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|
//! Rust format strings (`println!`, `format!`) -> Nim string expressions.
//!
//! Only the subset that is reproduced exactly is accepted. An unrecognised
//! format spec is an error, never a best-effort guess: `{:>8.3}` silently
//! rendered as `{}` would produce plausible-looking wrong output, which is the
//! failure mode this project exists to avoid.
/// One piece of a parsed format string.
#[derive(Debug, PartialEq)]
pub enum Piece {
Lit(String),
Arg { r#ref: Ref, spec: Spec },
}
#[derive(Debug, PartialEq)]
pub enum Ref {
/// `{}` — consumes the next positional argument.
Next,
/// `{0}` — an explicit index.
Index(usize),
/// `{name}` — an inline captured identifier.
Named(String),
}
#[derive(Debug, PartialEq, Default)]
pub struct Spec {
pub debug: bool,
/// `x`, `X`, `b`, `o` — `None` for Display/Debug.
pub radix: Option<char>,
pub width: usize,
pub zero: bool,
}
pub fn parse(s: &str) -> Result<Vec<Piece>, String> {
let mut out = Vec::new();
let mut lit = String::new();
let mut it = s.chars().peekable();
while let Some(c) = it.next() {
match c {
'{' if it.peek() == Some(&'{') => {
it.next();
lit.push('{');
}
'}' if it.peek() == Some(&'}') => {
it.next();
lit.push('}');
}
'}' => return Err("unmatched `}` in format string".into()),
'{' => {
if !lit.is_empty() {
out.push(Piece::Lit(std::mem::take(&mut lit)));
}
let mut body = String::new();
let mut closed = false;
for c in it.by_ref() {
if c == '}' {
closed = true;
break;
}
body.push(c);
}
if !closed {
return Err("unmatched `{` in format string".into());
}
let (name, spec) = match body.split_once(':') {
Some((n, s)) => (n, parse_spec(s)?),
None => (body.as_str(), Spec::default()),
};
let r#ref = if name.is_empty() {
Ref::Next
} else if let Ok(i) = name.parse::<usize>() {
Ref::Index(i)
} else if name.chars().all(|c| c.is_alphanumeric() || c == '_') {
Ref::Named(name.to_string())
} else {
return Err(format!("unsupported format argument `{name}`"));
};
out.push(Piece::Arg { r#ref, spec });
}
_ => lit.push(c),
}
}
if !lit.is_empty() {
out.push(Piece::Lit(lit));
}
Ok(out)
}
fn parse_spec(s: &str) -> Result<Spec, String> {
let mut spec = Spec::default();
let mut rest = s;
match rest.chars().last() {
Some('?') => {
spec.debug = true;
rest = &rest[..rest.len() - 1];
}
Some(r @ ('x' | 'X' | 'b' | 'o')) => {
spec.radix = Some(r);
rest = &rest[..rest.len() - 1];
}
_ => {}
}
if let Some(r) = rest.strip_prefix('0') {
spec.zero = true;
rest = r;
}
if !rest.is_empty() {
spec.width = rest
.parse::<usize>()
.map_err(|_| format!("unsupported format spec `:{s}` (precision, alignment and fill are not implemented)"))?;
}
Ok(spec)
}
/// Build the Nim expression for one argument, given its already-lowered value.
///
/// `integer` says whether the value is one of Nim's integer types. `{:x}` on
/// an integer formats its two's-complement bit pattern; on anything else it is
/// a call to that type's own `LowerHex`/`UpperHex` impl, which is a different
/// operation and a different proc.
pub fn render_arg(value: &str, spec: &Spec, integer: bool) -> String {
let core = match spec.radix {
Some(r) if integer => format!(
"rsRadix({}, {}, {})",
value,
match r {
'x' | 'X' => 16,
'b' => 2,
_ => 8,
},
r == 'X'
),
Some(r) => format!(
"{}({})",
match r {
'x' => "rsLowerHex",
'X' => "rsUpperHex",
'b' => "rsBinary",
_ => "rsOctal",
},
value
),
None if spec.debug => format!("rsDebug({value})"),
None => format!("rsDisplay({value})"),
};
if spec.width > 0 {
format!("rsPad({}, {}, {})", core, spec.width, spec.zero)
} else {
core
}
}
/// Nim string literal with Rust's escaping rules applied to the bytes we emit.
pub fn nim_str(s: &str) -> String {
let mut out = String::from("\"");
for c in s.chars() {
match c {
'"' => out.push_str("\\\""),
'\\' => out.push_str("\\\\"),
'\n' => out.push_str("\\n"),
'\t' => out.push_str("\\t"),
'\r' => out.push_str("\\r"),
c => out.push(c),
}
}
out.push('"');
out
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn braces_and_specs() {
assert_eq!(parse("a{{b").unwrap(), vec![Piece::Lit("a{b".into())]);
assert_eq!(
parse("{:02x}").unwrap(),
vec![Piece::Arg {
r#ref: Ref::Next,
spec: Spec { debug: false, radix: Some('x'), width: 2, zero: true }
}]
);
assert_eq!(
parse("{n:?}").unwrap(),
vec![Piece::Arg {
r#ref: Ref::Named("n".into()),
spec: Spec { debug: true, ..Spec::default() }
}]
);
}
#[test]
fn unsupported_specs_are_rejected_not_guessed() {
assert!(parse("{:>8}").is_err());
assert!(parse("{:.3}").is_err());
assert!(parse("{").is_err());
}
}
|