Added support for slicing

minijinja/src/compiler/ast.rs

@@ -91,6 +91,7 @@ impl<'a> fmt::Debug for Stmt<'a> {
 pub enum Expr<'a> {
     Var(Spanned<Var<'a>>),
     Const(Spanned<Const>),
+    Slice(Spanned<Slice<'a>>),
     UnaryOp(Spanned<UnaryOp<'a>>),
     BinOp(Spanned<BinOp<'a>>),
     IfExpr(Spanned<IfExpr<'a>>),
@@ -109,6 +110,7 @@ impl<'a> fmt::Debug for Expr<'a> {
         match self {
             Expr::Var(s) => fmt::Debug::fmt(s, f),
             Expr::Const(s) => fmt::Debug::fmt(s, f),
+            Expr::Slice(s) => fmt::Debug::fmt(s, f),
             Expr::UnaryOp(s) => fmt::Debug::fmt(s, f),
             Expr::BinOp(s) => fmt::Debug::fmt(s, f),
             Expr::IfExpr(s) => fmt::Debug::fmt(s, f),
@@ -228,6 +230,15 @@ pub struct Const {
     pub value: Value,
 }
 
+/// Represents a slice.
+#[cfg_attr(feature = "internal_debug", derive(Debug))]
+pub struct Slice<'a> {
+    pub expr: Expr<'a>,
+    pub start: Option<Expr<'a>>,
+    pub stop: Option<Expr<'a>>,
+    pub step: Option<Expr<'a>>,
+}
+
 /// A kind of unary operator.
 #[cfg_attr(feature = "internal_debug", derive(Debug))]
 pub enum UnaryOpKind {

minijinja/src/compiler/codegen.rs

@@ -402,6 +402,27 @@ impl<'source> CodeGenerator<'source> {
                 self.set_line_from_span(v.span());
                 self.add(Instruction::LoadConst(v.value.clone()));
             }
+            ast::Expr::Slice(s) => {
+                self.push_span(s.span());
+                self.compile_expr(&s.expr)?;
+                if let Some(ref start) = s.start {
+                    self.compile_expr(start)?;
+                } else {
+                    self.add(Instruction::LoadConst(Value::from(0)));
+                }
+                if let Some(ref stop) = s.stop {
+                    self.compile_expr(stop)?;
+                } else {
+                    self.add(Instruction::LoadConst(Value::from(())));
+                }
+                if let Some(ref step) = s.step {
+                    self.compile_expr(step)?;
+                } else {
+                    self.add(Instruction::LoadConst(Value::from(1)));
+                }
+                self.add(Instruction::Slice);
+                self.pop_span();
+            }
             ast::Expr::UnaryOp(c) => {
                 self.set_line_from_span(c.span());
                 self.compile_expr(&c.expr)?;

minijinja/src/compiler/instructions.rs

@@ -31,6 +31,9 @@ pub enum Instruction<'source> {
     /// Looks up an item.
     GetItem,
 
+    /// Performs a slice operation.
+    Slice,
+
     /// Loads a constant value.
     LoadConst(Value),
 

minijinja/src/compiler/parser.rs

@@ -315,15 +315,54 @@ impl<'a> Parser<'a> {
                 }
                 Some((Token::BracketOpen, _)) => {
                     self.stream.next()?;
-                    let subscript_expr = self.parse_expr()?;
+
+                    let mut start = None;
+                    let mut stop = None;
+                    let mut step = None;
+                    let mut is_slice = false;
+
+                    if !matches!(self.stream.current()?, Some((Token::Colon, _))) {
+                        start = Some(self.parse_expr()?);
+                    }
+                    if matches!(self.stream.current()?, Some((Token::Colon, _))) {
+                        is_slice = true;
+                        self.stream.next()?;
+                        if !matches!(
+                            self.stream.current()?,
+                            Some((Token::BracketClose | Token::Colon, _))
+                        ) {
+                            stop = Some(self.parse_expr()?);
+                        }
+                        if matches!(self.stream.current()?, Some((Token::Colon, _))) {
+                            self.stream.next()?;
+                            if !matches!(self.stream.current()?, Some((Token::BracketClose, _))) {
+                                step = Some(self.parse_expr()?);
+                            }
+                        }
+                    }
                     expect_token!(self, Token::BracketClose, "`]`")?;
-                    expr = ast::Expr::GetItem(Spanned::new(
-                        ast::GetItem {
-                            expr,
-                            subscript_expr,
-                        },
-                        self.stream.expand_span(span),
-                    ));
+
+                    if !is_slice {
+                        expr = ast::Expr::GetItem(Spanned::new(
+                            ast::GetItem {
+                                expr,
+                                subscript_expr: start.ok_or_else(|| {
+                                    Error::new(ErrorKind::SyntaxError, "empty subscript")
+                                })?,
+                            },
+                            self.stream.expand_span(span),
+                        ));
+                    } else {
+                        expr = ast::Expr::Slice(Spanned::new(
+                            ast::Slice {
+                                expr,
+                                start,
+                                stop,
+                                step,
+                            },
+                            self.stream.expand_span(span),
+                        ));
+                    }
                 }
                 Some((Token::ParenOpen, _)) => {
                     let args = self.parse_args()?;

minijinja/src/syntax.rs

@@ -136,6 +136,8 @@
 //!   which are treated like a dict syntax.  Eg: `foo(a=1, b=2)` is the same as
 //!   `foo({"a": 1, "b": 2})`.
 //! - ``.`` / ``[]``: Get an attribute of an object.
+//! - ``[start:stop]`` / ``[start:stop:step]``: slices a list or string.  All three expressions
+//!   are optional (`start`, `stop`, `step`).
 //!
 //! ### If Expressions
 //!

