path: root/askama_derive/src/generator.rs

use super::{get_template_source, Context, Heritage};
use input::TemplateInput;
use parser::{Cond, Expr, MatchParameter, MatchVariant, Node, Target, When, WS};
use shared::filters;

use proc_macro2::Span;

use quote::ToTokens;

use std::collections::{HashMap, HashSet};
use std::path::PathBuf;
use std::{cmp, hash, mem, str};

use syn;

use parser::parse;

pub(crate) fn generate(
    input: &TemplateInput,
    contexts: &HashMap<&PathBuf, Context>,
    heritage: &Option<Heritage>,
) -> String {
    Generator::new(input, contexts, heritage, SetChain::new()).build(&contexts[&input.path])
}

struct Generator<'a> {
    // The template input state: original struct AST and attributes
    input: &'a TemplateInput<'a>,
    // All contexts, keyed by the package-relative template path
    contexts: &'a HashMap<&'a PathBuf, Context<'a>>,
    // The heritage contains references to blocks and their ancestry
    heritage: &'a Option<Heritage<'a>>,
    // Variables accessible directly from the current scope (not redirected to context)
    locals: SetChain<'a, &'a str>,
    // Suffix whitespace from the previous literal. Will be flushed to the
    // output buffer unless suppressed by whitespace suppression on the next
    // non-literal.
    next_ws: Option<&'a str>,
    // Whitespace suppression from the previous non-literal. Will be used to
    // determine whether to flush prefix whitespace from the next literal.
    skip_ws: bool,
    // If currently in a block, this will contain the name of a potential parent block
    super_block: Option<(&'a str, usize)>,
    // buffer for writable
    buf_writable: Vec<Writable<'a>>,
    // Counter for write! hash named arguments
    named: usize,
}

impl<'a> Generator<'a> {
    fn new<'n>(
        input: &'n TemplateInput,
        contexts: &'n HashMap<&'n PathBuf, Context<'n>>,
        heritage: &'n Option<Heritage>,
        locals: SetChain<'n, &'n str>,
    ) -> Generator<'n> {
        Generator {
            input,
            contexts,
            heritage,
            locals,
            next_ws: None,
            skip_ws: false,
            super_block: None,
            buf_writable: vec![],
            named: 0,
        }
    }

    fn child(&mut self) -> Generator {
        let locals = SetChain::with_parent(&self.locals);
        Self::new(self.input, self.contexts, self.heritage, locals)
    }

    // Takes a Context and generates the relevant implementations.
    fn build(mut self, ctx: &'a Context) -> String {
        let mut buf = Buffer::new(0);
        if !ctx.blocks.is_empty() {
            if let Some(parent) = self.input.parent {
                self.deref_to_parent(&mut buf, parent);
            }
        };

        self.impl_template(ctx, &mut buf);
        self.impl_display(&mut buf);
        if cfg!(feature = "iron") {
            self.impl_modifier_response(&mut buf);
        }
        if cfg!(feature = "rocket") {
            self.impl_rocket_responder(&mut buf);
        }
        if cfg!(feature = "actix-web") {
            self.impl_actix_web_responder(&mut buf);
        }
        buf.buf
    }

    // Implement `Template` for the given context struct.
    fn impl_template(&mut self, ctx: &'a Context, buf: &mut Buffer) {
        self.write_header(buf, "::askama::Template", None);
        buf.writeln(
            "fn render_into(&self, writer: &mut ::std::fmt::Write) -> \
             ::askama::Result<()> {",
        );

        if let Some(heritage) = self.heritage {
            self.handle(heritage.root, heritage.root.nodes, buf, AstLevel::Top);
        } else {
            self.handle(ctx, &ctx.nodes, buf, AstLevel::Top);
        }

        self.flush_ws(WS(false, false));
        buf.writeln("Ok(())");
        buf.writeln("}");

        buf.writeln("fn extension() -> Option<&'static str> {");
        buf.writeln(&format!(
            "{:?}",
            self.input.path.extension().map(|s| s.to_str().unwrap())
        ));
        buf.writeln("}");

        buf.writeln("}");
    }

    // Implement `Deref<Parent>` for an inheriting context struct.
    fn deref_to_parent(&mut self, buf: &mut Buffer, parent_type: &syn::Type) {
        self.write_header(buf, "::std::ops::Deref", None);
        buf.writeln(&format!(
            "type Target = {};",
            parent_type.into_token_stream()
        ));
        buf.writeln("fn deref(&self) -> &Self::Target {");
        buf.writeln("&self._parent");
        buf.writeln("}");
        buf.writeln("}");
    }

