Diesel stops building with nightly-2025-12-10

[weiznich]

Code

I tried this code:

git clone https://github.com/diesel-rs/diesel
cd diesel
cargo check -F postgres --all-targets -p diesel_tests

I expected to see this happen: Code builds without errors
Instead, this happened: Compilation fails with errors

Version it worked on

It most recently worked on:

Version with regression

rustc --version --verbose:

rustc 1.94.0-nightly (c61a3a44d 2025-12-09)
binary: rustc
commit-hash: c61a3a44d1a5bee35914cada6c788a05e0808f5b
commit-date: 2025-12-09
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.5

bisec-rustc points to c61a3a4 (so #147984) for the commit/PR that introduced this regression.

The error message points to #114095
Unfortunately I see no meaningful way for diesel to address this without doing a significant breaking change. To address this without breaking change on our side it would be required to have tooling that lets us import only from a specific namespace (type, function, etc) if several items with the same name exists.

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

[bjorn3]

The actual error message as taken from https://github.com/diesel-rs/diesel/actions/runs/20098057029/job/57661673455?pr=4900:

error[E0252]: the name `max` is defined multiple times
   --> diesel_tests/tests/select_by.rs:170:9
    |
169 |     use diesel::dsl::{count_star, max, CountStar};
    |                                   --- previous import of the type `max` here
170 |     use diesel::helper_types::max;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^ `max` reimported here
    |
    = note: `max` must be defined only once in the type namespace of this block

error: `max` is ambiguous
   --> diesel_tests/tests/group_by.rs:33:22
    |
 33 |     use diesel::dsl::max;
    |                      ^^^ ambiguous name
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #114095 <https://github.com/rust-lang/rust/issues/114095>
    = note: ambiguous because of multiple glob imports of a name in the same module
note: `max` could refer to the type alias defined here
   --> /home/runner/work/diesel/diesel/diesel/src/lib.rs:341:13
    |
341 |     pub use crate::helper_types::*;
    |             ^^^^^^^^^^^^^^^^^^^
note: `max` could also refer to the type alias defined here
   --> /home/runner/work/diesel/diesel/diesel/src/lib.rs:344:13
    |
344 |     pub use crate::expression::dsl::*;
    |             ^^^^^^^^^^^^^^^^^^^^^^
    = note: `#[deny(ambiguous_glob_imports)]` (part of `#[deny(future_incompatible)]`) on by default

error: `max` is ambiguous
   --> diesel_tests/tests/select.rs:519:35
    |
519 |     use diesel::dsl::{count_star, max};
   --> /home/runner/work/diesel/diesel/diesel/src/lib.rs:344:13
    |
344 |     pub use crate::expression::dsl::*;
    |             ^^^^^^^^^^^^^^^^^^^^^^

error: `nth_value` is ambiguous
   --> diesel_derives/tests/auto_type.rs:722:9
    |
722 |         nth_value(users::id, 1_i32).over(),
    |         ^^^^^^^^^ ambiguous name
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #114095 <https://github.com/rust-lang/rust/issues/114095>
    = note: ambiguous because of multiple glob imports of a name in the same module
note: `nth_value` could refer to the type alias defined here
   --> /home/runner/work/diesel/diesel/diesel/src/lib.rs:341:13
    |
341 |     pub use crate::helper_types::*;
    |             ^^^^^^^^^^^^^^^^^^^
note: `nth_value` could also refer to the type alias defined here
   --> /home/runner/work/diesel/diesel/diesel/src/lib.rs:344:13
    |
344 |     pub use crate::expression::dsl::*;
    |             ^^^^^^^^^^^^^^^^^^^^^^

warning: unused import: `diesel::helper_types::*`
 --> diesel_derives/tests/auto_type.rs:3:5
  |
3 | use diesel::helper_types::*;
  |     ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

warning: `diesel_derives` (test "tests") generated 1 warning
error: could not compile `diesel_derives` (test "tests") due to 26 previous errors; 1 warning emitted
warning: build failed, waiting for other jobs to finish...
For more information about this error, try `rustc --explain E0252`.
warning: `diesel_tests` (test "integration_tests") generated 1 warning
error: could not compile `diesel_tests` (test "integration_tests") due to 9 previous errors; 1 warning emitted

[lqd]

cc @petrochenkov on that bisection

[petrochenkov]

From 74 regressions in #147984 (comment) (due to ambiguous_glob_imports being deny-by-default) a significant part is indeed from depending on diesel diesel::dsl::{max,sum,/*etc*/}.

I wondered why this hazard in diesel wasn't detected before, and it turns out it was, but diesel does #[allow(ambiguous_glob_reexports)] to ignore it.

[petrochenkov]

I'm trying to understand what's going on, but it's hard because diesel uses proc macros to generate code.

Why do both crate::helper_types::* and crate::expression::dsl::* contain fn max, but different ones?
The compiler doesn't know which one to choose -> ambiguous_glob_imports is reported, this is a legitimate error.
Which one is supposed to be chosen?

it would be required to have tooling that lets us import only from a specific namespace (type, function, etc) if several items with the same name exists

FWIW, the tooling exists, it's just not pretty.
I could make a fix if I understood what's going on there.

[petrochenkov]

Possible fix (at least the reproducer command line passes).
Single imports can be used to explicitly disambiguate the crate::helper_types::* vs crate::expression::dsl::* situation.

diff --git i/diesel/src/lib.rs w/diesel/src/lib.rs
index d6e26dc52..2dba34a76 100644
--- i/diesel/src/lib.rs
+++ w/diesel/src/lib.rs
@@ -339,6 +339,7 @@ pub mod dsl {

     #[doc(inline)]
     pub use crate::helper_types::*;
+    pub use crate::helper_types::{max, array_append};

     #[doc(inline)]
     pub use crate::expression::dsl::*;
diff --git i/diesel_tests/tests/select_by.rs w/diesel_tests/tests/select_by.rs
index 6adaf9717..554f5f27b 100644
--- i/diesel_tests/tests/select_by.rs
+++ w/diesel_tests/tests/select_by.rs
@@ -167,7 +167,6 @@ fn select_can_be_called_on_query_that_is_valid_subselect_but_invalid_query() {
 fn selecting_multiple_aggregate_expressions_without_group_by() {
     use self::users::dsl::*;
     use diesel::dsl::{count_star, max, CountStar};
-    use diesel::helper_types::max;

     #[derive(Queryable)]
     struct CountAndMax {