println/format strings only support a bare {} (no float precision, width or radix)

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还没有人认领这个 Issue。

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Issue 类型
功能
描述清晰度
基本清楚
活跃度
活跃
技术栈
rust
领域
compilers

调研方向

Start by locating the parser and formatter implementation for println format strings, then trace how placeholders are parsed and how f64 values are rendered. The issue gives a reproducer using {:.2} and describes desired precision, width, padding, alignment, and integer radix support. Add the supported formatting behavior and tests for those cases; done when the reproducer formats the float as requested and unsupported syntax is handled consistently.

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描述

Feature request: println / format strings accept only a bare {}. There is no way to control the number of fraction digits of a float, width or padding.

Repro

fun main() {
    var c: f64 = 0.1;
    println("{:.2}", c);
}

error[E2001]: failed to parse Wave source: unsupported format placeholder `{:.2}`

The error is clear, so this is not a diagnostics problem. The gap is that {} on an f64 always prints six fraction digits (0.100000), so a program that wants 0.10, or the full precision of a double, has to format the number by hand.

I hit this while writing a JSON library in Wave: I had to implement shortest round-trip and fixed-precision double formatting myself just to print a number the way I wanted.

Suggestion

Support at least {:.N} (precision) for floats, and ideally width/fill/alignment ({:5}, {:>8}, {:08}) and an integer radix ({:x}), or document that formatting is expected to be done in library code.

Version

wavec 0.2.1-pre-beta (LLVM 21.1.8), Ubuntu 26.04, x86_64.

主要语言
Rust
星标
56
派生
21
平均合并
23 小时 20 分钟
30 天内合并 PR
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环境准备

从这里开始

  1. 先读完整个 Issue，再读项目的贡献指南。
  2. 在 Issue 下留言说明你要接手 —— 这能避免两个人做同样的事。
  3. Fork 仓库，在一个分支上完成修改。
  4. 提交 Pull Request，并在描述里引用这个 Issue 编号。

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