rust/src/chains.rs

// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// Formatting of chained expressions, i.e. expressions which are chained by
// dots: struct and enum field access and method calls.
//
// Instead of walking these subexpressions one-by-one, as is our usual strategy
// for expression formatting, we collect maximal sequences of these expressions
// and handle them simultaneously.
//
// Whenever possible, the entire chain is put on a single line. If that fails,
// we put each subexpression on a separate, much like the (default) function
// argument function argument strategy.

use Indent;
use rewrite::{Rewrite, RewriteContext};
use utils::first_line_width;
use expr::rewrite_call;

use syntax::{ast, ptr};
use syntax::codemap::{mk_sp, Span};
use syntax::print::pprust;

pub fn rewrite_chain(mut expr: &ast::Expr,
                     context: &RewriteContext,
                     width: usize,
                     offset: Indent)
                     -> Option<String> {
    let total_span = expr.span;
    let mut subexpr_list = vec![expr];

    while let Some(subexpr) = pop_expr_chain(expr) {
        subexpr_list.push(subexpr);
        expr = subexpr;
    }

    let parent = subexpr_list.pop().unwrap();
    let parent_rewrite = try_opt!(expr.rewrite(context, width, offset));
    let (extra_indent, extend) = if !parent_rewrite.contains('\n') && is_continuable(parent) ||
                                    parent_rewrite.len() <= context.config.tab_spaces {
        (parent_rewrite.len(), true)
    } else {
        (context.config.tab_spaces, false)
    };
    let indent = offset + extra_indent;

    let max_width = try_opt!(width.checked_sub(extra_indent));
    let mut rewrites = try_opt!(subexpr_list.iter()
                                            .rev()
                                            .map(|e| {
                                                rewrite_chain_expr(e,
                                                                   total_span,
                                                                   context,
                                                                   max_width,
                                                                   indent)
                                            })
                                            .collect::<Option<Vec<_>>>());

    // Total of all items excluding the last.
    let almost_total = rewrites.split_last()
                               .unwrap()
                               .1
                               .iter()
                               .fold(0, |a, b| a + first_line_width(b)) +
                       parent_rewrite.len();
    let total_width = almost_total + first_line_width(rewrites.last().unwrap());
    let veto_single_line = if context.config.take_source_hints && subexpr_list.len() > 1 {
        // Look at the source code. Unless all chain elements start on the same
        // line, we won't consider putting them on a single line either.
        let first_line_no = context.codemap.lookup_char_pos(subexpr_list[0].span.lo).line;

        subexpr_list[1..]
            .iter()
            .any(|ex| context.codemap.lookup_char_pos(ex.span.hi).line != first_line_no)
    } else {
        false
    };

    let fits_single_line = !veto_single_line &&
                           match subexpr_list[0].node {
        ast::Expr_::ExprMethodCall(ref method_name, ref types, ref expressions)
            if context.config.chains_overflow_last => {
            let (last, init) = rewrites.split_last_mut().unwrap();

            if init.iter().all(|s| !s.contains('\n')) && total_width <= width {
                let last_rewrite = width.checked_sub(almost_total)
                                        .and_then(|inner_width| {
                                            rewrite_method_call(method_name.node,
                                                                types,
                                                                expressions,
                                                                total_span,
                                                                context,
                                                                inner_width,
                                                                offset + almost_total)
                                        });
                match last_rewrite {
                    Some(mut string) => {
                        ::std::mem::swap(&mut string, last);
                        true
                    }
                    None => false,
                }
            } else {
                false
            }
        }
        _ => total_width <= width && rewrites.iter().all(|s| !s.contains('\n')),
    };

    let connector = if fits_single_line {
        String::new()
    } else {
        format!("\n{}", indent.to_string(context.config))
    };

    let first_connector = if extend {
        ""
    } else {
        &connector[..]
    };

    Some(format!("{}{}{}", parent_rewrite, first_connector, rewrites.join(&connector)))
}

fn pop_expr_chain<'a>(expr: &'a ast::Expr) -> Option<&'a ast::Expr> {
    match expr.node {
        ast::Expr_::ExprMethodCall(_, _, ref expressions) => {
            Some(&expressions[0])
        }
        ast::Expr_::ExprTupField(ref subexpr, _) |
        ast::Expr_::ExprField(ref subexpr, _) => {
            Some(subexpr)
        }
        _ => None,
    }
}

fn rewrite_chain_expr(expr: &ast::Expr,
                      span: Span,
                      context: &RewriteContext,
                      width: usize,
                      offset: Indent)
                      -> Option<String> {
    match expr.node {
        ast::Expr_::ExprMethodCall(ref method_name, ref types, ref expressions) => {
            let inner = &RewriteContext { block_indent: offset, ..*context };
            rewrite_method_call(method_name.node, types, expressions, span, inner, width, offset)
        }
        ast::Expr_::ExprField(_, ref field) => {
            Some(format!(".{}", field.node))
        }
        ast::Expr_::ExprTupField(_, ref field) => {
            Some(format!(".{}", field.node))
        }
        _ => unreachable!(),
    }
}

// Determines we can continue formatting a given expression on the same line.
fn is_continuable(expr: &ast::Expr) -> bool {
    match expr.node {
        ast::Expr_::ExprPath(..) => true,
        _ => false,
    }
}

fn rewrite_method_call(method_name: ast::Ident,
                       types: &[ptr::P<ast::Ty>],
                       args: &[ptr::P<ast::Expr>],
                       span: Span,
                       context: &RewriteContext,
                       width: usize,
                       offset: Indent)
                       -> Option<String> {
    let type_str = if types.is_empty() {
        String::new()
    } else {
        let type_list = types.iter().map(|ty| pprust::ty_to_string(ty)).collect::<Vec<_>>();
        format!("::<{}>", type_list.join(", "))
    };

    let callee_str = format!(".{}{}", method_name, type_str);
    let span = mk_sp(args[0].span.hi, span.hi);

    rewrite_call(context, &callee_str, &args[1..], span, width, offset)
}