    // Implement `Display` for the given context struct.
    fn impl_display(&mut self, buf: &mut Buffer) {
        self.write_header(buf, "::std::fmt::Display", None);
        buf.writeln("fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {");
        buf.writeln("::askama::Template::render_into(self, f).map_err(|_| ::std::fmt::Error {})");
        buf.writeln("}");
        buf.writeln("}");
    }

    // Implement iron's Modifier<Response> if enabled
    fn impl_modifier_response(&mut self, buf: &mut Buffer) {
        self.write_header(
            buf,
            "::askama::iron::Modifier<::askama::iron::Response>",
            None,
        );
        buf.writeln("fn modify(self, res: &mut ::askama::iron::Response) {");
        buf.writeln(
            "res.body = Some(Box::new(::askama::Template::render(&self).unwrap().into_bytes()));",
        );

        let ext = self
            .input
            .path
            .extension()
            .map_or("", |s| s.to_str().unwrap_or(""));
        match ext {
            "html" | "htm" => {
                buf.writeln("::askama::iron::ContentType::html().0.modify(res);");
            }
            _ => (),
        };

        buf.writeln("}");
        buf.writeln("}");
    }

    // Implement Rocket's `Responder`.
    fn impl_rocket_responder(&mut self, buf: &mut Buffer) {
        let lifetime = syn::Lifetime::new("'askama", Span::call_site());
        let param = syn::GenericParam::Lifetime(syn::LifetimeDef::new(lifetime));
        self.write_header(
            buf,
            "::askama::rocket::Responder<'askama>",
            Some(vec![param]),
        );
        buf.writeln(
            "fn respond_to(self, _: &::askama::rocket::Request) \
             -> ::askama::rocket::Result<'askama> {",
        );

        let ext = match self.input.path.extension() {
            Some(s) => s.to_str().unwrap(),
            None => "txt",
        };
        buf.writeln(&format!("::askama::rocket::respond(&self, {:?})", ext));

        buf.writeln("}");
        buf.writeln("}");
    }

    // Implement Actix-web's `Responder`.
    fn impl_actix_web_responder(&mut self, buf: &mut Buffer) {
        self.write_header(buf, "::askama::actix_web::Responder", None);
        buf.writeln("type Item = ::askama::actix_web::HttpResponse;");
        buf.writeln("type Error = ::askama::actix_web::Error;");
        buf.writeln(
            "fn respond_to<S>(self, _req: &::askama::actix_web::HttpRequest<S>) \
             -> Result<Self::Item, Self::Error> {",
        );

        let ext = match self.input.path.extension() {
            Some(s) => s.to_str().unwrap(),
            None => "txt",
        };
        buf.writeln(&format!("::askama::actix_web::respond(&self, {:?})", ext));

        buf.writeln("}");
        buf.writeln("}");
    }

    // Writes header for the `impl` for `TraitFromPathName` or `Template`
    // for the given context struct.
    fn write_header(
        &mut self,
        buf: &mut Buffer,
        target: &str,
        params: Option<Vec<syn::GenericParam>>,
    ) {
        let mut generics = self.input.ast.generics.clone();
        if let Some(params) = params {
            for param in params {
                generics.params.push(param);
            }
        }
        let (_, orig_ty_generics, _) = self.input.ast.generics.split_for_impl();
        let (impl_generics, _, where_clause) = generics.split_for_impl();
        buf.writeln(
            format!(
                "{} {} for {}{} {{",
                quote!(impl#impl_generics),
                target,
                self.input.ast.ident,
                quote!(#orig_ty_generics #where_clause),
            ).as_ref(),
        );
    }

