rust/src/loops.rs

use rustc::lint::*;
use rustc_front::hir::*;
use reexport::*;
use rustc_front::intravisit::{Visitor, walk_expr, walk_block, walk_decl};
use rustc::middle::ty;
use rustc::middle::def::Def;
use consts::{constant_simple, Constant};
use rustc::front::map::Node::NodeBlock;
use std::borrow::Cow;
use std::collections::{HashSet, HashMap};

use utils::{snippet, span_lint, get_parent_expr, match_trait_method, match_type, in_external_macro, expr_block,
            span_help_and_lint, is_integer_literal, get_enclosing_block};
use utils::{HASHMAP_PATH, VEC_PATH, LL_PATH, OPTION_PATH, RESULT_PATH};

/// **What it does:** This lint checks for looping over the range of `0..len` of some collection just to get the values by index. It is `Warn` by default.
///
/// **Why is this bad?** Just iterating the collection itself makes the intent more clear and is probably faster.
///
/// **Known problems:** None
///
/// **Example:**
/// ```
/// for i in 0..vec.len() {
///     println!("{}", vec[i]);
/// }
/// ```
declare_lint!{ pub NEEDLESS_RANGE_LOOP, Warn,
               "for-looping over a range of indices where an iterator over items would do" }

/// **What it does:** This lint checks for loops on `x.iter()` where `&x` will do, and suggest the latter. It is `Warn` by default.
///
/// **Why is this bad?** Readability.
///
/// **Known problems:** False negatives. We currently only warn on some known types.
///
/// **Example:** `for x in y.iter() { .. }` (where y is a `Vec` or slice)
declare_lint!{ pub EXPLICIT_ITER_LOOP, Warn,
               "for-looping over `_.iter()` or `_.iter_mut()` when `&_` or `&mut _` would do" }

/// **What it does:** This lint checks for loops on `x.next()`. It is `Warn` by default.
///
/// **Why is this bad?** `next()` returns either `Some(value)` if there was a value, or `None` otherwise. The insidious thing is that `Option<_>` implements `IntoIterator`, so that possibly one value will be iterated, leading to some hard to find bugs. No one will want to write such code [except to win an Underhanded Rust Contest](https://www.reddit.com/r/rust/comments/3hb0wm/underhanded_rust_contest/cu5yuhr).
///
/// **Known problems:** None
///
/// **Example:** `for x in y.next() { .. }`
declare_lint!{ pub ITER_NEXT_LOOP, Warn,
               "for-looping over `_.next()` which is probably not intended" }

/// **What it does:** This lint checks for `for` loops over `Option` values. It is `Warn` by default.
///
/// **Why is this bad?** Readability. This is more clearly expressed as an `if let`.
///
/// **Known problems:** None
///
/// **Example:** `for x in option { .. }`. This should be `if let Some(x) = option { .. }`.
declare_lint!{ pub FOR_LOOP_OVER_OPTION, Warn,
               "for-looping over an `Option`, which is more clearly expressed as an `if let`" }

/// **What it does:** This lint checks for `for` loops over `Result` values. It is `Warn` by default.
///
/// **Why is this bad?** Readability. This is more clearly expressed as an `if let`.
///
/// **Known problems:** None
///
/// **Example:** `for x in result { .. }`. This should be `if let Ok(x) = result { .. }`.
declare_lint!{ pub FOR_LOOP_OVER_RESULT, Warn,
               "for-looping over a `Result`, which is more clearly expressed as an `if let`" }

/// **What it does:** This lint detects `loop + match` combinations that are easier written as a `while let` loop. It is `Warn` by default.
///
/// **Why is this bad?** The `while let` loop is usually shorter and more readable
///
/// **Known problems:** Sometimes the wrong binding is displayed (#383)
///
/// **Example:**
///
/// ```
/// loop {
///     let x = match y {
///         Some(x) => x,
///         None => break,
///     }
///     // .. do something with x
/// }
/// // is easier written as
/// while let Some(x) = y {
///     // .. do something with x
/// }
/// ```
declare_lint!{ pub WHILE_LET_LOOP, Warn,
               "`loop { if let { ... } else break }` can be written as a `while let` loop" }

