bad diagnostic suggestion for self access in a format string

[ehuss]

Given the following code:

struct Foo {
    value: i32
}

impl Foo {
    fn f(&self) -> String {
        format!("{value}")
    }
}

The current output is:

error[E0425]: cannot find value `value` in this scope
 --> src/lib.rs:7:19
  |
7 |         format!("{value}")
  |                   ^^^^^ help: you might have meant to use the available field: `self.value`

Applying the suggestion doesn't work since format strings don't allow non-identifier expressions. Fortunately applying the suggestion and compiling again will lead to another suggestion with the correct syntax. Ideally the original diagnostic should provide the correct syntax of format!("{}", self.value)

[chenyukang]

Similar issue with #104884

@estebank
We may have bad diagnostic suggestions for macro code, this may happened in many similar reports.
Do we have some centralized way to handle it well?

[estebank]

@chenyukang for macros in general we do get quite a few issues around spans, particularly for bindings that go through some transformation inside the macro (like borrowing) making us point at the whole macro instead of the binding directly. For this case in particular, we can focus only on the behavior of the format! macros. I believe fixing #96999 would be enough to mitigate the issue here, but it should still be fixed independently (not suggest self.value, instead suggest adding a new named arg).

[jendrikw]

Current output is even more explicit about the wrong suggestion.

error[E0425]: cannot find value `value` in this scope
  --> src/lib.rs:39:52
  |
7 |         format!("{value}")
  |                   ^^^^^
  |
help: you might have meant to use the available field
  |
7 |         format!("{self.value}")
  |                   +++++

[estebank]

@jendrikw do note that when writing that you get

error: invalid format string: field access isn't supported
 --> src/lib.rs:9:19
  |
9 |         format!("{self.value}")
  |                   ^^^^^^^^^^ not supported in format string
  |
help: consider using a positional formatting argument instead
  |
9 |         format!("{0}", self.value)
  |                   ~  ++++++++++++

We consider suggestions that get you towards working code to be fine. We do fix invalid suggestions when possible, but if there are technical reasons that make fixing a suggestion that gets you a subsequent correct suggestion, we allow ourselves to be incorrect in the short term.