derive: refactor parser

`parser.rs` was a single file containing almost 2000 lines.

This PR split the file into multiple, smaller files. `Expr`, `Node`, and
`Target` each get an own file. Each struct gets a `parse()` method that
return `Result<Self>`, and every other detail is private to the file.

This PR should make this essential part of Askama more easy to
understand, and make future modifications easier.

1 files changed, 346 insertions, 0 deletions

diff --git a/askama_derive/src/parser/expr.rs b/askama_derive/src/parser/expr.rs
new file mode 100644
index 0000000..4546e04
--- /dev/null
+++ b/askama_derive/src/parser/expr.rs
@@ -0,0 +1,346 @@
+use std::str;
+
+use nom::branch::alt;
+use nom::bytes::complete::{tag, take_till};
+use nom::character::complete::char;
+use nom::combinator::{cut, map, not, opt, peek, recognize};
+use nom::multi::{fold_many0, many0, separated_list0, separated_list1};
+use nom::sequence::{delimited, pair, preceded, terminated, tuple};
+use nom::IResult;
+
+use super::{
+    bool_lit, char_lit, identifier, nested_parenthesis, not_ws, num_lit, path, str_lit, ws,
+};
+
+#[derive(Debug, PartialEq)]
+pub(crate) enum Expr<'a> {
+    BoolLit(&'a str),
+    NumLit(&'a str),
+    StrLit(&'a str),
+    CharLit(&'a str),
+    Var(&'a str),
+    Path(Vec<&'a str>),
+    Array(Vec<Expr<'a>>),
+    Attr(Box<Expr<'a>>, &'a str),
+    Index(Box<Expr<'a>>, Box<Expr<'a>>),
+    Filter(&'a str, Vec<Expr<'a>>),
+    Unary(&'a str, Box<Expr<'a>>),
+    BinOp(&'a str, Box<Expr<'a>>, Box<Expr<'a>>),
+    Range(&'a str, Option<Box<Expr<'a>>>, Option<Box<Expr<'a>>>),
+    Group(Box<Expr<'a>>),
+    Tuple(Vec<Expr<'a>>),
+    Call(Box<Expr<'a>>, Vec<Expr<'a>>),
+    RustMacro(&'a str, &'a str),
+    Try(Box<Expr<'a>>),
+}
+
+impl Expr<'_> {
+    pub(super) fn parse(i: &str) -> IResult<&str, Expr<'_>> {
+        expr_any(i)
+    }
+
+    pub(super) fn parse_arguments(i: &str) -> IResult<&str, Vec<Expr<'_>>> {
+        arguments(i)
+    }
+
+    /// Returns `true` if enough assumptions can be made,
+    /// to determine that `self` is copyable.
+    pub(crate) fn is_copyable(&self) -> bool {
+        self.is_copyable_within_op(false)
+    }
+
+    fn is_copyable_within_op(&self, within_op: bool) -> bool {
+        use Expr::*;
+        match self {
+            BoolLit(_) | NumLit(_) | StrLit(_) | CharLit(_) => true,
+            Unary(.., expr) => expr.is_copyable_within_op(true),
+            BinOp(_, lhs, rhs) => {
+                lhs.is_copyable_within_op(true) && rhs.is_copyable_within_op(true)
+            }
+            Range(..) => true,
+            // The result of a call likely doesn't need to be borrowed,
+            // as in that case the call is more likely to return a
+            // reference in the first place then.
+            Call(..) | Path(..) => true,
+            // If the `expr` is within a `Unary` or `BinOp` then
+            // an assumption can be made that the operand is copy.
+            // If not, then the value is moved and adding `.clone()`
+            // will solve that issue. However, if the operand is
+            // implicitly borrowed, then it's likely not even possible
+            // to get the template to compile.
+            _ => within_op && self.is_attr_self(),
+        }
+    }
+
+    /// Returns `true` if this is an `Attr` where the `obj` is `"self"`.
+    pub(crate) fn is_attr_self(&self) -> bool {
+        match self {
+            Expr::Attr(obj, _) if matches!(obj.as_ref(), Expr::Var("self")) => true,
+            Expr::Attr(obj, _) if matches!(obj.as_ref(), Expr::Attr(..)) => obj.is_attr_self(),
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if the outcome of this expression may be used multiple times in the same
+    /// `write!()` call, without evaluating the expression again, i.e. the expression should be
+    /// side-effect free.
+    pub(crate) fn is_cachable(&self) -> bool {
+        match self {
+            // Literals are the definition of pure:
