Long type name emitted if the type name is part of a suggestion

[weiznich]

Code

use diesel::prelude::*; //diesel = { version = "2.2.6", default-features = false, features = ["postgres"] }

diesel::table! {
    foo (id) {
        id -> Int4,
        name -> Text,
    }
}

fn main() {
    diesel::debug_query(
        &foo::table
            .filter(foo::id.eq(42))
            .order(foo::name.desc())
            .limit(10)
            .offset(42),
    );
}

Current output

error[E0282]: type annotations needed
  --> src/main.rs:11:5
   |
11 |     diesel::debug_query(
   |     ^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `DB` declared on the function `debug_query`
   |
help: consider specifying the generic arguments
   |
11 |     diesel::debug_query::<DB, SelectStatement<FromClause<table>, query_builder::select_clause::DefaultSelectClause<FromClause<table>>, query_builder::distinct_clause::NoDistinctClause, query_builder::where_clause::WhereClause<diesel::expression::grouped::Grouped<diesel::expression::operators::Eq<columns::id, diesel::expression::bound::Bound<Integer, i32>>>>, query_builder::order_clause::OrderClause<diesel::expression::operators::Desc<columns::name>>, LimitOffsetClause<LimitClause<diesel::expression::bound::Bound<BigInt, i64>>, OffsetClause<diesel::expression::bound::Bound<BigInt, i64>>>>>(
   |                        ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Desired output

error[E0282]: type annotations needed
  --> src/main.rs:11:5
   |
11 |     diesel::debug_query(
   |     ^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `DB` declared on the function `debug_query`
   |
help: consider specifying the generic arguments
   |
11 |     diesel::debug_query::<DB, _>(
   |                           ++ 

Rationale and extra context

Emitting the query type there is not helpful for the users as it is long, makes the error message harder to read and ultimately is not even required to fix the code snippet. Fixing the code snippet only requires to specify the DB generic type there. Ideally rustc would be able to suggest possible types for DB there, given the existing constraint on debug_query. In the example above that's even only satisfied by one type (diesel::pg::Pg).

Other cases

If you change the exact query that you pass to debug_query the output changes. You can make the query more complex and therefore get a much more complex and longer query type in the compiler output.

Rust Version

rustc 1.86.0-nightly (649b995a9 2025-01-22)
binary: rustc
commit-hash: 649b995a9febd658b2570160703dff6fdc038ab2
commit-date: 2025-01-22
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.7

Anything else?

cc @estebank as they expressed interested in such cases

List of cases

• E0282 suggestion In "specify type" suggestion, skip type params that are already known #135965
• E0308 "this expression" type Use short type string in E0308 secondary span label #135949
• E0277 message Rework "long type names" printing logic #136328
• E0382 "move occurs because binding has type" Use short ty string for move errors #136018
• "missing type for const item" (not really addressable unless we suggest incomplete code)
• E0369 Use short ty string for binop and unop errors #136315
• E0609 Teach structured errors to display short Ty<'_> #137201
• E0618 Teach structured errors to display short Ty<'_> #137201
• E0529 Teach structured errors to display short Ty<'_> #137201
• E0614 Teach structured errors to display short Ty<'_> #137201

[theemathas]

Similar self-contained reproducer:

type A = (i32, i32, i32, i32);
type B = (A, A, A, A);
type C = (B, B, B, B);
type D = (C, C, C, C);

fn foo(x: D) {
    let () = x;
}

Error:

error[E0308]: mismatched types
 --> src/lib.rs:7:9
  |
7 |     let () = x;
  |         ^^   - this expression has type `((((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))))`
  |         |
  |         expected `((..., ..., ..., ...), ..., ..., ...)`, found `()`
  |
  = note:  expected tuple `(((..., ..., ..., ...), ..., ..., ...), ..., ..., ...)`
          found unit type `()`
  = note: the full type name has been written to '/playground/target/debug/deps/playground-392933b699e17266.long-type-8011729504781541518.txt'
  = note: consider using `--verbose` to print the full type name to the console

For more information about this error, try `rustc --explain E0308`.

