Rust的char具有 to_lowercase 函数，该函数似乎返回struct ToLowercase ，该结构似乎是一个始终带有一个元素的迭代器。
直接返回char会更自然，更简单吗？
最佳答案
自然，简单和错误。 Unicode太复杂了，无法使用。基本问题是，对于“字符”的某些定义，char不足以始终代表一个逻辑上完整的“字符”。
通过运行一个简单的程序将每个有效的Unicode代码点大小写，可以简单地证明这是错误的。该程序:
/*!
Add the following to a `Cargo.toml` file:
```cargo
[dependencies]
arrayvec = "0.3.15"
```
*/
extern crate arrayvec;
use arrayvec::ArrayVec;
fn main() {
let mut expanded_lcs = 0;
let mut expanded_ucs = 0;
let usvs = (0..0xd7ff).chain(0xe000..0x10ffff)
.flat_map(|v| std::char::from_u32(v).into_iter());
for c in usvs {
let lc: ArrayVec<[_; 4]> = c.to_lowercase().collect();
let uc: ArrayVec<[_; 4]> = c.to_uppercase().collect();
if lc.len() != 1 {
expanded_lcs += 1;
print!("'{}' U+{:04X} L -> ", c, c as u32);
for c in lc {
print!("'{}' U+{:04X} ", c, c as u32);
}
println!("");
}
if uc.len() != 1 {
expanded_ucs += 1;
print!("'{}' U+{:04X} U -> ", c, c as u32);
for c in uc {
print!("'{}' U+{:04X} ", c, c as u32);
}
println!("");
}
}
println!("\n-----\n");
println!("Found {} chars with expanded lowercase conversions.", expanded_lcs);
println!("Found {} chars with expanded uppercase conversions.", expanded_ucs);
}
及其输出，每晚给出
rustc 1.8:'ß' U+00DF U -> 'S' U+0053 'S' U+0053
'İ' U+0130 L -> 'i' U+0069 '̇' U+0307
'ʼn' U+0149 U -> 'ʼ' U+02BC 'N' U+004E
'ǰ' U+01F0 U -> 'J' U+004A '̌' U+030C
'ΐ' U+0390 U -> 'Ι' U+0399 '̈' U+0308 '́' U+0301
'ΰ' U+03B0 U -> 'Υ' U+03A5 '̈' U+0308 '́' U+0301
'և' U+0587 U -> 'Ե' U+0535 'Ւ' U+0552
'ẖ' U+1E96 U -> 'H' U+0048 '̱' U+0331
'ẗ' U+1E97 U -> 'T' U+0054 '̈' U+0308
'ẘ' U+1E98 U -> 'W' U+0057 '̊' U+030A
'ẙ' U+1E99 U -> 'Y' U+0059 '̊' U+030A
'ẚ' U+1E9A U -> 'A' U+0041 'ʾ' U+02BE
'ὐ' U+1F50 U -> 'Υ' U+03A5 '̓' U+0313
'ὒ' U+1F52 U -> 'Υ' U+03A5 '̓' U+0313 '̀' U+0300
'ὔ' U+1F54 U -> 'Υ' U+03A5 '̓' U+0313 '́' U+0301
'ὖ' U+1F56 U -> 'Υ' U+03A5 '̓' U+0313 '͂' U+0342