+            Expr::BoolLit(_) => true,
+            Expr::NumLit(_) => true,
+            Expr::StrLit(_) => true,
+            Expr::CharLit(_) => true,
+            // fmt::Display should have no effects:
+            Expr::Var(_) => true,
+            Expr::Path(_) => true,
+            // Check recursively:
+            Expr::Array(args) => args.iter().all(|arg| arg.is_cachable()),
+            Expr::Attr(lhs, _) => lhs.is_cachable(),
+            Expr::Index(lhs, rhs) => lhs.is_cachable() && rhs.is_cachable(),
+            Expr::Filter(_, args) => args.iter().all(|arg| arg.is_cachable()),
+            Expr::Unary(_, arg) => arg.is_cachable(),
+            Expr::BinOp(_, lhs, rhs) => lhs.is_cachable() && rhs.is_cachable(),
+            Expr::Range(_, lhs, rhs) => {
+                lhs.as_ref().map_or(true, |v| v.is_cachable())
+                    && rhs.as_ref().map_or(true, |v| v.is_cachable())
+            }
+            Expr::Group(arg) => arg.is_cachable(),
+            Expr::Tuple(args) => args.iter().all(|arg| arg.is_cachable()),
+            // We have too little information to tell if the expression is pure:
+            Expr::Call(_, _) => false,
+            Expr::RustMacro(_, _) => false,
+            Expr::Try(_) => false,
+        }
+    }
+}
+
+fn expr_bool_lit(i: &str) -> IResult<&str, Expr<'_>> {
+    map(bool_lit, Expr::BoolLit)(i)
+}
+
+fn expr_num_lit(i: &str) -> IResult<&str, Expr<'_>> {
+    map(num_lit, Expr::NumLit)(i)
+}
+
+fn expr_array_lit(i: &str) -> IResult<&str, Expr<'_>> {
+    delimited(
+        ws(char('[')),
+        map(separated_list1(ws(char(',')), expr_any), Expr::Array),
+        ws(char(']')),
+    )(i)
+}
+
+fn expr_str_lit(i: &str) -> IResult<&str, Expr<'_>> {
+    map(str_lit, Expr::StrLit)(i)
+}
+
+fn expr_char_lit(i: &str) -> IResult<&str, Expr<'_>> {
+    map(char_lit, Expr::CharLit)(i)
+}
+
+fn expr_var(i: &str) -> IResult<&str, Expr<'_>> {
+    map(identifier, Expr::Var)(i)
+}
+
+fn expr_path(i: &str) -> IResult<&str, Expr<'_>> {
+    let (i, path) = path(i)?;
+    Ok((i, Expr::Path(path)))
+}
+
+fn expr_group(i: &str) -> IResult<&str, Expr<'_>> {
+    let (i, expr) = preceded(ws(char('(')), opt(expr_any))(i)?;
+    let expr = match expr {
+        Some(expr) => expr,
+        None => {
+            let (i, _) = char(')')(i)?;
+            return Ok((i, Expr::Tuple(vec![])));
+        }
+    };
+
+    let (i, comma) = ws(opt(peek(char(','))))(i)?;
+    if comma.is_none() {
+        let (i, _) = char(')')(i)?;
+        return Ok((i, Expr::Group(Box::new(expr))));
+    }
+
+    let mut exprs = vec![expr];
+    let (i, _) = fold_many0(
+        preceded(char(','), ws(expr_any)),
+        || (),
+        |_, expr| {
+            exprs.push(expr);
+        },
+    )(i)?;
+    let (i, _) = pair(ws(opt(char(','))), char(')'))(i)?;
+    Ok((i, Expr::Tuple(exprs)))
+}
+
+fn expr_single(i: &str) -> IResult<&str, Expr<'_>> {
+    alt((
+        expr_bool_lit,
+        expr_num_lit,
+        expr_str_lit,
+        expr_char_lit,
+        expr_path,
+        expr_rust_macro,
+        expr_array_lit,
+        expr_var,
+        expr_group,
+    ))(i)
+}
+
+enum Suffix<'a> {
+    Attr(&'a str),
+    Index(Expr<'a>),
+    Call(Vec<Expr<'a>>),
+    Try,
+}
+
+fn expr_attr(i: &str) -> IResult<&str, Suffix<'_>> {
+    map(
+        preceded(
+            ws(pair(char('.'), not(char('.')))),
+            cut(alt((num_lit, identifier))),
+        ),
+        Suffix::Attr,
+    )(i)
+}
+
+fn expr_index(i: &str) -> IResult<&str, Suffix<'_>> {
+    map(
+        preceded(ws(char('[')), cut(terminated(expr_any, ws(char(']'))))),
+        Suffix::Index,
+    )(i)
+}
+
+fn expr_call(i: &str) -> IResult<&str, Suffix<'_>> {
+    map(arguments, Suffix::Call)(i)
+}
+
+fn expr_try(i: &str) -> IResult<&str, Suffix<'_>> {
+    map(preceded(take_till(not_ws), char('?')), |_| Suffix::Try)(i)
+}
+
+fn filter(i: &str) -> IResult<&str, (&str, Option<Vec<Expr<'_>>>)> {
+    let (i, (_, fname, args)) = tuple((char('|'), ws(identifier), opt(arguments)))(i)?;