    /* Helper methods for handling node types */

    fn handle(&mut self, ctx: &'a Context, nodes: &'a [Node], buf: &mut Buffer, level: AstLevel) {
        for n in nodes {
            match *n {
                Node::Lit(lws, val, rws) => {
                    self.visit_lit(lws, val, rws);
                }
                Node::Comment(ws) => {
                    self.write_comment(ws);
                }
                Node::Expr(ws, ref val) => {
                    self.write_expr(ws, val);
                }
                Node::LetDecl(ws, ref var) => {
                    self.write_let_decl(buf, ws, var);
                }
                Node::Let(ws, ref var, ref val) => {
                    self.write_let(buf, ws, var, val);
                }
                Node::Cond(ref conds, ws) => {
                    self.write_cond(ctx, buf, conds, ws);
                }
                Node::Match(ws1, ref expr, inter, ref arms, ws2) => {
                    self.write_match(ctx, buf, ws1, expr, inter, arms, ws2);
                }
                Node::Loop(ws1, ref var, ref iter, ref body, ws2) => {
                    self.write_loop(ctx, buf, ws1, var, iter, body, ws2);
                }
                Node::BlockDef(ws1, name, _, ws2) => {
                    if AstLevel::Nested == level {
                        panic!(
                            "blocks ('{}') are only allowed at the top level of a template \
                             or another block",
                            name
                        );
                    }
                    let outer = WS(ws1.0, ws2.1);
                    self.write_block(buf, Some(name), outer);
                }
                Node::Include(ws, path) => {
                    self.handle_include(ctx, buf, ws, path);
                }
                Node::Call(ws, scope, name, ref args) => {
                    self.write_call(ctx, buf, ws, scope, name, args);
                }
                Node::Macro(_, ref m) => {
                    if level != AstLevel::Top {
                        panic!("macro blocks only allowed at the top level");
                    }
                    self.flush_ws(m.ws1);
                    self.prepare_ws(m.ws2);
                }
                Node::Import(ws, _, _) => {
                    if level != AstLevel::Top {
                        panic!("import blocks only allowed at the top level");
                    }
                    self.handle_ws(ws);
                }
                Node::Extends(_) => {
                    if level != AstLevel::Top {
                        panic!("extend blocks only allowed at the top level");
                    }
                    // No whitespace handling: child template top-level is not used,
                    // except for the blocks defined in it.
                }
            }
        }

        if AstLevel::Top == level {
            self.write_buf_writable(buf);
        }
    }

    fn write_cond(&mut self, ctx: &'a Context, buf: &mut Buffer, conds: &'a [Cond], ws: WS) {
        for (i, &(cws, ref cond, ref nodes)) in conds.iter().enumerate() {
            self.handle_ws(cws);
            self.write_buf_writable(buf);
            match *cond {
                Some(ref expr) => {
                    let expr_code = self.visit_expr_root(expr);
                    if i == 0 {
                        buf.write("if ");
                    } else {
                        self.write_buf_writable(buf);
                        buf.dedent();
                        buf.write("} else if ");
                    }
                    buf.write(&expr_code);
                }
                None => {
                    self.write_buf_writable(buf);
                    buf.dedent();
                    buf.write("} else");
                }
            }
            buf.writeln(" {");
            self.locals.push();
            self.handle(ctx, nodes, buf, AstLevel::Nested);
            self.locals.pop();
        }
        self.handle_ws(ws);
        self.write_buf_writable(buf);
        buf.writeln("}");
    }

    fn write_match(
        &mut self,
        ctx: &'a Context,
        buf: &mut Buffer,
        ws1: WS,
        expr: &Expr,
        inter: Option<&'a str>,
        arms: &'a [When],
        ws2: WS,
    ) {
        self.flush_ws(ws1);
        self.write_buf_writable(buf);
        if let Some(inter) = inter {
            if !inter.is_empty() {
                self.next_ws = Some(inter);
            }
        }

        let expr_code = self.visit_expr_root(expr);
        buf.writeln(&format!("match &{} {{", expr_code));
        for arm in arms {
            let &(ws, ref variant, ref params, ref body) = arm;
            self.locals.push();
            match *variant {
                Some(ref param) => {
                    self.visit_match_variant(buf, param);
                }
                None => buf.write("_"),
            };
            if !params.is_empty() {
                buf.write("(");
                for (i, param) in params.iter().enumerate() {
                    if let MatchParameter::Name(p) = *param {
                        self.locals.insert(p);
                    }
                    if i > 0 {
                        buf.write(", ");
                    }
                    self.visit_match_param(buf, param);
                }
                buf.write(")");
            }
            buf.writeln(" => {");
            self.handle_ws(ws);
            self.handle(ctx, body, buf, AstLevel::Nested);
            self.write_buf_writable(buf);
            buf.writeln("}");
            self.locals.pop();
        }

        buf.writeln("}");
        self.handle_ws(ws2);
    }

    fn write_loop(
        &mut self,
        ctx: &'a Context,
        buf: &mut Buffer,
        ws1: WS,
        var: &'a Target,
        iter: &Expr,
        body: &'a [Node],
        ws2: WS,
    ) {
        self.handle_ws(ws1);
        self.locals.push();

        let expr_code = self.visit_expr_root(iter);