/// **What it does:** This lint checks for using `collect()` on an iterator without using the result. It is `Warn` by default.
///
/// **Why is this bad?** It is more idiomatic to use a `for` loop over the iterator instead.
///
/// **Known problems:** None
///
/// **Example:** `vec.iter().map(|x| /* some operation returning () */).collect::<Vec<_>>();`
declare_lint!{ pub UNUSED_COLLECT, Warn,
               "`collect()`ing an iterator without using the result; this is usually better \
                written as a for loop" }

/// **What it does:** This lint checks for loops over ranges `x..y` where both `x` and `y` are constant and `x` is greater or equal to `y`, unless the range is reversed or has a negative `.step_by(_)`. It is `Warn` by default.
///
/// **Why is it bad?** Such loops will either be skipped or loop until wrap-around (in debug code, this may `panic!()`). Both options are probably not intended.
///
/// **Known problems:** The lint cannot catch loops over dynamically defined ranges. Doing this would require simulating all possible inputs and code paths through the program, which would be complex and error-prone.
///
/// **Examples**: `for x in 5..10-5 { .. }` (oops, stray `-`)
declare_lint!{ pub REVERSE_RANGE_LOOP, Warn,
               "Iterating over an empty range, such as `10..0` or `5..5`" }

/// **What it does:** This lint checks `for` loops over slices with an explicit counter and suggests the use of `.enumerate()`. It is `Warn` by default.
///
/// **Why is it bad?** Not only is the version using `.enumerate()` more readable, the compiler is able to remove bounds checks which can lead to faster code in some instances.
///
/// **Known problems:** None.
///
/// **Example:** `for i in 0..v.len() { foo(v[i]); }` or `for i in 0..v.len() { bar(i, v[i]); }`
declare_lint!{ pub EXPLICIT_COUNTER_LOOP, Warn,
               "for-looping with an explicit counter when `_.enumerate()` would do" }

/// **What it does:** This lint checks for empty `loop` expressions. It is `Warn` by default.
///
/// **Why is this bad?** Those busy loops burn CPU cycles without doing anything. Think of the environment and either block on something or at least make the thread sleep for some microseconds.
///
/// **Known problems:** None
///
/// **Example:** `loop {}`
declare_lint!{ pub EMPTY_LOOP, Warn, "empty `loop {}` detected" }

/// **What it does:** This lint checks for `while let` expressions on iterators. It is `Warn` by default.
///
/// **Why is this bad?** Readability. A simple `for` loop is shorter and conveys the intent better.
///
/// **Known problems:** None
///
/// **Example:** `while let Some(val) = iter() { .. }`
declare_lint!{ pub WHILE_LET_ON_ITERATOR, Warn, "using a while-let loop instead of a for loop on an iterator" }

#[derive(Copy, Clone)]
pub struct LoopsPass;

impl LintPass for LoopsPass {
    fn get_lints(&self) -> LintArray {
        lint_array!(NEEDLESS_RANGE_LOOP,
                    EXPLICIT_ITER_LOOP,
                    ITER_NEXT_LOOP,
                    WHILE_LET_LOOP,
                    UNUSED_COLLECT,
                    REVERSE_RANGE_LOOP,
                    EXPLICIT_COUNTER_LOOP,
                    EMPTY_LOOP,
                    WHILE_LET_ON_ITERATOR)
    }
}

impl LateLintPass for LoopsPass {
    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
        if let Some((pat, arg, body)) = recover_for_loop(expr) {
            check_for_loop(cx, pat, arg, body, expr);
        }
        // check for `loop { if let {} else break }` that could be `while let`
        // (also matches an explicit "match" instead of "if let")
        // (even if the "match" or "if let" is used for declaration)
        if let ExprLoop(ref block, _) = expr.node {
            // also check for empty `loop {}` statements
            if block.stmts.is_empty() && block.expr.is_none() {
                span_lint(cx,
                          EMPTY_LOOP,
                          expr.span,
                          "empty `loop {}` detected. You may want to either use `panic!()` or add \
                           `std::thread::sleep(..);` to the loop body.");
            }

