lit-mod 0.2.0

lit-mod is a collection of procedural macros for working with string literals.
Documentation 
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macro_rules! slice_like {
    ($name:literal) => {
        use crate::util::unwind_expr_to_isize;
        use syn::{ExprRange, LitStr, RangeLimits, Token};
        use syn::parse::{Parse, ParseStream, Result};

        struct Slice {
            source: LitStr,
            ranges: Vec<(Option<isize>, Option<isize>)>,
        }

        impl Parse for Slice {
            fn parse(input: ParseStream) -> Result<Self> {
                let source = input.parse::<LitStr>()
                    .map_err(|_| input.error(concat!($name, " expects a string as the first argument")))?;
                input.parse::<Token![,]>()
                    .map_err(|_| input.error(concat!($name, " expects at least one range")))?;

                let mut ranges = Vec::new();

                while !input.is_empty() {
                    let range = input.parse::<ExprRange>()?;
                        let start = unwind_expr_to_isize(range.start)
                            .map_err(|err| input.error(format!("{} {}", $name, err)))?;
                        let end = unwind_expr_to_isize(range.end)
                            .map_err(|err| input.error(format!("{} {}", $name, err)))?;

                    if let (Some(start), Some(end)) = (start, end) {
                        if start > end {
                            return Err(input.error(concat!($name, " expects valid ranges")));
                        }
                    }

                    match range.limits {
                        RangeLimits::HalfOpen(_) => {
                            ranges.push((start, end));
                        }
                        RangeLimits::Closed(_) => {
                            ranges.push((start, end.map(|e| e + 1)));
                        }
                    }
                }

                Ok(Self { source, ranges })
            }
        }
    };
}

pub(crate) use slice_like;
use syn::{Expr, ExprLit, ExprUnary, Lit};

pub(crate) fn unwind_expr_to_isize(expr: Option<Box<Expr>>) -> Result<Option<isize>, &'static str> {
    match expr {
        Some(boxed) => match *boxed {
            Expr::Lit(ExprLit {
                lit: Lit::Int(lit), ..
            }) => Ok(lit
                .base10_parse::<isize>()
                .map_err(|_| "expects ranges of integers")?
                .into()),
            Expr::Unary(ExprUnary {
                op: syn::UnOp::Neg(_),
                expr,
                ..
            }) => match *expr {
                Expr::Lit(ExprLit {
                    lit: Lit::Int(lit), ..
                }) => Ok(lit
                    .base10_parse::<isize>()
                    .map_err(|_| "expects ranges of integers")?
                    .checked_neg()
                    .ok_or("expects ranges of integers")?
                    .into()),
                _ => Err("expects ranges of integers"),
            },
            _ => Err("expects ranges of integers"),
        },
        None => Ok(None),
    }
}

pub fn start_to_usize(source_len: usize, index: Option<isize>) -> usize {
    match index {
        Some(index) if index < 0 => source_len - index.unsigned_abs(),
        Some(index) => index as usize,
        None => 0,
    }
}

pub fn end_to_usize(source_len: usize, index: Option<isize>) -> usize {
    match index {
        Some(index) if index < 0 => source_len - index.unsigned_abs(),
        Some(index) => index as usize,
        None => source_len,
    }
}