        self.write_buf_writable(buf);
        buf.write("for (_loop_index, ");
        let targets = self.visit_target(var);
        for name in &targets {
            self.locals.insert(name);
            buf.write(name);
        }
        match iter {
            Expr::Range(_, _, _) => buf.writeln(&format!(") in ({}).enumerate() {{", expr_code)),
            _ => buf.writeln(&format!(") in (&{}).into_iter().enumerate() {{", expr_code)),
        };

        self.handle(ctx, body, buf, AstLevel::Nested);
        self.handle_ws(ws2);

        self.write_buf_writable(buf);
        buf.writeln("}");
        self.locals.pop();
    }

    fn write_call(
        &mut self,
        ctx: &'a Context,
        buf: &mut Buffer,
        ws: WS,
        scope: Option<&str>,
        name: &str,
        args: &[Expr],
    ) {
        if name == "super" {
            self.write_block(buf, None, ws);
            return;
        }

        let (def, own_ctx) = if let Some(s) = scope {
            let path = ctx
                .imports
                .get(s)
                .unwrap_or_else(|| panic!("no import found for scope '{}'", s));
            let mctx = self
                .contexts
                .get(path)
                .unwrap_or_else(|| panic!("context for '{:?}' not found", path));
            (
                mctx.macros
                    .get(name)
                    .unwrap_or_else(|| panic!("macro '{}' not found in scope '{}'", s, name)),
                mctx,
            )
        } else {
            (
                ctx.macros
                    .get(name)
                    .unwrap_or_else(|| panic!("macro '{}' not found", name)),
                ctx,
            )
        };

        self.flush_ws(ws); // Cannot handle_ws() here: whitespace from macro definition comes first
        self.locals.push();
        self.write_buf_writable(buf);
        buf.writeln("{");
        self.prepare_ws(def.ws1);

        for (i, arg) in def.args.iter().enumerate() {
            let expr_code = self.visit_expr_root(
                args.get(i)
                    .unwrap_or_else(|| panic!("macro '{}' takes more than {} arguments", name, i)),
            );
            buf.writeln(&format!("let {} = &{};", arg, expr_code));
            self.locals.insert(arg);
        }

        self.handle(own_ctx, &def.nodes, buf, AstLevel::Nested);

        self.flush_ws(def.ws2);
        self.write_buf_writable(buf);
        buf.writeln("}");
        self.locals.pop();
        self.prepare_ws(ws);
    }

    fn handle_include(&mut self, ctx: &'a Context, buf: &mut Buffer, ws: WS, path: &str) {
        self.flush_ws(ws);
        self.write_buf_writable(buf);
        let path = self
            .input
            .config
            .find_template(path, Some(&self.input.path));
        let src = get_template_source(&path);
        let nodes = parse(&src, self.input.syntax);
        {
            // Since nodes must not outlive the Generator, we instantiate
            // a nested Generator here to handle the include's nodes.
            let mut gen = self.child();
            gen.handle(ctx, &nodes, buf, AstLevel::Nested);
            gen.write_buf_writable(buf);
        }
        self.prepare_ws(ws);
    }

    fn write_let_decl(&mut self, buf: &mut Buffer, ws: WS, var: &'a Target) {
        self.handle_ws(ws);
        self.write_buf_writable(buf);
        buf.write("let ");
        match *var {
            Target::Name(name) => {
                self.locals.insert(name);
                buf.write(name);
            }
        }
        buf.writeln(";");
    }

    fn write_let(&mut self, buf: &mut Buffer, ws: WS, var: &'a Target, val: &Expr) {
        self.handle_ws(ws);
        let mut expr_buf = Buffer::new(0);
        self.visit_expr(&mut expr_buf, val);

        match *var {
            Target::Name(name) => {
                if !self.locals.contains(name) {
                    buf.write("let ");
                    self.locals.insert(name);
                }
                buf.write(name);
            }
        }
        buf.writeln(&format!(" = {};", &expr_buf.buf));
    }

    // If `name` is `Some`, this is a call to a block definition, and we have to find
    // the first block for that name from the ancestry chain. If name is `None`, this
    // is from a `super()` call, and we can get the name from `self.super_block`.
    fn write_block(&mut self, buf: &mut Buffer, name: Option<&'a str>, outer: WS) {
        // Flush preceding whitespace according to the outer WS spec
        self.flush_ws(outer);

        let prev_block = self.super_block;
        let cur = match (name, prev_block) {
            // The top-level context contains a block definition
            (Some(cur_name), None) => (cur_name, 0),
            // A block definition contains a block definition of the same name
            (Some(cur_name), Some((prev_name, _))) if cur_name == prev_name => {
                panic!("cannot define recursive blocks ({})", cur_name)
            }
            // A block definition contains a definition of another block
            (Some(cur_name), Some((_, _))) => (cur_name, 0),
            // `super()` was called inside a block
            (None, Some((prev_name, gen))) => (prev_name, gen + 1),
            // `super()` is called from outside a block
            (None, None) => panic!("cannot call 'super()' outside block"),
        };
        self.super_block = Some(cur);