            // extract the expression from the first statement (if any) in a block
            let inner_stmt_expr = extract_expr_from_first_stmt(block);
            // or extract the first expression (if any) from the block
            if let Some(inner) = inner_stmt_expr.or_else(|| extract_first_expr(block)) {
                if let ExprMatch(ref matchexpr, ref arms, ref source) = inner.node {
                    // collect the remaining statements below the match
                    let mut other_stuff = block.stmts
                                               .iter()
                                               .skip(1)
                                               .map(|stmt| format!("{}", snippet(cx, stmt.span, "..")))
                                               .collect::<Vec<String>>();
                    if inner_stmt_expr.is_some() {
                        // if we have a statement which has a match,
                        if let Some(ref expr) = block.expr {
                            // then collect the expression (without semicolon) below it
                            other_stuff.push(format!("{}", snippet(cx, expr.span, "..")));
                        }
                    }

                    // ensure "if let" compatible match structure
                    match *source {
                        MatchSource::Normal | MatchSource::IfLetDesugar{..} => {
                            if arms.len() == 2 && arms[0].pats.len() == 1 && arms[0].guard.is_none() &&
                               arms[1].pats.len() == 1 && arms[1].guard.is_none() &&
                               is_break_expr(&arms[1].body) {
                                if in_external_macro(cx, expr.span) {
                                    return;
                                }
                                let loop_body = if inner_stmt_expr.is_some() {
                                    // FIXME: should probably be an ellipsis
                                    // tabbing and newline is probably a bad idea, especially for large blocks
                                    Cow::Owned(format!("{{\n    {}\n}}", other_stuff.join("\n    ")))
                                } else {
                                    expr_block(cx, &arms[0].body, Some(other_stuff.join("\n    ")), "..")
                                };
                                span_help_and_lint(cx,
                                                   WHILE_LET_LOOP,
                                                   expr.span,
                                                   "this loop could be written as a `while let` loop",
                                                   &format!("try\nwhile let {} = {} {}",
                                                            snippet(cx, arms[0].pats[0].span, ".."),
                                                            snippet(cx, matchexpr.span, ".."),
                                                            loop_body));
                            }
                        }
                        _ => (),
                    }
                }
            }
        }
        if let ExprMatch(ref match_expr, ref arms, MatchSource::WhileLetDesugar) = expr.node {
            let pat = &arms[0].pats[0].node;
            if let (&PatEnum(ref path, Some(ref pat_args)),
                    &ExprMethodCall(method_name, _, ref method_args)) = (pat, &match_expr.node) {
                let iter_expr = &method_args[0];
                if let Some(lhs_constructor) = path.segments.last() {
                    if method_name.node.as_str() == "next" &&
                       match_trait_method(cx, match_expr, &["core", "iter", "Iterator"]) &&
                       lhs_constructor.identifier.name.as_str() == "Some" &&
                       !is_iterator_used_after_while_let(cx, iter_expr) {
                        let iterator = snippet(cx, method_args[0].span, "_");
                        let loop_var = snippet(cx, pat_args[0].span, "_");
                        span_help_and_lint(cx,
                                           WHILE_LET_ON_ITERATOR,
                                           expr.span,
                                           "this loop could be written as a `for` loop",
                                           &format!("try\nfor {} in {} {{...}}", loop_var, iterator));
                    }
                }
            }
        }
    }

    fn check_stmt(&mut self, cx: &LateContext, stmt: &Stmt) {
        if let StmtSemi(ref expr, _) = stmt.node {
            if let ExprMethodCall(ref method, _, ref args) = expr.node {
                if args.len() == 1 && method.node.as_str() == "collect" &&
                   match_trait_method(cx, expr, &["core", "iter", "Iterator"]) {
                    span_lint(cx,
                              UNUSED_COLLECT,
                              expr.span,
                              &format!("you are collect()ing an iterator and throwing away the result. Consider \
                                        using an explicit for loop to exhaust the iterator"));
                }
            }
        }
    }
}

fn check_for_loop(cx: &LateContext, pat: &Pat, arg: &Expr, body: &Expr, expr: &Expr) {
    check_for_loop_range(cx, pat, arg, body, expr);
    check_for_loop_reverse_range(cx, arg, expr);
    check_for_loop_arg(cx, pat, arg, expr);
    check_for_loop_explicit_counter(cx, arg, body, expr);
}

