Constify methods that transmute

* AsciiChar::from_ascii_unchecked()
* AsciiChar::as_printable_char()

Author: tormol

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [1.46.0, stable, beta, nightly]
+        rust: [1.56.1, stable, beta, nightly]
     steps:
       - uses: actions/checkout@v2
       - uses: hecrj/setup-rust-action@v1

README.md

@@ -35,7 +35,7 @@ ascii = { version = "1.1", default-features = false, features = ["alloc"] }
 
 ## Minimum supported Rust version
 
-The minimum Rust version for 1.2.\* releases is 1.46.0.
+The minimum Rust version for 1.2.\* releases is 1.56.1.
 Later 1.y.0 releases might require newer Rust versions, but the three most
 recent stable releases at the time of publishing will always be supported.  
 For example this means that if the current stable Rust version is 1.70 when

src/ascii_char.rs

@@ -375,9 +375,9 @@ impl AsciiChar {
     /// and `Some(AsciiChar::from_ascii_unchecked(128))` might be `None`.
     #[inline]
     #[must_use]
-    pub unsafe fn from_ascii_unchecked(ch: u8) -> Self {
+    pub const unsafe fn from_ascii_unchecked(ch: u8) -> Self {
         // SAFETY: Caller guarantees `ch` is within bounds of ascii.
-        unsafe { ch.to_ascii_char_unchecked() }
+        unsafe { mem::transmute(ch) }
     }
 
     /// Converts an ASCII character into a `u8`.
@@ -659,14 +659,15 @@ impl AsciiChar {
     /// assert_eq!(AsciiChar::new('p').as_printable_char(), 'p');
     /// ```
     #[must_use]
-    pub fn as_printable_char(self) -> char {
+    pub const fn as_printable_char(self) -> char {
+        #![allow(clippy::transmute_int_to_char)] // from_utf32_unchecked() is not const fn yet.
         match self as u8 {
             // Non printable characters
             // SAFETY: From codepoint 0x2400 ('␀') to 0x241f (`␟`), there are characters representing
             //         the unprintable characters from 0x0 to 0x1f, ordered correctly.
             //         As `b` is guaranteed to be within 0x0 to 0x1f, the conversion represents a
             //         valid character.
-            b @ 0x0..=0x1f => unsafe { char::from_u32_unchecked(u32::from('␀') + u32::from(b)) },
+            b @ 0x0..=0x1f => unsafe { mem::transmute('␀' as u32 + b as u32) },
 
             // 0x7f (delete) has it's own character at codepoint 0x2420, not 0x247f, so it is special
             // cased to return it's character

src/lib.rs

@@ -15,7 +15,7 @@
 //!
 //! # Minimum supported Rust version
 //!
-//! The minimum Rust version for 1.2.\* releases is 1.46.0.
+//! The minimum Rust version for 1.2.\* releases is 1.56.1.
 //! Later 1.y.0 releases might require newer Rust versions, but the three most
 //! recent stable releases at the time of publishing will always be supported.  
 //! For example this means that if the current stable Rust version is 1.70 when