        // Get the block definition from the heritage chain
        let heritage = self
            .heritage
            .as_ref()
            .unwrap_or_else(|| panic!("no block ancestors available"));
        let (ctx, def) = heritage.blocks[cur.0]
            .get(cur.1)
            .unwrap_or_else(|| match name {
                None => panic!("no super() block found for block '{}'", cur.0),
                Some(name) => panic!("no block found for name '{}'", name),
            });

        // Get the nodes and whitespace suppression data from the block definition
        let (ws1, nodes, ws2) = if let Node::BlockDef(ws1, _, nodes, ws2) = def {
            (ws1, nodes, ws2)
        } else {
            unreachable!()
        };

        // Handle inner whitespace suppression spec and process block nodes
        self.prepare_ws(*ws1);
        self.locals.push();
        self.handle(ctx, nodes, buf, AstLevel::Block);
        self.locals.pop();
        self.flush_ws(*ws2);

        // Restore original block context and set whitespace suppression for
        // succeeding whitespace according to the outer WS spec
        self.super_block = prev_block;
        self.prepare_ws(outer);
    }

    fn write_expr(&mut self, ws: WS, s: &'a Expr<'a>) {
        self.handle_ws(ws);
        self.buf_writable.push(Writable::Expr(s));
    }

    // Write expression buffer and empty
    fn write_buf_writable(&mut self, buf: &mut Buffer) {
        if self.buf_writable.is_empty() {
            return;
        } else {
            if self.buf_writable.iter().all(|w| match w {
                Writable::Lit(_) => true,
                _ => false,
            }) {
                let mut buf_lit = Buffer::new(0);
                for s in mem::replace(&mut self.buf_writable, vec![]) {
                    if let Writable::Lit(s) = s {
                        buf_lit.write(s);
                    };
                }

                buf.writeln(&format!("writer.write_str({:#?})?;", &buf_lit.buf));
            } else {
                let mut buf_format = Buffer::new(0);
                let mut buf_expr = Buffer::new(buf.indent + 1);
                let mut expr_cache = HashMap::with_capacity(self.buf_writable.len());
                for s in mem::replace(&mut self.buf_writable, vec![]) {
                    match s {
                        Writable::Lit(s) => {
                            buf_format.write(&s.replace("{", "{{").replace("}", "}}"));
                        }
                        Writable::Expr(s) => {
                            use self::DisplayWrap::*;
                            use super::input::EscapeMode::*;
                            let mut expr_buf = Buffer::new(0);
                            let wrapped = self.visit_expr(&mut expr_buf, s);
                            let expression = match (wrapped, &self.input.escaping) {
                                (Wrapped, &Html) | (Wrapped, &None) | (Unwrapped, &None) => {
                                    expr_buf.buf.clone()
                                }
                                (Unwrapped, &Html) => {
                                    format!(
                                        "::askama::MarkupDisplay::from(&{})",
                                        expr_buf.buf.clone()
                                    )
                                }
                            };

                            let id = expr_cache.entry(expression).or_insert_with(|| {
                                let id = self.named;
                                self.named += 1;

                                buf_expr.write(&format!("a{}=", id));
                                buf_expr.write("&");
                                buf_expr.write(&expr_buf.buf);
                                buf_expr.writeln(",");

                                id
                            });

                            buf_format.write(&format!("{{a{}}}", id));
                        }
                    }
                }

                buf.writeln("write!(");
                buf.indent();
                buf.writeln("writer,");
                buf.writeln(&format!("{:#?},", &buf_format.buf));
                buf.write(buf_expr.buf.trim());
                buf.dedent();
                buf.writeln(")?;");
            }
        }
    }

    fn visit_lit(&mut self, lws: &'a str, val: &'a str, rws: &'a str) {
        assert!(self.next_ws.is_none());
        if !lws.is_empty() {
            if self.skip_ws {
                self.skip_ws = false;
            } else if val.is_empty() {
                assert!(rws.is_empty());
                self.next_ws = Some(lws);
            } else {
                self.buf_writable.push(Writable::Lit(lws));
            }
        }

