On format!() arg count mismatch provide extra info

[estebank]

When positional width and precision formatting flags are present in a
formatting string that has an argument count mismatch, provide extra
information pointing at them making it easiser to understand where the
problem may lay:

error: 4 positional arguments in format string, but there are 3 arguments
  --> $DIR/ifmt-bad-arg.rs:78:15
   |
LL |     println!("{} {:.*} {}", 1, 3.2, 4);
   |               ^^ ^^--^ ^^      --- this parameter corresponds to the precision flag
   |                    |
   |                    this precision flag adds an extra required argument at position 1, which is why there are 4 arguments expected
   |
   = note: positional arguments are zero-based
   = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html

error: 4 positional arguments in format string, but there are 3 arguments
  --> $DIR/ifmt-bad-arg.rs:81:15
   |
LL |     println!("{} {:07$.*} {}", 1, 3.2, 4);
   |               ^^ ^^-----^ ^^      --- this parameter corresponds to the precision flag
   |                    |  |
   |                    |  this precision flag adds an extra required argument at position 1, which is why there are 4 arguments expected
   |                    this width flag expects an `usize` argument at position 7, but there are 3 arguments
   |
   = note: positional arguments are zero-based
   = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html

error: invalid reference to positional argument 7 (there are 3 arguments)
  --> $DIR/ifmt-bad-arg.rs:84:18
   |
LL |     println!("{} {:07$} {}", 1, 3.2, 4);
   |                  ^^^--^
   |                     |
   |                     this width flag expects an `usize` argument at position 7, but there are 3 arguments
   |
   = note: positional arguments are zero-based
   = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html

Fix #49384.

[rust-highfive]

r? @nikomatsakis

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[estebank]

r? @Centril

[Centril]

Unfortunately I'm not really familiar with this part of the compiler so I don't feel comfortable doing the main review but I left some comments above.

r? @alexcrichton cc @Mark-Simulacrum

[alexcrichton]

Looks reasonable to me! I tend to not review the error-generation code too too carefully, but the error messages look fantastic and I've largely just got one stylistic request on the fmt_macros crate

[alexcrichton]

This seems like it's pretty tailored in the implementation, but perhaps this could be somewhat more generalized? For example instead of modifying existing methods and having some which return a span and some which don't (in addition to passing around start spans and such) could one new function be added liek:

fn span<R>(&mut self, f: impl FnOnce(&mut Self) -> R) -> (R, Span) {
    // ...
}

and that's called like let (count, span) = self.span(|me| me.count()); or something like that?

[estebank]

The thing is that the actual span for the thing being composed requires some passing of location between different methods. count in particular is only used in two places and both care about the span. These methods are meant to only be used from a single module, which is why I didn't bother with an externally pleasing api surface for them.

[alexcrichton]

@bors: r+

Ok, sounds reasonable to me!

[bors]

📌 Commit 22ea38d has been approved by alexcrichton