'ᾀ' U+1F80 U -> 'Ἀ' U+1F08 'Ι' U+0399
'ᾁ' U+1F81 U -> 'Ἁ' U+1F09 'Ι' U+0399
'ᾂ' U+1F82 U -> 'Ἂ' U+1F0A 'Ι' U+0399
'ᾃ' U+1F83 U -> 'Ἃ' U+1F0B 'Ι' U+0399
'ᾄ' U+1F84 U -> 'Ἄ' U+1F0C 'Ι' U+0399
'ᾅ' U+1F85 U -> 'Ἅ' U+1F0D 'Ι' U+0399
'ᾆ' U+1F86 U -> 'Ἆ' U+1F0E 'Ι' U+0399
'ᾇ' U+1F87 U -> 'Ἇ' U+1F0F 'Ι' U+0399
'ᾈ' U+1F88 U -> 'Ἀ' U+1F08 'Ι' U+0399
'ᾉ' U+1F89 U -> 'Ἁ' U+1F09 'Ι' U+0399
'ᾊ' U+1F8A U -> 'Ἂ' U+1F0A 'Ι' U+0399
'ᾋ' U+1F8B U -> 'Ἃ' U+1F0B 'Ι' U+0399
'ᾌ' U+1F8C U -> 'Ἄ' U+1F0C 'Ι' U+0399
'ᾍ' U+1F8D U -> 'Ἅ' U+1F0D 'Ι' U+0399
'ᾎ' U+1F8E U -> 'Ἆ' U+1F0E 'Ι' U+0399
'ᾏ' U+1F8F U -> 'Ἇ' U+1F0F 'Ι' U+0399
'ᾐ' U+1F90 U -> 'Ἠ' U+1F28 'Ι' U+0399
'ᾑ' U+1F91 U -> 'Ἡ' U+1F29 'Ι' U+0399
'ᾒ' U+1F92 U -> 'Ἢ' U+1F2A 'Ι' U+0399
'ᾓ' U+1F93 U -> 'Ἣ' U+1F2B 'Ι' U+0399
'ᾔ' U+1F94 U -> 'Ἤ' U+1F2C 'Ι' U+0399
'ᾕ' U+1F95 U -> 'Ἥ' U+1F2D 'Ι' U+0399
'ᾖ' U+1F96 U -> 'Ἦ' U+1F2E 'Ι' U+0399
'ᾗ' U+1F97 U -> 'Ἧ' U+1F2F 'Ι' U+0399
'ᾘ' U+1F98 U -> 'Ἠ' U+1F28 'Ι' U+0399
'ᾙ' U+1F99 U -> 'Ἡ' U+1F29 'Ι' U+0399
'ᾚ' U+1F9A U -> 'Ἢ' U+1F2A 'Ι' U+0399
'ᾛ' U+1F9B U -> 'Ἣ' U+1F2B 'Ι' U+0399
'ᾜ' U+1F9C U -> 'Ἤ' U+1F2C 'Ι' U+0399
'ᾝ' U+1F9D U -> 'Ἥ' U+1F2D 'Ι' U+0399
'ᾞ' U+1F9E U -> 'Ἦ' U+1F2E 'Ι' U+0399
'ᾟ' U+1F9F U -> 'Ἧ' U+1F2F 'Ι' U+0399
'ᾠ' U+1FA0 U -> 'Ὠ' U+1F68 'Ι' U+0399
'ᾡ' U+1FA1 U -> 'Ὡ' U+1F69 'Ι' U+0399
'ᾢ' U+1FA2 U -> 'Ὢ' U+1F6A 'Ι' U+0399
'ᾣ' U+1FA3 U -> 'Ὣ' U+1F6B 'Ι' U+0399
'ᾤ' U+1FA4 U -> 'Ὤ' U+1F6C 'Ι' U+0399
'ᾥ' U+1FA5 U -> 'Ὥ' U+1F6D 'Ι' U+0399
'ᾦ' U+1FA6 U -> 'Ὦ' U+1F6E 'Ι' U+0399
'ᾧ' U+1FA7 U -> 'Ὧ' U+1F6F 'Ι' U+0399
'ᾨ' U+1FA8 U -> 'Ὠ' U+1F68 'Ι' U+0399
'ᾩ' U+1FA9 U -> 'Ὡ' U+1F69 'Ι' U+0399
'ᾪ' U+1FAA U -> 'Ὢ' U+1F6A 'Ι' U+0399
'ᾫ' U+1FAB U -> 'Ὣ' U+1F6B 'Ι' U+0399
'ᾬ' U+1FAC U -> 'Ὤ' U+1F6C 'Ι' U+0399
'ᾭ' U+1FAD U -> 'Ὥ' U+1F6D 'Ι' U+0399
'ᾮ' U+1FAE U -> 'Ὦ' U+1F6E 'Ι' U+0399
'ᾯ' U+1FAF U -> 'Ὧ' U+1F6F 'Ι' U+0399