        if !val.is_empty() {
            self.buf_writable.push(Writable::Lit(val));
        }

        if !rws.is_empty() {
            self.next_ws = Some(rws);
        }
    }

    fn write_comment(&mut self, ws: WS) {
        self.handle_ws(ws);
    }

    /* Visitor methods for expression types */

    fn visit_expr_root(&mut self, expr: &Expr) -> String {
        let mut buf = Buffer::new(0);
        self.visit_expr(&mut buf, expr);
        buf.buf
    }

    fn visit_expr(&mut self, buf: &mut Buffer, expr: &Expr) -> DisplayWrap {
        match *expr {
            Expr::NumLit(s) => self.visit_num_lit(buf, s),
            Expr::StrLit(s) => self.visit_str_lit(buf, s),
            Expr::Var(s) => self.visit_var(buf, s),
            Expr::Path(ref path) => self.visit_path(buf, path),
            Expr::Array(ref elements) => self.visit_array(buf, elements),
            Expr::Attr(ref obj, name) => self.visit_attr(buf, obj, name),
            Expr::Index(ref obj, ref key) => self.visit_index(buf, obj, key),
            Expr::Filter(name, ref args) => self.visit_filter(buf, name, args),
            Expr::Unary(op, ref inner) => self.visit_unary(buf, op, inner),
            Expr::BinOp(op, ref left, ref right) => self.visit_binop(buf, op, left, right),
            Expr::Range(op, ref left, ref right) => self.visit_range(buf, op, left, right),
            Expr::Group(ref inner) => self.visit_group(buf, inner),
            Expr::MethodCall(ref obj, method, ref args) => {
                self.visit_method_call(buf, obj, method, args)
            }
            Expr::RustMacro(name, ref args) => self.visit_rust_macro(buf, name, args),
        }
    }

    fn visit_rust_macro(&mut self, buf: &mut Buffer, name: &str, args: &[Expr]) -> DisplayWrap {
        buf.write(name);
        buf.write("!(");
        self._visit_args(buf, args);
        buf.write(")");

        DisplayWrap::Unwrapped
    }

    fn visit_match_variant(&mut self, buf: &mut Buffer, param: &MatchVariant) -> DisplayWrap {
        let mut expr_buf = Buffer::new(0);
        let wrapped = match *param {
            MatchVariant::StrLit(s) => {
                expr_buf.write("&");
                self.visit_str_lit(&mut expr_buf, s)
            }
            MatchVariant::NumLit(s) => self.visit_num_lit(&mut expr_buf, s),
            MatchVariant::Name(s) => {
                expr_buf.write(s);
                DisplayWrap::Unwrapped
            }
            MatchVariant::Path(ref s) => {
                expr_buf.write(&s.join("::"));
                DisplayWrap::Unwrapped
            }
        };
        buf.write(&expr_buf.buf);
        wrapped
    }

    fn visit_match_param(&mut self, buf: &mut Buffer, param: &MatchParameter) -> DisplayWrap {
        let mut expr_buf = Buffer::new(0);
        let wrapped = match *param {
            MatchParameter::NumLit(s) => self.visit_num_lit(&mut expr_buf, s),
            MatchParameter::StrLit(s) => self.visit_str_lit(&mut expr_buf, s),
            MatchParameter::Name(s) => {
                expr_buf.write(s);
                DisplayWrap::Unwrapped
            }
        };
        buf.write(&expr_buf.buf);
        wrapped
    }

    fn visit_filter(&mut self, buf: &mut Buffer, name: &str, args: &[Expr]) -> DisplayWrap {
        if name == "format" {
            self._visit_format_filter(buf, args);
            return DisplayWrap::Unwrapped;
        } else if name == "join" {
            self._visit_join_filter(buf, args);
            return DisplayWrap::Unwrapped;
        }

        if filters::BUILT_IN_FILTERS.contains(&name) {
            buf.write(&format!("::askama::filters::{}(&", name));
        } else {
            buf.write(&format!("filters::{}(&", name));
        }

        self._visit_args(buf, args);
        buf.write(")?");
        if name == "safe" || name == "escape" || name == "e" || name == "json" {
            DisplayWrap::Wrapped
        } else {
            DisplayWrap::Unwrapped
        }
    }

    fn _visit_format_filter(&mut self, buf: &mut Buffer, args: &[Expr]) {
        buf.write("format!(");
        self._visit_args(buf, args);
        buf.write(")");
    }

