format-dynamic-fill

text/3394-format-dynamic-fill.md

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+- Feature Name: `format_dynamic_fill`
+- Start Date: 2023-02-23
+- RFC PR: [rust-lang/rfcs#3394](https://github.com/rust-lang/rfcs/pull/3394)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Allow specifying the *fill* character of a format string, matching behavior of *width* and *precision*.
+
+# Motivation
+[motivation]: #motivation
+
+To allow changing the *fill* character at run-time. Due to not being able to create or modify [`Formatter`](https://doc.rust-lang.org/std/fmt/struct.Formatter.html), this is the only way to allow this.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Extending on [`std::fmt`#Fill/Alignment](https://doc.rust-lang.org/std/fmt/index.html#fillalignment):
+
+The value for the fill can also be provided as a [`char`](https://doc.rust-lang.org/std/primitive.char.html) in the list of parameters by adding a postfix `$` indicating that the *n*th or named argument is a char specifying the fill.
+
+Referring to an argument with the dollar syntax does not affect the “next argument” counter, so it’s usually a good idea to refer to arguments by position, or use named arguments.
+
+```rs
+assert_eq!(format!("Hello {:fill$>5}!", "x", fill='+'), "Hello x++++!");
+assert_eq!(format!("Hello {1:0$<5}!", '-', "x"),        "Hello x----!");
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Implementation should be matching that of width and precision.
+
+The fill and alignment is the first specified format parameter, so if the `:` is followed by:
+- `"{:-^}"` a single character *c* (this includes `$`) followed by an *alignment* ⇒ the fill is *c*
+- `"{:ident$>}"` a valid rust identifier *ident* followed by a `$` and an *alignment* ⇒ the fill is the named argument/variable *ident*
+- `"{:1$<}"` an usize *n* followed by a `$` and an *alignment* ⇒ the fill is the *n*th argument (this does not affect the "next argument" counter matching [width](https://doc.rust-lang.org/std/fmt/index.html#width))
+- `"{:width$}"` a valid rust identifier or usize followed by a `$` and not followed by an *alignment* ⇒ this is the width
+
+(*alignment* is one of `<^>`)
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+It complicates the format spec, and it is unclear how much it might be used.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- If a minimum of 2 characters was enforced the `$` could be omitted, but this only makes this easier to mess up and is unnecessarily restrictive, as well as inconsistent with other parameters that do require `$`.
+- This cannot be implemented in a library, as the `Formatter` used by the `Display` trait does not expose a mechanism to change the fill character, and any other mechanism would not allow using the `Display` implementations throughout the ecosystem.
+
+# Prior art
+[prior-art]: #prior-art
+
+The implementation of width and precision.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+Create a stable interface to specify all possible format arguments.