When single impl can satisfy inference error, suggest type by estebank · Pull Request #153727 · rust-lang/rust

estebank · 2026-03-11T16:39:20Z

When encountering an inference error where a return type must be known, like when calling Iterator::sum::<T>() without specifying T, look for all T that would satisfy Sum<S>. If only one, suggest it.

error[E0283]: type annotations needed
  --> $DIR/cannot-infer-iterator-sum-return-type.rs:4:9
   |
LL |     let sum = v
   |         ^^^
...
LL |         .sum(); // `sum::<T>` needs `T` to be specified
   |          --- type must be known at this point
   |
   = note: cannot satisfy `_: Sum<i32>`
help: the trait `Sum` is implemented for `i32`
  --> $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL
  ::: $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL
   |
   = note: in this macro invocation
note: required by a bound in `std::iter::Iterator::sum`
  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
   = note: this error originates in the macro `integer_sum_product` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider giving `sum` an explicit type, where the type for type parameter `S` is specified
   |
LL |     let sum: i32 = v
   |            +++++

Fix #100802.

rustbot · 2026-03-11T16:39:23Z

Some changes occurred in need_type_info.rs

cc @lcnr

rustbot · 2026-03-11T16:39:26Z

r? @JohnTitor

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Owners of files modified in this PR: compiler, types
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estebank · 2026-03-11T16:41:38Z

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

+            let mut specific_candidates = candidates.clone();
+            specific_candidates.retain(|(tr, _)| {
+                tr.with_replaced_self_ty(self.tcx, trait_pred.skip_binder().self_ty())
+                    == trait_pred.skip_binder().trait_ref
+            });
+            if !specific_candidates.is_empty() {
+                // We have found a subset of impls that fully satisfy the expected trait, only
+                // mention those types.
+                candidates = specific_candidates;
+            }


This gives us the

help: the trait `Sum` is implemented for `i32` --> library/core/src/iter/traits/accum.rs:48:9 | 48 | impl Sum for $a { | ^^^^^^^^^^^^^^^ ... 204 | integer_sum_product! { i8 i16 i32 i64 i128 isize u8 u16 u32 u64 u128 usize } | ---------------------------------------------------------------------------- in this macro invocation

instead of

= help: the following types implement trait `Sum<A>`: `Duration` implements `Sum<&'a Duration>` `Duration` implements `Sum` `Option<T>` implements `Sum<Option<U>>` `Result<T, E>` implements `Sum<Result<U, E>>` `Saturating<u128>` implements `Sum<&'a Saturating<u128>>` `Saturating<u128>` implements `Sum` `Saturating<u16>` implements `Sum<&'a Saturating<u16>>` `Saturating<u16>` implements `Sum` and 88 others

JohnTitor · 2026-03-13T11:55:23Z

compiler/rustc_trait_selection/src/error_reporting/infer/need_type_info.rs

+                if let Some(t) = self.ty
+                    && ty.has_infer()
+                {
+                    ty = t;


IIUC the logic on ambiguity.rs doesn't check what's the local type at all, right?
I wonder sometimes (inferred) self type isn't same as the local type when it has an outer type unrelated to the trait resolution, e.g. Box<T>, so overwriting seems a bit risky.
What's happen on suggestions for such a case?

Note that the overwriting is purely local and affecting the diagnostic: we're changing what will be included in the LetBinding ty field below not affecting the type associated to local.hir_id, for that we'd have to go through typeck.adjustments_mut().

I see, thank you for clarifying!

When encountering an inference error where a return type must be known, like when calling `Iterator::sum::<T>()` without specifying `T`, look for all `T` that would satisfy `Sum<S>`. If only one, suggest it. ``` error[E0283]: type annotations needed --> $DIR/cannot-infer-iterator-sum-return-type.rs:4:9 | LL | let sum = v | ^^^ ... LL | .sum(); // `sum::<T>` needs `T` to be specified | --- type must be known at this point | = note: cannot satisfy `_: Sum<i32>` help: the trait `Sum` is implemented for `i32` --> $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL ::: $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL | = note: in this macro invocation note: required by a bound in `std::iter::Iterator::sum` --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL = note: this error originates in the macro `integer_sum_product` (in Nightly builds, run with -Z macro-backtrace for more info) help: consider giving `sum` an explicit type, where the type for type parameter `S` is specified | LL | let sum: i32 = v | +++++ ```

rustbot · 2026-03-16T18:43:31Z

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.

JohnTitor

LGTM! @bors r+

View changes since this review

rust-bors · 2026-03-17T21:55:01Z

📌 Commit 42709d4 has been approved by JohnTitor

It is now in the queue for this repository.

rust-bors · 2026-03-18T03:23:22Z

⌛ Testing commit 42709d4 with merge 67c31c2...