    // Force type coercion on first argument to `join` filter (see #39).
    fn _visit_join_filter(&mut self, buf: &mut Buffer, args: &[Expr]) {
        buf.write("::askama::filters::join((&");
        for (i, arg) in args.iter().enumerate() {
            if i > 0 {
                buf.write(", &");
            }
            self.visit_expr(buf, arg);
            if i == 0 {
                buf.write(").into_iter()");
            }
        }
        buf.write(")?");
    }

    fn _visit_args(&mut self, buf: &mut Buffer, args: &[Expr]) {
        for (i, arg) in args.iter().enumerate() {
            if i > 0 {
                buf.write(", &");
            }

            let scoped = match *arg {
                Expr::Filter(_, _) | Expr::MethodCall(_, _, _) => true,
                _ => false,
            };

            if scoped {
                buf.writeln("{");
                self.visit_expr(buf, arg);
                buf.writeln("}");
            } else {
                self.visit_expr(buf, arg);
            }
        }
    }

    fn visit_attr(&mut self, buf: &mut Buffer, obj: &Expr, attr: &str) -> DisplayWrap {
        if let Expr::Var(name) = *obj {
            if name == "loop" {
                if attr == "index" {
                    buf.write("(_loop_index + 1)");
                    return DisplayWrap::Unwrapped;
                } else if attr == "index0" {
                    buf.write("_loop_index");
                    return DisplayWrap::Unwrapped;
                } else if attr == "first" {
                    buf.write("(_loop_index == 0)");
                    return DisplayWrap::Unwrapped;
                } else {
                    panic!("unknown loop variable");
                }
            }
        }
        self.visit_expr(buf, obj);
        buf.write(&format!(".{}", attr));
        DisplayWrap::Unwrapped
    }

    fn visit_index(&mut self, buf: &mut Buffer, obj: &Expr, key: &Expr) -> DisplayWrap {
        buf.write("&");
        self.visit_expr(buf, obj);
        buf.write("[");
        self.visit_expr(buf, key);
        buf.write("]");
        DisplayWrap::Unwrapped
    }

    fn visit_method_call(
        &mut self,
        buf: &mut Buffer,
        obj: &Expr,
        method: &str,
        args: &[Expr],
    ) -> DisplayWrap {
        if let Expr::Var("self") = obj {
            buf.write("self");
        } else {
            self.visit_expr(buf, obj);
        }

        buf.write(&format!(".{}(", method));
        self._visit_args(buf, args);
        buf.write(")");
        DisplayWrap::Unwrapped
    }

    fn visit_unary(&mut self, buf: &mut Buffer, op: &str, inner: &Expr) -> DisplayWrap {
        buf.write(op);
        self.visit_expr(buf, inner);
        DisplayWrap::Unwrapped
    }

    fn visit_range(
        &mut self,
        buf: &mut Buffer,
        op: &str,
        left: &Option<Box<Expr>>,
        right: &Option<Box<Expr>>,
    ) -> DisplayWrap {
        if let Some(left) = left {
            self.visit_expr(buf, left);
        }
        buf.write(op);
        if let Some(right) = right {
            self.visit_expr(buf, right);
        }
        DisplayWrap::Unwrapped
    }

    fn visit_binop(
        &mut self,
        buf: &mut Buffer,
        op: &str,
        left: &Expr,
        right: &Expr,
    ) -> DisplayWrap {
        self.visit_expr(buf, left);
        buf.write(&format!(" {} ", op));
        self.visit_expr(buf, right);
        DisplayWrap::Unwrapped
    }

    fn visit_group(&mut self, buf: &mut Buffer, inner: &Expr) -> DisplayWrap {
        buf.write("(");
        self.visit_expr(buf, inner);
        buf.write(")");
        DisplayWrap::Unwrapped
    }

    fn visit_array(&mut self, buf: &mut Buffer, elements: &[Expr]) -> DisplayWrap {
        buf.write("[");
        for (i, el) in elements.iter().enumerate() {
            if i > 0 {
                buf.write(", ");
            }
            self.visit_expr(buf, el);
        }
        buf.write("]");
        DisplayWrap::Unwrapped
    }

    fn visit_path(&mut self, buf: &mut Buffer, path: &[&str]) -> DisplayWrap {
        for (i, part) in path.iter().enumerate() {
            if i > 0 {
                buf.write("::");
            }
            buf.write(part);
        }
        DisplayWrap::Unwrapped
    }

    fn visit_var(&mut self, buf: &mut Buffer, s: &str) -> DisplayWrap {
        if self.locals.contains(s) {
            buf.write(s);
        } else {
            buf.write("self.");
            buf.write(s);
        }
        DisplayWrap::Unwrapped
    }

