How do I convert between String, &str, Vec<u8> and &[u8]?

A new Rustacean like me struggles with juggling these types: String, &str, Vec, &[u8].

In time, I hope to have an epiphany and suddenly get why some library calls use one or the other. Until then, I need help to map out each idiomatic transition.

Given these types:

let st: &str = ...;
let s:  String = ...;
let u:  &[u8] = ...;
let v:  Vec = ...;

I think I have figured these out, but are they idiomatic?
&str -> String String::from(st)
&str -> &[u8] st.as_bytes()
String -> &str s.as_str()
&[u8] -> &str str::from_utf8(u)
Vec -> String String::from_utf8(v)


Ultimately I want a complete table of transitions for these types:
&str -> String
&str -> &[u8]
&str -> Vec
String -> &str
String -> &[u8]
String -> Vec
&[u8] -> &str
&[u8] -> String
&[u8] -> Vec
Vec -> &str
Vec -> String
Vec -> &[u8]


From &str

• &str -> String has many equally valid methods: String::from(st), st.to_string(), st.to_owned().

• But I suggest you stick with one of them within a single project. The major advantage of String::from is that you can use it as an argument to a map method. So instead of x.map(|s| String::from(s)) you can often use x.map(String::from).

• &str -> &[u8] is done by st.as_bytes()

• &str -> Vec is a combination of &str -> &[u8] -> Vec, i.e. st.as_bytes().to_vec() or st.as_bytes().to_owned()

From String

• String -> &str should just be &s where coercion is available or s.as_str() where it is not.

• String -> &[u8] is the same as &str -> &[u8]: s.as_bytes()

• String -> Vec has a custom method: s.into_bytes()

From &[u8]

• &[u8] -> Vec is done by u.to_owned() or u.to_vec(). They do the same thing, but to_vec has the slight advantage of being unambiguous about the type it returns.

• &[u8] -> &str doesn't actually exist, that would be &[u8] -> Result, provided via str::from_utf8(u)

• str::from_utf8(u).unwrap() works, but you should prefer better error handling (see Error handling - The Result type).

• &[u8] -> String is the combination of &[u8] -> Result -> Result

• String::from_utf8(u).unwrap() works, but prefer better error handling (see Error handling - The Result type and also Result::map.

From Vec

• Vec -> &[u8] should be just &v where coercion is available, or as_slice where it's not.

• Vec -> &str is the same as Vec -> &[u8] -> Result i.e. str::from_utf8(&v)

• str::from_utf8(&v).unwrap() works, but prefer better error handling (see Error handling - The Result type)

• Vec -> String doesn't actually exist, that would be Vec -> Result via String::from_utf8(v)

• String::from_utf8(v).unwrap() works, but prefer better error handling (see Error handling - The Result type).

Coercion is available whenever the target is not generic but explicitly typed as &str or &[u8], respectively. The Rustonomicon has a chapter on coercions with more details about coercion sites.

tl;dr

&str -> String | String::from(s) or s.to_string() or s.to_owned()
&str -> &[u8] | s.as_bytes()
&str -> Vec | s.as_bytes().to_vec() or s.as_bytes().to_owned()
String -> &str | &s if possible* else s.as_str()
String -> &[u8] | s.as_bytes()
String -> Vec | s.into_bytes()
&[u8] -> &str | s.to_vec() or s.to_owned()
&[u8] -> String | std::str::from_utf8(s).unwrap(), but don't**
&[u8] -> Vec | String::from_utf8(s).unwrap(), but don't**
Vec -> &str | &s if possible* else s.as_slice()
Vec -> String | std::str::from_utf8(&s).unwrap(), but don't**
Vec -> &[u8] | String::from_utf8(s).unwrap(), but don't**

* target should have explicit type (i.e., checker can't infer that)

** handle the error properly instead


Stack Overflow Q#41034635, score: 220