Workflow: https://github.com/rust-lang/rust/actions/runs/23227589667

When single impl can satisfy inference error, suggest type When encountering an inference error where a return type must be known, like when calling `Iterator::sum::<T>()` without specifying `T`, look for all `T` that would satisfy `Sum<S>`. If only one, suggest it. ``` error[E0283]: type annotations needed --> $DIR/cannot-infer-iterator-sum-return-type.rs:4:9 | LL | let sum = v | ^^^ ... LL | .sum(); // `sum::<T>` needs `T` to be specified | --- type must be known at this point | = note: cannot satisfy `_: Sum<i32>` help: the trait `Sum` is implemented for `i32` --> $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL ::: $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL | = note: in this macro invocation note: required by a bound in `std::iter::Iterator::sum` --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL = note: this error originates in the macro `integer_sum_product` (in Nightly builds, run with -Z macro-backtrace for more info) help: consider giving `sum` an explicit type, where the type for type parameter `S` is specified | LL | let sum: i32 = v | +++++ ``` Fix #100802.

When single impl can satisfy inference error, suggest type When encountering an inference error where a return type must be known, like when calling `Iterator::sum::<T>()` without specifying `T`, look for all `T` that would satisfy `Sum<S>`. If only one, suggest it. ``` error[E0283]: type annotations needed --> $DIR/cannot-infer-iterator-sum-return-type.rs:4:9 | LL | let sum = v | ^^^ ... LL | .sum(); // `sum::<T>` needs `T` to be specified | --- type must be known at this point | = note: cannot satisfy `_: Sum<i32>` help: the trait `Sum` is implemented for `i32` --> $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL ::: $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL | = note: in this macro invocation note: required by a bound in `std::iter::Iterator::sum` --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL = note: this error originates in the macro `integer_sum_product` (in Nightly builds, run with -Z macro-backtrace for more info) help: consider giving `sum` an explicit type, where the type for type parameter `S` is specified | LL | let sum: i32 = v | +++++ ``` Fix rust-lang#100802.

Zalathar · 2026-03-18T03:24:59Z

@bors yield (to enclosing rollup)

rust-bors · 2026-03-18T03:25:02Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/23227589667

The next pull request likely to be tested is #154030.

Rollup of 3 pull requests Successful merges: - #153727 (When single impl can satisfy inference error, suggest type) - #153998 (Move query-stack-frame spans into `QueryStackFrame`) - #154026 (Remove unused types `UnusedGenericParams` and `FiniteBitSet`)

rust-bors · 2026-03-18T03:55:53Z

⌛ Testing commit 42709d4 with merge 3047ff7...

Workflow: https://github.com/rust-lang/rust/actions/runs/23228345838

When single impl can satisfy inference error, suggest type When encountering an inference error where a return type must be known, like when calling `Iterator::sum::<T>()` without specifying `T`, look for all `T` that would satisfy `Sum<S>`. If only one, suggest it. ``` error[E0283]: type annotations needed --> $DIR/cannot-infer-iterator-sum-return-type.rs:4:9 | LL | let sum = v | ^^^ ... LL | .sum(); // `sum::<T>` needs `T` to be specified | --- type must be known at this point | = note: cannot satisfy `_: Sum<i32>` help: the trait `Sum` is implemented for `i32` --> $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL ::: $SRC_DIR/core/src/iter/traits/accum.rs:LL:COL | = note: in this macro invocation note: required by a bound in `std::iter::Iterator::sum` --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL = note: this error originates in the macro `integer_sum_product` (in Nightly builds, run with -Z macro-backtrace for more info) help: consider giving `sum` an explicit type, where the type for type parameter `S` is specified | LL | let sum: i32 = v | +++++ ``` Fix #100802.

Zalathar · 2026-03-18T04:08:31Z

Failed in rollup: #154030 (comment)

@bors r-

rust-bors · 2026-03-18T04:08:38Z

This pull request was unapproved.

This PR was contained in a rollup (#154030), which was unapproved.

Auto build was cancelled due to unapproval. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/23228345838

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 Mar 11, 2026

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estebank commented Mar 11, 2026

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estebank force-pushed the issue-100802 branch from b0bf031 to 5a9406d Compare March 11, 2026 18:34

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estebank force-pushed the issue-100802 branch from 5a9406d to 42709d4 Compare March 16, 2026 18:43

JohnTitor approved these changes Mar 17, 2026

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Zalathar mentioned this pull request Mar 18, 2026

Rollup of 3 pull requests #154030

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