+    Ok((i, (fname, args)))
+}
+
+fn expr_filtered(i: &str) -> IResult<&str, Expr<'_>> {
+    let (i, (obj, filters)) = tuple((expr_prefix, many0(filter)))(i)?;
+
+    let mut res = obj;
+    for (fname, args) in filters {
+        res = Expr::Filter(fname, {
+            let mut args = match args {
+                Some(inner) => inner,
+                None => Vec::new(),
+            };
+            args.insert(0, res);
+            args
+        });
+    }
+
+    Ok((i, res))
+}
+
+fn expr_prefix(i: &str) -> IResult<&str, Expr<'_>> {
+    let (i, (ops, mut expr)) = pair(many0(ws(alt((tag("!"), tag("-"))))), expr_suffix)(i)?;
+    for op in ops.iter().rev() {
+        expr = Expr::Unary(op, Box::new(expr));
+    }
+    Ok((i, expr))
+}
+
+fn expr_suffix(i: &str) -> IResult<&str, Expr<'_>> {
+    let (mut i, mut expr) = expr_single(i)?;
+    loop {
+        let (j, suffix) = opt(alt((expr_attr, expr_index, expr_call, expr_try)))(i)?;
+        i = j;
+        match suffix {
+            Some(Suffix::Attr(attr)) => expr = Expr::Attr(expr.into(), attr),
+            Some(Suffix::Index(index)) => expr = Expr::Index(expr.into(), index.into()),
+            Some(Suffix::Call(args)) => expr = Expr::Call(expr.into(), args),
+            Some(Suffix::Try) => expr = Expr::Try(expr.into()),
+            None => break,
+        }
+    }
+    Ok((i, expr))
+}
+
+fn macro_arguments(i: &str) -> IResult<&str, &str> {
+    delimited(char('('), recognize(nested_parenthesis), char(')'))(i)
+}
+
+fn expr_rust_macro(i: &str) -> IResult<&str, Expr<'_>> {
+    let (i, (mname, _, args)) = tuple((identifier, char('!'), macro_arguments))(i)?;
+    Ok((i, Expr::RustMacro(mname, args)))
+}
+
+macro_rules! expr_prec_layer {
+    ( $name:ident, $inner:ident, $op:expr ) => {
+        fn $name(i: &str) -> IResult<&str, Expr<'_>> {
+            let (i, left) = $inner(i)?;
+            let (i, right) = many0(pair(
+                ws(tag($op)),
+                $inner,
+            ))(i)?;
+            Ok((
+                i,
+                right.into_iter().fold(left, |left, (op, right)| {
+                    Expr::BinOp(op, Box::new(left), Box::new(right))
+                }),
+            ))
+        }
+    };
+    ( $name:ident, $inner:ident, $( $op:expr ),+ ) => {
+        fn $name(i: &str) -> IResult<&str, Expr<'_>> {
+            let (i, left) = $inner(i)?;
+            let (i, right) = many0(pair(
+                ws(alt(($( tag($op) ),+,))),
+                $inner,
+            ))(i)?;
+            Ok((
+                i,
+                right.into_iter().fold(left, |left, (op, right)| {
+                    Expr::BinOp(op, Box::new(left), Box::new(right))
+                }),
+            ))
+        }
+    }
+}
+
+expr_prec_layer!(expr_muldivmod, expr_filtered, "*", "/", "%");
+expr_prec_layer!(expr_addsub, expr_muldivmod, "+", "-");
+expr_prec_layer!(expr_shifts, expr_addsub, ">>", "<<");
+expr_prec_layer!(expr_band, expr_shifts, "&");
+expr_prec_layer!(expr_bxor, expr_band, "^");
+expr_prec_layer!(expr_bor, expr_bxor, "|");
+expr_prec_layer!(expr_compare, expr_bor, "==", "!=", ">=", ">", "<=", "<");
+expr_prec_layer!(expr_and, expr_compare, "&&");
+expr_prec_layer!(expr_or, expr_and, "||");
+
+fn expr_any(i: &str) -> IResult<&str, Expr<'_>> {
+    let range_right = |i| pair(ws(alt((tag("..="), tag("..")))), opt(expr_or))(i);
+    alt((
+        map(range_right, |(op, right)| {
+            Expr::Range(op, None, right.map(Box::new))
+        }),
+        map(
+            pair(expr_or, opt(range_right)),
+            |(left, right)| match right {
+                Some((op, right)) => Expr::Range(op, Some(Box::new(left)), right.map(Box::new)),
+                None => left,
+            },
+        ),
+    ))(i)
+}
+
+fn arguments(i: &str) -> IResult<&str, Vec<Expr<'_>>> {
+    delimited(
+        ws(char('(')),
+        separated_list0(char(','), ws(expr_any)),
+        ws(char(')')),
+    )(i)
+}