Incorrect suggestion for function parameter of type str

fn foo(bar: str) {}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `str` cannot be known at compilation time
 --> src/lib.rs:1:8
  |
1 | fn foo(bar: str) {}
  |        ^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `str`
help: function arguments must have a statically known size, borrowed types always have a known size
  |
1 | fn foo(&bar: str) {}
  |        ^

error: aborting due to previous error

Applying the suggestion leads to yet more errors. The new errors do suggest using &str, but they still suggest invalid code:

error[E0308]: mismatched types
 --> src/lib.rs:1:8
  |
1 | fn foo(&bar: str) {}
  |        ^^^^-----
  |        |     |
  |        |     expected due to this
  |        expected `str`, found reference
  |        help: did you mean `bar`: `&str`
  |
  = note:   expected type `str`
          found reference `&_`

error[E0277]: the size for values of type `str` cannot be known at compilation time
 --> src/lib.rs:1:8
  |
1 | fn foo(&bar: str) {}
  |        ^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `str`
help: function arguments must have a statically known size, borrowed types always have a known size
  |
1 | fn foo(&bar: &str) {}
  |              ^

error: aborting due to 2 previous errors

After applying the new suggestions, I get:

error[E0277]: the size for values of type `str` cannot be known at compilation time
 --> src/lib.rs:1:9
  |
1 | fn foo(&bar: &str) {}
  |         ^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `str`
  = note: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature

error: aborting due to previous error

This seems likely to send new users in circles.

Assigning P-medium and removing I-prioritize as discussed in the prioritization working group.

I've bisected to this nightly range:

+nightly-2020-07-12-x86_64-unknown-linux-gnu 346aec9b0 => old error msg
+nightly-2020-07-26-x86_64-unknown-linux-gnu d6953df14 => new error message

(I cannot bisect further because in between of that range I cannot install nightlies 🤷‍♂️ )

Funny enough, when using _ as variable name, the suggestion will get displayed correctly:

fn f(a: str) {}
fn g(_: str) {}

   Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `str` cannot be known at compilation time
 --> src/lib.rs:1:6
  |
1 | fn f(a: str) {}
  |      ^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `str`
help: function arguments must have a statically known size, borrowed types always have a known size
  |
1 | fn f(&a: str) {}
  |      ^

error[E0277]: the size for values of type `str` cannot be known at compilation time
 --> src/lib.rs:2:6
  |
2 | fn g(_: str) {}
  |      ^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `str`
help: function arguments must have a statically known size, borrowed types always have a known size
  |
2 | fn g(_: &str) {}

@camelid

This seems to be fixed on nighlty, but not on beta...

somewhere between 5a4ab26...9d9c2c9

Fixed in 9d9c2c9

Fixed by #84313

So this can be closed @camelid 🎉

But, as a sidenote: How could this ever be commited to master? I mean, it's obviosly wrong so say &a: str :/

I hope, that in the future the review process can be improved to detect such errors early on.

The error was introduced in #78152, where the test has been adapted to match the wrong syntax, but nobody has seen this :( Let's hope in the future it will get noticed right away 😊

The error was introduced in #78152, where the test has been adapted to match the wrong syntax, but nobody has seen this :( Let's hope in the future it will get noticed right away 😊

It looks like that PR caused a bunch of small diagnostics changes, so the reviewers probably just missed that bug.

@rustbot

Assigning myself for adding a test. @rustbot claim

Rollup merge of rust-lang#84728 - camelid:sized-param-sugg-test, r=Ma…

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Incorrect suggestion for function parameter of type `str` #82820

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Incorrect suggestion for function parameter of type str #82820

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camelid commented on Mar 6, 2021

apiraino commented on Mar 6, 2021

hellow554 commented on Apr 28, 2021

hellow554 commented on Apr 28, 2021

hellow554 commented on Apr 28, 2021

camelid commented on Apr 29, 2021

camelid commented on Apr 29, 2021

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Incorrect suggestion for function parameter of type `str` #82820