minijinja/src/value/ops.rs

@@ -1,4 +1,4 @@
-use std::convert::TryFrom;
+use std::convert::{TryFrom, TryInto};
 use std::fmt::Write;
 
 use crate::error::{Error, ErrorKind};
@@ -48,6 +48,73 @@ pub fn coerce(a: &Value, b: &Value) -> Option<CoerceResult> {
     }
 }
 
+fn get_offset_and_len<F: FnOnce() -> usize>(
+    start: i64,
+    stop: Option<i64>,
+    end: F,
+) -> (usize, usize) {
+    if start < 0 || stop.map_or(true, |x| x < 0) {
+        let end = end();
+        let start = if start < 0 {
+            (end as i64 + start) as usize
+        } else {
+            start as usize
+        };
+        let stop = match stop {
+            None => end,
+            Some(x) if x < 0 => (end as i64 + x) as usize,
+            Some(x) => x as usize,
+        };
+        (start, stop.saturating_sub(start))
+    } else {
+        (
+            start as usize,
+            (stop.unwrap() as usize).saturating_sub(start as usize),
+        )
+    }
+}
+
+pub fn slice(value: Value, start: Value, stop: Value, step: Value) -> Result<Value, Error> {
+    let start: i64 = if start.is_none() {
+        0
+    } else {
+        start.try_into()?
+    };
+    let stop: Option<i64> = if stop.is_none() {
+        None
+    } else {
+        Some(stop.try_into()?)
+    };
+    let step = if step.is_none() {
+        1
+    } else {
+        u64::try_from(step)? as usize
+    };
+
+    if let Some(s) = value.as_str() {
+        let (start, len) = get_offset_and_len(start, stop, || s.chars().count());
+        return Ok(Value::from(
+            s.chars()
+                .skip(start)
+                .take(len)
+                .step_by(step)
+                .collect::<String>(),
+        ));
+    }
+
+    let slice = value.as_slice()?;
+    let (start, len) = get_offset_and_len(start, stop, || slice.len());
+    Ok(Value::from(
+        slice
+            .iter()
+            .skip(start)
+            .take(len)
+            .step_by(step)
+            .cloned()
+            .collect::<Vec<_>>(),
+    ))
+}
+
 fn int_as_value(val: i128) -> Value {
     if val as i64 as i128 == val {
         (val as i64).into()

minijinja/src/vm/mod.rs

@@ -146,6 +146,13 @@ impl<'env> Vm<'env> {
                     let value = stack.pop();
                     stack.push(try_ctx!(value.get_item(&attr)));
                 }
+                Instruction::Slice => {
+                    let step = stack.pop();
+                    let stop = stack.pop();
+                    let start = stack.pop();
+                    let value = stack.pop();
+                    stack.push(try_ctx!(ops::slice(value, start, stop, step)));
+                }
                 Instruction::LoadConst(value) => {
                     stack.push(value.clone());
                 }

minijinja/tests/inputs/slicing.txt

@@ -0,0 +1,17 @@
+{
+  "hello": "Hello World",
+  "intrange": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+}
+---
+{{ hello[:] }}
+{{ hello[1:] }}
+{{ hello[1:-1] }}
+{{ hello[::2] }}
+{{ hello[2:10] }}
+{{ hello[2:10:2] }}
+{{ intrange[:] }}
+{{ intrange[1:] }}
+{{ intrange[1:-1] }}
+{{ intrange[::2] }}
+{{ intrange[2:10] }}
+{{ intrange[2:10:2] }}

minijinja/tests/snapshots/test_templates__vm@slicing.txt.snap

@@ -0,0 +1,31 @@
+---
+source: minijinja/tests/test_templates.rs
+description: "{{ hello[:] }}\n{{ hello[1:] }}\n{{ hello[1:-1] }}\n{{ hello[::2] }}\n{{ hello[2:10] }}\n{{ hello[2:10:2] }}\n{{ intrange[:] }}\n{{ intrange[1:] }}\n{{ intrange[1:-1] }}\n{{ intrange[::2] }}\n{{ intrange[2:10] }}\n{{ intrange[2:10:2] }}"
+info:
+  hello: Hello World
+  intrange:
+    - 0
+    - 1
+    - 2
+    - 3
+    - 4
+    - 5
+    - 6
+    - 7
+    - 8
+    - 9
+input_file: minijinja/tests/inputs/slicing.txt
+---
+Hello World
+ello World
+ello Worl
+HloWrd
+llo Worl
+loWr
+[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+[1, 2, 3, 4, 5, 6, 7, 8, 9]
+[1, 2, 3, 4, 5, 6, 7, 8]
+[0, 2, 4, 6, 8]
+[2, 3, 4, 5, 6, 7, 8, 9]
+[2, 4, 6, 8]
+