    fn visit_str_lit(&mut self, buf: &mut Buffer, s: &str) -> DisplayWrap {
        buf.write(&format!("\"{}\"", s));
        DisplayWrap::Unwrapped
    }

    fn visit_num_lit(&mut self, buf: &mut Buffer, s: &str) -> DisplayWrap {
        buf.write(s);
        DisplayWrap::Unwrapped
    }

    fn visit_target_single<'t>(&mut self, name: &'t str) -> Vec<&'t str> {
        vec![name]
    }

    fn visit_target<'t>(&mut self, target: &'t Target) -> Vec<&'t str> {
        match *target {
            Target::Name(s) => self.visit_target_single(s),
        }
    }

    /* Helper methods for dealing with whitespace nodes */

    // Combines `flush_ws()` and `prepare_ws()` to handle both trailing whitespace from the
    // preceding literal and leading whitespace from the succeeding literal.
    fn handle_ws(&mut self, ws: WS) {
        self.flush_ws(ws);
        self.prepare_ws(ws);
    }

    // If the previous literal left some trailing whitespace in `next_ws` and the
    // prefix whitespace suppressor from the given argument, flush that whitespace.
    // In either case, `next_ws` is reset to `None` (no trailing whitespace).
    fn flush_ws(&mut self, ws: WS) {
        if self.next_ws.is_some() && !ws.0 {
            let val = self.next_ws.unwrap();
            if !val.is_empty() {
                self.buf_writable.push(Writable::Lit(val));
            }
        }
        self.next_ws = None;
    }

    // Sets `skip_ws` to match the suffix whitespace suppressor from the given
    // argument, to determine whether to suppress leading whitespace from the
    // next literal.
    fn prepare_ws(&mut self, ws: WS) {
        self.skip_ws = ws.1;
    }
}

struct Buffer {
    // The buffer to generate the code into
    buf: String,
    // The current level of indentation (in spaces)
    indent: u8,
    // Whether the output buffer is currently at the start of a line
    start: bool,
}

impl Buffer {
    fn new(indent: u8) -> Self {
        Self {
            buf: String::new(),
            indent,
            start: true,
        }
    }

    fn writeln(&mut self, s: &str) {
        if s == "}" {
            self.dedent();
        }
        if !s.is_empty() {
            self.write(s);
        }
        self.buf.push('\n');
        if s.ends_with('{') {
            self.indent();
        }
        self.start = true;
    }

    fn write(&mut self, s: &str) {
        if self.start {
            for _ in 0..(self.indent * 4) {
                self.buf.push(' ');
            }
            self.start = false;
        }
        self.buf.push_str(s);
    }

    fn indent(&mut self) {
        self.indent += 1;
    }

    fn dedent(&mut self) {
        if self.indent == 0 {
            panic!("dedent() called while indentation == 0");
        }
        self.indent -= 1;
    }
}

struct SetChain<'a, T: 'a>
where
    T: cmp::Eq + hash::Hash,
{
    parent: Option<&'a SetChain<'a, T>>,
    scopes: Vec<HashSet<T>>,
}

impl<'a, T: 'a> SetChain<'a, T>
where
    T: cmp::Eq + hash::Hash,
{
    fn new() -> SetChain<'a, T> {
        SetChain {
            parent: None,
            scopes: vec![HashSet::new()],
        }
    }
    fn with_parent<'p>(parent: &'p SetChain<T>) -> SetChain<'p, T> {
        SetChain {
            parent: Some(parent),
            scopes: vec![HashSet::new()],
        }
    }
    fn contains(&self, val: T) -> bool {
        self.scopes.iter().rev().any(|set| set.contains(&val)) || match self.parent {
            Some(set) => set.contains(val),
            None => false,
        }
    }
    fn insert(&mut self, val: T) {
        self.scopes.last_mut().unwrap().insert(val);
    }
    fn push(&mut self) {
        self.scopes.push(HashSet::new());
    }
    fn pop(&mut self) {
        self.scopes.pop().unwrap();
        assert!(!self.scopes.is_empty());
    }
}

#[derive(Clone, PartialEq)]
enum AstLevel {
    Top,
    Block,
    Nested,
}

impl Copy for AstLevel {}

#[derive(Clone)]
enum DisplayWrap {
    Wrapped,
    Unwrapped,
}

impl Copy for DisplayWrap {}

#[derive(Debug)]
enum Writable<'a> {
    Lit(&'a str),
    Expr(&'a Expr<'a>),
}