Apparent duplicated diagnostic note for type mismatch involving type with defaulted type parameter.

[zachs18]

Given the following code: (playground)

pub struct DefaultedParam<A = ()>(A);

fn takes_default_param(_: DefaultedParam) {}
fn calls_default_param_with_ref(x: &DefaultedParam) {
    takes_default_param(x); // (apparent) diagnostic duplication
}
fn returns_default_param(x: DefaultedParam) -> DefaultedParam {
    x; // (apparent) diagnostic duplication
}

pub struct WithParam<A>(A);

fn takes_with_param(_: WithParam<()>) {}
fn calls_with_param_with_ref(x: &WithParam<()>) {
    takes_with_param(x); // somewhat diagnostic duplication
}
fn returns_with_param(x: WithParam<()>) -> WithParam<()> {
    x; // somewhat diagnostic duplication
}

pub struct NoParam;

fn takes_no_param(_: NoParam) {}
fn calls_no_param_with_ref(x: &NoParam) {
    takes_no_param(x); // no diagnostic duplication
}
fn returns_no_param(x: NoParam) -> NoParam {
    x; // no diagnostic duplication
}

The current output is:

Current output

(*_no_param show the ideal diagnostics, *_default_param show cases where the diagnostic appears to the user to be completely duplicated, *_with_param show cases where the diagnostic appears to the user to be somewhat duplicated)

   Compiling rust-default-type-param-changes-diagnostics v0.1.0 (/home/zachary/Programming/mwe/rust-default-type-param-changes-diagnostics)
error[E0308]: mismatched types
 --> src/lib.rs:5:25
  |
5 |     takes_default_param(x); // (apparent) diagnostic duplication
  |     ------------------- ^ expected struct `DefaultedParam`, found `&DefaultedParam`
  |     |
  |     arguments to this function are incorrect
  |
  = note: expected struct `DefaultedParam`
          found reference `&DefaultedParam`
note: function defined here
 --> src/lib.rs:3:4
  |
3 | fn takes_default_param(_: DefaultedParam) {}
  |    ^^^^^^^^^^^^^^^^^^^ -----------------

error[E0308]: mismatched types
 --> src/lib.rs:7:48
  |
7 | fn returns_default_param(x: DefaultedParam) -> DefaultedParam {
  |    ---------------------                       ^^^^^^^^^^^^^^ expected struct `DefaultedParam`, found `()`
  |    |
  |    implicitly returns `()` as its body has no tail or `return` expression
8 |     x; // (apparent) diagnostic duplication
  |      - help: remove this semicolon
  |
  = note: expected struct `DefaultedParam`
          found unit type `()`

error[E0308]: mismatched types
  --> src/lib.rs:15:22
   |
15 |     takes_with_param(x); // somewhat diagnostic duplication
   |     ---------------- ^ expected struct `WithParam`, found `&WithParam<()>`
   |     |
   |     arguments to this function are incorrect
   |
   = note: expected struct `WithParam<()>`
           found reference `&WithParam<()>`
note: function defined here
  --> src/lib.rs:13:4
   |
13 | fn takes_with_param(_: WithParam<()>) {}
   |    ^^^^^^^^^^^^^^^^ ----------------

error[E0308]: mismatched types
  --> src/lib.rs:17:44
   |
17 | fn returns_with_param(x: WithParam<()>) -> WithParam<()> {
   |    ------------------                      ^^^^^^^^^^^^^ expected struct `WithParam`, found `()`
   |    |
   |    implicitly returns `()` as its body has no tail or `return` expression
18 |     x; // somewhat diagnostic duplication
   |      - help: remove this semicolon
   |
   = note: expected struct `WithParam<()>`
           found unit type `()`

error[E0308]: mismatched types
  --> src/lib.rs:25:20
   |
25 |     takes_no_param(x); // no diagnostic duplication
   |     -------------- ^ expected struct `NoParam`, found `&NoParam`
   |     |
   |     arguments to this function are incorrect
   |
note: function defined here
  --> src/lib.rs:23:4
   |
23 | fn takes_no_param(_: NoParam) {}
   |    ^^^^^^^^^^^^^^ ----------

error[E0308]: mismatched types
  --> src/lib.rs:27:36
   |
27 | fn returns_no_param(x: NoParam) -> NoParam {
   |    ----------------                ^^^^^^^ expected struct `NoParam`, found `()`
   |    |
   |    implicitly returns `()` as its body has no tail or `return` expression
28 |     x; // no diagnostic duplication
   |      - help: remove this semicolon

For more information about this error, try `rustc --explain E0308`.
error: could not compile `rust-default-type-param-changes-diagnostics` due to 6 previous errors

Ideally the output should look like:

Ideal output (diff)

Essentailly, have the diagnostics for calls_default_param_with_ref and calls_with_param_with_ref be the same as calls_no_param_with_ref, and the diagnostics for returns_default_param and returns_with_param be the same as returns_no_param (with the corresponding types).

(The diagnostics for *_with_param are technically not duplicated, since the note does give more information (the type parameter) that is not given by the error message. I have included them here since they are still mostly duplicated when all types involved are known, but it's debatable.)

