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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estebank · 2026-03-11T16:41:38Z

+            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

+                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!

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

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 · 2026-03-18T18:16:58Z

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.

estebank · 2026-03-18T18:44:48Z

@bors r=JohnTitor

rust-bors · 2026-03-18T18:44:53Z

📌 Commit 49bb371 has been approved by JohnTitor

It is now in the queue for this repository.

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.

…uwer Rollup of 9 pull requests Successful merges: - #153957 (Add bootstrap step for stdarch-verify) - #153727 (When single impl can satisfy inference error, suggest type) - #153308 (Add hygiene annotations for tokens in `macro_rules!` bodies) - #153557 (fix inference variables leaking into HIR const literal lowering logic) - #153913 (Fix some suggestions of the `for-loops-over-fallibles` lint) - #153987 (mGCA: Lower const generic args to infer when needed) - #153992 (bootstrap: Optionally print a backtrace if a command fails) - #154036 (borrowck/type_check: remove helper left-over from unsized locals) - #154038 (merge `regions-outlives-nominal-type-*` tests into one file)

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.

…uwer Rollup of 9 pull requests Successful merges: - #153957 (Add bootstrap step for stdarch-verify) - #153727 (When single impl can satisfy inference error, suggest type) - #153308 (Add hygiene annotations for tokens in `macro_rules!` bodies) - #153557 (fix inference variables leaking into HIR const literal lowering logic) - #153913 (Fix some suggestions of the `for-loops-over-fallibles` lint) - #153987 (mGCA: Lower const generic args to infer when needed) - #153992 (bootstrap: Optionally print a backtrace if a command fails) - #154036 (borrowck/type_check: remove helper left-over from unsized locals) - #154038 (merge `regions-outlives-nominal-type-*` tests into one file)

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.

…uwer Rollup of 19 pull requests Successful merges: - #152870 (Do not enable split debuginfo for windows-gnu) - #153333 (Fix bootstrap rust build failure for vxworks) - #153681 (ci: add runners for vanilla LLVM 22) - #153727 (When single impl can satisfy inference error, suggest type) - #153824 (Add `Wake` diagnostic item for `alloc::task::Wake`) - #154077 (Optimize 128-bit integer formatting) - #154078 (`define_callbacks` tweaks) - #151905 (Split the `dec2flt::RawFloat` trait for easier reuse) - #153170 (Add is_disconnected functions to mpsc and mpmc channels) - #153308 (Add hygiene annotations for tokens in `macro_rules!` bodies) - #153557 (fix inference variables leaking into HIR const literal lowering logic) - #153804 (Derive Macro Eq: link to more detailed documentation) - #153913 (Fix some suggestions of the `for-loops-over-fallibles` lint) - #153974 (Point at return type when it is the source of the type expectation) - #153987 (mGCA: Lower const generic args to infer when needed) - #154018 (simplify and remove `tests/ui/kindck` and related tests) - #154036 (borrowck/type_check: remove helper left-over from unsized locals) - #154038 (merge `regions-outlives-nominal-type-*` tests into one file) - #154041 (bootstrap: Allow `--bless`ing changes to editor settings files)

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