'ᾲ' U+1FB2 U -> 'Ὰ' U+1FBA 'Ι' U+0399
'ᾳ' U+1FB3 U -> 'Α' U+0391 'Ι' U+0399
'ᾴ' U+1FB4 U -> 'Ά' U+0386 'Ι' U+0399
'ᾶ' U+1FB6 U -> 'Α' U+0391 '͂' U+0342
'ᾷ' U+1FB7 U -> 'Α' U+0391 '͂' U+0342 'Ι' U+0399
'ᾼ' U+1FBC U -> 'Α' U+0391 'Ι' U+0399
'ῂ' U+1FC2 U -> 'Ὴ' U+1FCA 'Ι' U+0399
'ῃ' U+1FC3 U -> 'Η' U+0397 'Ι' U+0399
'ῄ' U+1FC4 U -> 'Ή' U+0389 'Ι' U+0399
'ῆ' U+1FC6 U -> 'Η' U+0397 '͂' U+0342
'ῇ' U+1FC7 U -> 'Η' U+0397 '͂' U+0342 'Ι' U+0399
'ῌ' U+1FCC U -> 'Η' U+0397 'Ι' U+0399
'ῒ' U+1FD2 U -> 'Ι' U+0399 '̈' U+0308 '̀' U+0300
'ΐ' U+1FD3 U -> 'Ι' U+0399 '̈' U+0308 '́' U+0301
'ῖ' U+1FD6 U -> 'Ι' U+0399 '͂' U+0342
'ῗ' U+1FD7 U -> 'Ι' U+0399 '̈' U+0308 '͂' U+0342
'ῢ' U+1FE2 U -> 'Υ' U+03A5 '̈' U+0308 '̀' U+0300
'ΰ' U+1FE3 U -> 'Υ' U+03A5 '̈' U+0308 '́' U+0301
'ῤ' U+1FE4 U -> 'Ρ' U+03A1 '̓' U+0313
'ῦ' U+1FE6 U -> 'Υ' U+03A5 '͂' U+0342
'ῧ' U+1FE7 U -> 'Υ' U+03A5 '̈' U+0308 '͂' U+0342
'ῲ' U+1FF2 U -> 'Ὼ' U+1FFA 'Ι' U+0399
'ῳ' U+1FF3 U -> 'Ω' U+03A9 'Ι' U+0399
'ῴ' U+1FF4 U -> 'Ώ' U+038F 'Ι' U+0399
'ῶ' U+1FF6 U -> 'Ω' U+03A9 '͂' U+0342
'ῷ' U+1FF7 U -> 'Ω' U+03A9 '͂' U+0342 'Ι' U+0399
'ῼ' U+1FFC U -> 'Ω' U+03A9 'Ι' U+0399
'ff' U+FB00 U -> 'F' U+0046 'F' U+0046
'fi' U+FB01 U -> 'F' U+0046 'I' U+0049
'fl' U+FB02 U -> 'F' U+0046 'L' U+004C
'ffi' U+FB03 U -> 'F' U+0046 'F' U+0046 'I' U+0049
'ffl' U+FB04 U -> 'F' U+0046 'F' U+0046 'L' U+004C
'ſt' U+FB05 U -> 'S' U+0053 'T' U+0054
'st' U+FB06 U -> 'S' U+0053 'T' U+0054
'ﬓ' U+FB13 U -> 'Մ' U+0544 'Ն' U+0546
'ﬔ' U+FB14 U -> 'Մ' U+0544 'Ե' U+0535
'ﬕ' U+FB15 U -> 'Մ' U+0544 'Ի' U+053B
'ﬖ' U+FB16 U -> 'Վ' U+054E 'Ն' U+0546
'ﬗ' U+FB17 U -> 'Մ' U+0544 'Խ' U+053D
-----
Found 1 chars with expanded lowercase conversions.
Found 102 chars with expanded uppercase conversions.
请注意，这没有考虑到语言环境，这可能会更改输出。
关于rust - Rust的ToLowercase的动机是什么，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/35716159/