/// Check for looping over a range and then indexing a sequence with it.
/// The iteratee must be a range literal.
fn check_for_loop_range(cx: &LateContext, pat: &Pat, arg: &Expr, body: &Expr, expr: &Expr) {
    if let ExprRange(Some(ref l), ref r) = arg.node {
        // the var must be a single name
        if let PatIdent(_, ref ident, _) = pat.node {
            let mut visitor = VarVisitor {
                cx: cx,
                var: ident.node.name,
                indexed: HashSet::new(),
                nonindex: false,
            };
            walk_expr(&mut visitor, body);
            // linting condition: we only indexed one variable
            if visitor.indexed.len() == 1 {
                let indexed = visitor.indexed
                                     .into_iter()
                                     .next()
                                     .expect("Len was nonzero, but no contents found");

                let starts_at_zero = is_integer_literal(l, 0);

                let skip: Cow<_> = if starts_at_zero {
                    "".into()
                } else {
                    format!(".skip({})", snippet(cx, l.span, "..")).into()
                };

                let take: Cow<_> = if let Some(ref r) = *r {
                    if !is_len_call(&r, &indexed) {
                        format!(".take({})", snippet(cx, r.span, "..")).into()
                    } else {
                        "".into()
                    }
                } else {
                    "".into()
                };

                if visitor.nonindex {
                    span_lint(cx,
                              NEEDLESS_RANGE_LOOP,
                              expr.span,
                              &format!("the loop variable `{}` is used to index `{}`. \
                                        Consider using `for ({}, item) in {}.iter().enumerate(){}{}` or similar iterators",
                                        ident.node.name,
                                        indexed,
                                        ident.node.name,
                                        indexed,
                                        take,
                                        skip));
                } else {
                    let repl = if starts_at_zero && take.is_empty() {
                        format!("&{}", indexed)
                    } else {
                        format!("{}.iter(){}{}", indexed, take, skip)
                    };

                    span_lint(cx,
                              NEEDLESS_RANGE_LOOP,
                              expr.span,
                              &format!("the loop variable `{}` is only used to index `{}`. \
                                        Consider using `for item in {}` or similar iterators",
                                        ident.node.name,
                                        indexed,
                                        repl));
                }
            }
        }
    }
}

fn is_len_call(expr: &Expr, var: &Name) -> bool {
    if_let_chain! {[
        let ExprMethodCall(method, _, ref len_args) = expr.node,
        len_args.len() == 1,
        method.node.as_str() == "len",
        let ExprPath(_, ref path) = len_args[0].node,
        path.segments.len() == 1,
        &path.segments[0].identifier.name == var
    ], {
        return true;
    }}

    false
}

fn check_for_loop_reverse_range(cx: &LateContext, arg: &Expr, expr: &Expr) {
    // if this for loop is iterating over a two-sided range...
    if let ExprRange(Some(ref start_expr), Some(ref stop_expr)) = arg.node {
        // ...and both sides are compile-time constant integers...
        if let Some(start_idx @ Constant::Int(..)) = constant_simple(start_expr) {
            if let Some(stop_idx @ Constant::Int(..)) = constant_simple(stop_expr) {
                // ...and the start index is greater than the stop index,
                // this loop will never run. This is often confusing for developers
                // who think that this will iterate from the larger value to the
                // smaller value.
                if start_idx > stop_idx {
                    span_help_and_lint(cx,
                                       REVERSE_RANGE_LOOP,
                                       expr.span,
                                       "this range is empty so this for loop will never run",
                                       &format!("Consider using `({}..{}).rev()` if you are attempting to iterate \
                                                 over this range in reverse",
                                                stop_idx,
                                                start_idx));
                } else if start_idx == stop_idx {
                    // if they are equal, it's also problematic - this loop
                    // will never run.
                    span_lint(cx,
                              REVERSE_RANGE_LOOP,
                              expr.span,
                              "this range is empty so this for loop will never run");
                }
            }
        }
    }
}