[theemathas]

Another self-contained reproducer:

type A = (i32, i32, i32, i32);
type B = (A, A, A, A);
type C = (B, B, B, B);
type D = (C, C, C, C);

fn foo(x: D) {
    let y = Err(x);
}

Error:

error[E0282]: type annotations needed for `Result<_, (((..., ..., ..., ...), ..., ..., ...), ..., ..., ...)>`
 --> src/lib.rs:7:9
  |
7 |     let y = Err(x);
  |         ^   ------ type must be known at this point
  |
  = note: the full type name has been written to '/playground/target/debug/deps/playground-6ee3eef46ddd6e99.long-type-14742862580756300638.txt'
help: consider giving `y` an explicit type, where the type for type parameter `T` is specified
  |
7 |     let y: Result<T, ((((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))))> = Err(x);
  |          +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

For more information about this error, try `rustc --explain E0282`.

[estebank]

The first comment is addressed in #135949. The second comment and original concern is addressed in #135965. Note that it doesn't shorten all long types, only the ones where some type parameters can be elided. It should work well enough for most cases, and pathological cases where the entire type is "important" for the suggestion... well, you kind of have no other alternative than spelling out the full type other than maybe break things up.

[theemathas]

Yet another version

type A = (i32, i32, i32, i32);
type B = (A, A, A, A);
type C = (B, B, B, B);
type D = (C, C, C, C);

trait Trait {}

fn require_trait<T: Trait>() {}

fn foo() {
    require_trait::<D>();
}

error[E0277]: the trait bound `((((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)))): Trait` is not satisfied
  --> src/lib.rs:11:21
   |
11 |     require_trait::<D>();
   |                     ^ the trait `Trait` is not implemented for `(((..., ..., ..., ...), ..., ..., ...), ..., ..., ...)`
   |
help: this trait has no implementations, consider adding one
  --> src/lib.rs:6:1
   |
6  | trait Trait {}
   | ^^^^^^^^^^^
note: required by a bound in `require_trait`
  --> src/lib.rs:8:21
   |
8  | fn require_trait<T: Trait>() {}
   |                     ^^^^^ required by this bound in `require_trait`

For more information about this error, try `rustc --explain E0277`.

[theemathas]

Another version

type A = (String, String, String, String);
type B = (A, A, A, A);
type C = (B, B, B, B);
type D = (C, C, C, C);

trait Trait {}

fn require_trait<T: Trait>() {}

fn foo(x: D) {
    let _a = x;
    let _b = x;
}

error[E0382]: use of moved value: `x`
  --> src/lib.rs:12:14
   |
10 | fn foo(x: D) {
   |        - move occurs because `x` has type `((((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String))), (((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String))), (((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String))), (((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String)), ((String, String, String, String), (String, String, String, String), (String, String, String, String), (String, String, String, String))))`, which does not implement the `Copy` trait
11 |     let _a = x;
   |              - value moved here
12 |     let _b = x;
   |              ^ value used here after move
   |
help: consider cloning the value if the performance cost is acceptable
   |
11 |     let _a = x.clone();
   |               ++++++++

For more information about this error, try `rustc --explain E0382`.

[theemathas]

Another version

type A = (i32, i32, i32, i32);
type B = (A, A, A, A);
type C = (B, B, B, B);
type D = (C, C, C, C);

const X = None::<D>;

error: missing type for `const` item
 --> src/lib.rs:6:8
  |
6 | const X = None::<D>;
  |        ^ help: provide a type for the constant: `: Option<((((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))))>`

[estebank]

For the 3rd case in the comments it's going to be hard to address because we don't have an equivalent to short_ty_string for bounds, and trying to replicate the output of bounds printing by taking the trait and the type breaks down when it comes to for lifetimes (they end up in the wrong place). The solution for that is to write a new short_bound_string method that can do that correctly, but it might take a while. For the 5th case right above, I don't think there's anything we can do to make that less ugly, beyond correctly tracking types and using them when possible. type erasure has been a long standing issue that we need to address, but given the vintage of the ticket I wouldn't hold my breath on it being done anytime soon ^_^'