Detect `;` -> `,` typo on `vec!["_", 10]`

[estebank]

Only when writing vec!["_", 10] we could suggest replacing the , with ;.

[osa1]

Is it possible to do this in a declarative macro?

I wonder if we could have a "peephole warning/suggestion generator" for this kind of thing where we pattern match in terms in different contexts (e.g. types, expressions, items, perhaps with the errors in the spans) and suggest alternatives.

[estebank]

Today it is possible to do this in a proc-macro (by explicitly looking for cases), and in the compiler (rustc today when encountering a macro call that doesn't match any arm will look for places to add a comma token and see if that would match any macro arm. If so, it suggests the comma in the right place). In declarative macros we don't have a good way of doing this, other than maybe using std::compile_error as shown in its docs.

Edit before submitting: Actually, this has to be in the compiler because macros do not have access to the type information, which you need to properly handle this. The first field can be anything that is Copy, while the second field has to be an integer literal or usize.

[henryboisdequin]

@estebank Would this help message only be emitted when the code fails and/or when the program compiles successfully (if there is a way to do such a thing)?

[estebank]

Only on error. The only "suspect" cases would be like vec![1, 42] and at that point that is fine, there are plenty of valid reasons for that code to be written. Only in cases where there's an error being emitted it makes sense to help, as the typo might not be obvious.

[henryboisdequin]

@rustbot claim

[henryboisdequin]

@estebank Do you know roughly where I need to search for cases such as this one? Thanks

[veera-sivarajan]

@rustbot claim