--- err.out	2022-09-18 13:17:40.476817923 -0500
+++ better_err.out	2022-09-17 17:49:25.510427120 -0500
@@ -1,85 +1,75 @@
    Compiling rust-default-type-param-changes-diagnostics v0.1.0 (/home/zachary/Programming/mwe/rust-default-type-param-changes-diagnostics)
 error[E0308]: mismatched types
  --> src/lib.rs:5:25
   |
 5 |     takes_default_param(x); // (apparent) diagnostic duplication
   |     ------------------- ^ expected struct `DefaultedParam`, found `&DefaultedParam`
   |     |
   |     arguments to this function are incorrect
   |
-  = note: expected struct `DefaultedParam`
-          found reference `&DefaultedParam`
 note: function defined here
  --> src/lib.rs:3:4
   |
 3 | fn takes_default_param(_: DefaultedParam) {}
   |    ^^^^^^^^^^^^^^^^^^^ -----------------
 
 error[E0308]: mismatched types
  --> src/lib.rs:7:48
   |
 7 | fn returns_default_param(x: DefaultedParam) -> DefaultedParam {
   |    ---------------------                       ^^^^^^^^^^^^^^ expected struct `DefaultedParam`, found `()`
   |    |
   |    implicitly returns `()` as its body has no tail or `return` expression
 8 |     x; // (apparent) diagnostic duplication
   |      - help: remove this semicolon
-  |
-  = note: expected struct `DefaultedParam`
-          found unit type `()`
 
 error[E0308]: mismatched types
   --> src/lib.rs:15:22
    |
 15 |     takes_with_param(x); // somewhat diagnostic duplication
-   |     ---------------- ^ expected struct `WithParam`, found `&WithParam<()>`
+   |     ---------------- ^ expected struct `WithParam<()>`, found `&WithParam<()>`
    |     |
    |     arguments to this function are incorrect
    |
-   = note: expected struct `WithParam<()>`
-           found reference `&WithParam<()>`
 note: function defined here
   --> src/lib.rs:13:4
    |
 13 | fn takes_with_param(_: WithParam<()>) {}
    |    ^^^^^^^^^^^^^^^^ ----------------
 
 error[E0308]: mismatched types
   --> src/lib.rs:17:44
    |
 17 | fn returns_with_param(x: WithParam<()>) -> WithParam<()> {
-   |    ------------------                      ^^^^^^^^^^^^^ expected struct `WithParam`, found `()`
+   |    ------------------                      ^^^^^^^^^^^^^ expected struct `WithParam<()>`, found `()`
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
 18 |     x; // somewhat diagnostic duplication
    |      - help: remove this semicolon
-   |
-   = note: expected struct `WithParam<()>`
-           found unit type `()`
 
 error[E0308]: mismatched types
   --> src/lib.rs:25:20
    |
 25 |     takes_no_param(x); // no diagnostic duplication
    |     -------------- ^ expected struct `NoParam`, found `&NoParam`
    |     |
    |     arguments to this function are incorrect
    |
 note: function defined here
   --> src/lib.rs:23:4
    |
 23 | fn takes_no_param(_: NoParam) {}
    |    ^^^^^^^^^^^^^^ ----------
 
 error[E0308]: mismatched types
   --> src/lib.rs:27:36
    |
 27 | fn returns_no_param(x: NoParam) -> NoParam {
    |    ----------------                ^^^^^^^ expected struct `NoParam`, found `()`
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
 28 |     x; // no diagnostic duplication
    |      - help: remove this semicolon
 
 For more information about this error, try `rustc --explain E0308`.
 error: could not compile `rust-default-type-param-changes-diagnostics` due to 6 previous errors

Explanation: When there is a type mismatch where the error message already completely describes the two types, no note is printed (see the NoParam functions above)¹. When a type mismatch occurs where the error message does not already completely describe the two types, a note is printed with the two complete types (see the WithParam functions above).

In the case where the only type parameter of a type has a default, and that default is used, this causes the note that is printed to contain the "same" types as the error message².

(The diagnostics for *_with_param are technically not duplicated, since the note does give more information (the type parameter) that is not given by the error message. I have included them here since they are still mostly duplicated when all types involved are known, but it's debatable.)

---

I found this difference when adding a defaulted allocator type parameter to String in #101551, which added these notes to the outputs of a lot of UI tests.

The results are the same in all editions, and on stable and nightly (and 1.41.0, but not 1.40.0, which prints the duplicate note even for NoParam ¹).

Rust version

Stable:

rustc 1.63.0 (4b91a6ea7 2022-08-08)
binary: rustc
commit-hash: 4b91a6ea7258a947e59c6522cd5898e7c0a6a88f
commit-date: 2022-08-08
host: x86_64-unknown-linux-gnu
release: 1.63.0
LLVM version: 14.0.5

Nightly:

rustc 1.65.0-nightly (750bd1a7f 2022-09-14)
binary: rustc
commit-hash: 750bd1a7ff3e010611b97ee75d30b7cbf5f3a03c
commit-date: 2022-09-14
host: x86_64-unknown-linux-gnu
release: 1.65.0-nightly
LLVM version: 15.0.0

Footnotes

1. Note that this was introduced 1.41.0: (godbolt.org link). Previously, the note with duplicate information appears to have always been printed. ↩ ↩²

2. I say "same" because what is printed is the same, but I think the error messages for *_default_param are really saying expected DefaultParam<_>, found ..., and the notes are saying expected DefaultParam<()>, found .... ↩