fn check_for_loop_arg(cx: &LateContext, pat: &Pat, arg: &Expr, expr: &Expr) {
    let mut next_loop_linted = false; // whether or not ITER_NEXT_LOOP lint was used
    if let ExprMethodCall(ref method, _, ref args) = arg.node {
        // just the receiver, no arguments
        if args.len() == 1 {
            let method_name = method.node;
            // check for looping over x.iter() or x.iter_mut(), could use &x or &mut x
            if method_name.as_str() == "iter" || method_name.as_str() == "iter_mut" {
                if is_ref_iterable_type(cx, &args[0]) {
                    let object = snippet(cx, args[0].span, "_");
                    span_lint(cx,
                              EXPLICIT_ITER_LOOP,
                              expr.span,
                              &format!("it is more idiomatic to loop over `&{}{}` instead of `{}.{}()`",
                                       if method_name.as_str() == "iter_mut" {
                                           "mut "
                                       } else {
                                           ""
                                       },
                                       object,
                                       object,
                                       method_name));
                }
            } else if method_name.as_str() == "next" && match_trait_method(cx, arg, &["core", "iter", "Iterator"]) {
                span_lint(cx,
                          ITER_NEXT_LOOP,
                          expr.span,
                          "you are iterating over `Iterator::next()` which is an Option; this will compile but is \
                           probably not what you want");
                next_loop_linted = true;
            }
        }
    }
    if !next_loop_linted {
        check_arg_type(cx, pat, arg);
    }
}

/// Check for `for` loops over `Option`s and `Results`
fn check_arg_type(cx: &LateContext, pat: &Pat, arg: &Expr) {
    let ty = cx.tcx.expr_ty(arg);
    if match_type(cx, ty, &OPTION_PATH) {
        span_help_and_lint(
            cx,
            FOR_LOOP_OVER_OPTION,
            arg.span,
            &format!("for loop over `{0}`, which is an `Option`. This is more readably written as \
                      an `if let` statement.", snippet(cx, arg.span, "_")),
            &format!("consider replacing `for {0} in {1}` with `if let Some({0}) = {1}`",
                     snippet(cx, pat.span, "_"), snippet(cx, arg.span, "_"))
        );
    }
    else if match_type(cx, ty, &RESULT_PATH) {
        span_help_and_lint(
            cx,
            FOR_LOOP_OVER_RESULT,
            arg.span,
            &format!("for loop over `{0}`, which is a `Result`. This is more readably written as \
                      an `if let` statement.", snippet(cx, arg.span, "_")),
            &format!("consider replacing `for {0} in {1}` with `if let Ok({0}) = {1}`",
                     snippet(cx, pat.span, "_"), snippet(cx, arg.span, "_"))
        );
    }
}

fn check_for_loop_explicit_counter(cx: &LateContext, arg: &Expr, body: &Expr, expr: &Expr) {
    // Look for variables that are incremented once per loop iteration.
    let mut visitor = IncrementVisitor {
        cx: cx,
        states: HashMap::new(),
        depth: 0,
        done: false,
    };
    walk_expr(&mut visitor, body);

    // For each candidate, check the parent block to see if
    // it's initialized to zero at the start of the loop.
    let map = &cx.tcx.map;
    let parent_scope = map.get_enclosing_scope(expr.id).and_then(|id| map.get_enclosing_scope(id));
    if let Some(parent_id) = parent_scope {
        if let NodeBlock(block) = map.get(parent_id) {
            for (id, _) in visitor.states.iter().filter(|&(_, v)| *v == VarState::IncrOnce) {
                let mut visitor2 = InitializeVisitor {
                    cx: cx,
                    end_expr: expr,
                    var_id: *id,
                    state: VarState::IncrOnce,
                    name: None,
                    depth: 0,
                    past_loop: false,
                };
                walk_block(&mut visitor2, block);

                if visitor2.state == VarState::Warn {
                    if let Some(name) = visitor2.name {
                        span_lint(cx,
                                  EXPLICIT_COUNTER_LOOP,
                                  expr.span,
                                  &format!("the variable `{0}` is used as a loop counter. Consider using `for ({0}, \
                                            item) in {1}.enumerate()` or similar iterators",
                                           name,
                                           snippet(cx, arg.span, "_")));
                    }
                }
            }
        }
    }
}

