Add from_str_const constructor for ArrayString

Author: bluss

src/array_string.rs

@@ -19,7 +19,6 @@ use crate::utils::MakeMaybeUninit;
 #[cfg(feature="serde")]
 use serde::{Serialize, Deserialize, Serializer, Deserializer};
 
-
 /// A string with a fixed capacity.
 ///
 /// The `ArrayString` is a string backed by a fixed size array. It keeps track
@@ -108,6 +107,42 @@ impl<const CAP: usize> ArrayString<CAP>
         Ok(arraystr)
     }
 
+    /// Create a new `ArrayString` from a `str`, suitable for const context
+    ///
+    /// Capacity is inferred from the type parameter.
+    ///
+    /// **Panics** or causes a **const error** if the backing array is not large enough to fit the
+    /// string.
+    ///
+    /// ```
+    /// use arrayvec::ArrayString;
+    ///
+    /// const S: ArrayString<3> = ArrayString::from_str_const("");
+    /// ```
+    ///
+    /// A compile-time error will occur - in constants - if the input is too long:
+    ///
+    /// ```compile_fail
+    /// # use arrayvec::ArrayString;
+    /// const S1: ArrayString<3> = ArrayString::from_str_const("too long for the capacity");
+    /// ```
+    pub const fn from_str_const(s: &str) -> Self {
+        let bytes = s.as_bytes();
+        let len = bytes.len();
+        assert_length_lt_capacity_const!(len, CAP);
+
+        let mut vec = Self::new_const();
+        let mut i = 0;
+        while i < len {
+            vec.xs[i] = MaybeUninit::new(bytes[i]);
+            i += 1;
+        }
+
+        // Safety: we know len <= CAP and elements < len are initialized
+        vec.len = len as u32;
+        vec
+    }
+
     /// Create a new `ArrayString` from a byte string literal.
     ///
     /// **Errors** if the byte string literal is not valid UTF-8.

src/lib.rs

@@ -46,6 +46,14 @@ macro_rules! assert_capacity_limit_const {
     }
 }
 
+macro_rules! assert_length_lt_capacity_const {
+    ($len:expr, $cap:expr) => {
+        if $len > 0 {
+            let _len_check = [(); $cap /*ArrayVec/ArrayString: insufficient capacity */][$len - 1];
+        }
+    }
+}
+
 mod arrayvec_impl;
 mod arrayvec;
 mod array_string;

tests/tests.rs

@@ -784,10 +784,23 @@ fn test_arraystring_const_constructible() {
     assert_eq!(var, *"hello");
 }
 
+#[test]
+fn test_arraystring_from_str_const() {
+    const AS: ArrayString<10> = ArrayString::from_str_const("0123456789");
+
+    let mut var = AS;
+    assert_eq!(&*var, "0123456789");
+    assert!(var.try_push_str("1").is_err());
+
+    var.clear();
+    var.push_str("9876543210");
+    assert_eq!(&*var, "9876543210");
+}
+
 
 #[test]
 fn test_arraystring_zero_filled_has_some_sanity_checks() {
     let string = ArrayString::<4>::zero_filled();
     assert_eq!(string.as_str(), "\0\0\0\0");
     assert_eq!(string.len(), 4);
-}
+}