Add a detailed note for missing comma typo w/ FRU syntax

[compiler-errors]

Thanks to @pierwill for working on this with me!

Fixes #104373, perhaps @alice-i-cecile can comment on the new error for the example provided on that issue -- feedback is welcome.

error[E0063]: missing field `defaulted` in initializer of `Outer`
  --> $DIR/multi-line-fru-suggestion.rs:14:5
   |
LL |     Outer {
   |     ^^^^^ missing `defaulted`
   |
note: this expression may have been misinterpreted as a `..` range expression
  --> $DIR/multi-line-fru-suggestion.rs:16:16
   |
LL |           inner: Inner {
   |  ________________^
LL | |             a: 1,
LL | |             b: 2,
LL | |         }
   | |_________^ this expression does not end in a comma...
LL |           ..Default::default()
   |           ^^^^^^^^^^^^^^^^^^^^ ... so this is interpreted as a `..` range expression, instead of functional record update syntax
help: to set the remaining fields from `Default::default()`, separate the last named field with a comma
   |
LL |         }, 
   |          +

error: aborting due to previous error

For more information about this error, try `rustc --explain E0063`.

[rustbot]

r? @lcnr

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[rustbot]

rustc_error_messages was changed

cc @davidtwco, @compiler-errors, @JohnTitor, @estebank, @TaKO8Ki

[alice-i-cecile]

"functional record update syntax" is pretty heavy on jargon (and this is an error beginners will regularly see). Google search for it turns up nothing unless you add Rust, in which case you get this RFC: https://rust-lang.github.io/rfcs/2528-type-changing-struct-update-syntax.html

[alice-i-cecile]

Suggested change

	hir_typeck_fru_expr2 = ... so this is interpreted as a `..` range expression, instead of functional record update syntax
	hir_typeck_fru_expr2 = ... so this is interpreted as a `..` range expression, instead of functional record update syntax, used to set the remaining fields of a struct

[compiler-errors]

This part of the error message is already pretty long, and it doesn't wrap well -- if you're against the jargon, I would rather remove everything after instead of.

[compiler-errors]

I'm also not too keen on the duplication of "used to set the remaining fields of a struct" between the note and the suggestion's "to set the remaining fields"

[alice-i-cecile]

Hmm, I see your point! I think it's better left as is then.

[Noratrieb]

Maybe "struct update syntax"? I've heard that quite often when talking about FRU.

[alice-i-cecile]

As someone new to this part of the codebase, this line is unclear to me. What's s, and why do we filter it like this? Why 25?

[compiler-errors]

This is to avoid rendering very long expressions in "to set the remaining fields in $EXPR"

[compiler-errors]

25 is an arbitrary number.

[compiler-errors]

And is_control is to avoid rendering something like:

to set the remaining fields in `a
.b`

[alice-i-cecile]

Makes sense :) Perhaps leave a comment to this effect?

[compiler-errors]

Sure thing.

[estebank]

r? @estebank
@bors r+

[bors]

📌 Commit b85b47cfd6c17acf21c8e3d6de3f543f3dbef942 has been approved by estebank

It is now in the queue for this repository.

[compiler-errors]

@bors r- i still got some changes to make

[compiler-errors]

@bors r=estebank

[bors]

📌 Commit bb0cb9a has been approved by estebank

It is now in the queue for this repository.