathanBrouwer mentioned this pull request Feb 5, 2026

Rollup of 11 pull requests #152185

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jieyouxu mentioned this pull request Feb 6, 2026

Rollup of 4 pull requests #152200

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rust-bors bot merged commit 7b821d1 into rust-lang:main Feb 6, 2026
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