custom_format/
compile_time.rs

1//! Provides types associated to compile-time formatting.
2
3use core::fmt;
4
5/// Trait for custom formatting with compile-time format checking
6pub trait CustomFormat<const SPEC: u128> {
7    /// Formats the value using the given formatter.
8    ///
9    /// # Examples
10    ///
11    /// ```rust
12    /// use custom_format as cfmt;
13    /// use custom_format::compile_time::{spec, CustomFormat};
14    ///
15    /// use core::fmt;
16    ///
17    /// #[derive(Debug)]
18    /// struct Hex(u8);
19    ///
20    /// impl CustomFormat<{ spec("x") }> for Hex {
21    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
22    ///         write!(f, "{:#02x}", self.0)
23    ///     }
24    /// }
25    ///
26    /// impl CustomFormat<{ spec("X") }> for Hex {
27    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
28    ///         write!(f, "{:#02X}", self.0)
29    ///     }
30    /// }
31    ///
32    /// assert_eq!(cfmt::format!("{0:X?}, {0 :x}, {0 :X}", Hex(0xAB)), "Hex(AB), 0xab, 0xAB");
33    /// ```
34    ///
35    /// The following statement doesn't compile since `"z"` is not a valid format specifier:
36    ///
37    /// ```rust,compile_fail
38    /// # use custom_format as cfmt;
39    /// # use custom_format::compile_time::{spec, CustomFormat};
40    /// # use core::fmt;
41    /// # struct Hex(u8);
42    /// # impl CustomFormat<{ cfmt::spec("x") }> for Hex {
43    /// #     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
44    /// #         write!(f, "{:#02x}", self.0)
45    /// #     }
46    /// # }
47    /// # impl CustomFormat<{ cfmt::spec("X") }> for Hex {
48    /// #     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
49    /// #         write!(f, "{:#02X}", self.0)
50    /// #     }
51    /// # }
52    /// cfmt::println!("{ :z}", Hex(0));
53    /// ```
54    ///
55    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result;
56}
57
58/// Wrapper for custom formatting via its [`Display`](core::fmt::Display) trait.
59///
60/// The format specifier is a const-generic parameter and is part of the type.
61///
62#[derive(Debug, Clone)]
63pub struct CustomFormatter<'a, T, const SPEC: u128> {
64    /// Value to format
65    value: &'a T,
66}
67
68impl<'a, T, const SPEC: u128> CustomFormatter<'a, T, SPEC> {
69    /// Construct a new [`CustomFormatter`] value
70    pub fn new(value: &'a T) -> Self {
71        Self { value }
72    }
73}
74
75/// Helper macro for constructing a new [`compile_time::CustomFormatter`](CustomFormatter) value from a format specifier
76#[macro_export]
77macro_rules! custom_formatter {
78    ($spec:literal, $value:expr) => {{
79        $crate::compile_time::CustomFormatter::<_, { $crate::compile_time::spec($spec) }>::new($value)
80    }};
81}
82pub use custom_formatter;
83
84impl<T: CustomFormat<SPEC>, const SPEC: u128> fmt::Display for CustomFormatter<'_, T, SPEC> {
85    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
86        CustomFormat::fmt(self.value, f)
87    }
88}
89
90/// Convert a format specifier to a [`u128`], used as a const-generic parameter
91pub const fn spec(s: &str) -> u128 {
92    let bytes = s.as_bytes();
93    let len = s.len();
94
95    if len > 16 {
96        #[allow(unconditional_panic)]
97        let _ = ["format specifier is limited to 16 bytes"][usize::MAX];
98    }
99
100    let mut result = [0u8; 16];
101
102    let mut i = 0;
103    while i < len {
104        result[i] = bytes[i];
105        i += 1;
106    }
107
108    u128::from_le_bytes(result)
109}
custom_format/compile_time.rs

custom_format/
compile_time.rs