/// Recover the essential nodes of a desugared for loop:
/// `for pat in arg { body }` becomes `(pat, arg, body)`.
fn recover_for_loop(expr: &Expr) -> Option<(&Pat, &Expr, &Expr)> {
    if_let_chain! {
        [
            let ExprMatch(ref iterexpr, ref arms, _) = expr.node,
            let ExprCall(_, ref iterargs) = iterexpr.node,
            iterargs.len() == 1 && arms.len() == 1 && arms[0].guard.is_none(),
            let ExprLoop(ref block, _) = arms[0].body.node,
            block.stmts.is_empty(),
            let Some(ref loopexpr) = block.expr,
            let ExprMatch(_, ref innerarms, MatchSource::ForLoopDesugar) = loopexpr.node,
            innerarms.len() == 2 && innerarms[0].pats.len() == 1,
            let PatEnum(_, Some(ref somepats)) = innerarms[0].pats[0].node,
            somepats.len() == 1
        ], {
            return Some((&somepats[0],
                         &iterargs[0],
                         &innerarms[0].body));
        }
    }
    None
}

struct VarVisitor<'v, 't: 'v> {
    cx: &'v LateContext<'v, 't>, // context reference
    var: Name, // var name to look for as index
    indexed: HashSet<Name>, // indexed variables
    nonindex: bool, // has the var been used otherwise?
}

impl<'v, 't> Visitor<'v> for VarVisitor<'v, 't> {
    fn visit_expr(&mut self, expr: &'v Expr) {
        if let ExprPath(None, ref path) = expr.node {
            if path.segments.len() == 1 && path.segments[0].identifier.name == self.var {
                // we are referencing our variable! now check if it's as an index
                if_let_chain! {
                    [
                        let Some(parexpr) = get_parent_expr(self.cx, expr),
                        let ExprIndex(ref seqexpr, _) = parexpr.node,
                        let ExprPath(None, ref seqvar) = seqexpr.node,
                        seqvar.segments.len() == 1
                    ], {
                        self.indexed.insert(seqvar.segments[0].identifier.name);
                        return;  // no need to walk further
                    }
                }
                // we are not indexing anything, record that
                self.nonindex = true;
                return;
            }
        }
        walk_expr(self, expr);
    }
}

fn is_iterator_used_after_while_let(cx: &LateContext, iter_expr: &Expr) -> bool {
    let def_id = match var_def_id(cx, iter_expr) {
        Some(id) => id,
        None => return false,
    };
    let mut visitor = VarUsedAfterLoopVisitor {
        cx: cx,
        def_id: def_id,
        iter_expr_id: iter_expr.id,
        past_while_let: false,
        var_used_after_while_let: false,
    };
    if let Some(enclosing_block) = get_enclosing_block(cx, def_id) {
        walk_block(&mut visitor, enclosing_block);
    }
    visitor.var_used_after_while_let
}

struct VarUsedAfterLoopVisitor<'v, 't: 'v> {
    cx: &'v LateContext<'v, 't>,
    def_id: NodeId,
    iter_expr_id: NodeId,
    past_while_let: bool,
    var_used_after_while_let: bool,
}

impl<'v, 't> Visitor<'v> for VarUsedAfterLoopVisitor<'v, 't> {
    fn visit_expr(&mut self, expr: &'v Expr) {
        if self.past_while_let {
            if Some(self.def_id) == var_def_id(self.cx, expr) {
                self.var_used_after_while_let = true;
            }
        } else if self.iter_expr_id == expr.id {
            self.past_while_let = true;
        }
        walk_expr(self, expr);
    }
}


/// Return true if the type of expr is one that provides IntoIterator impls
/// for &T and &mut T, such as Vec.
fn is_ref_iterable_type(cx: &LateContext, e: &Expr) -> bool {
    // no walk_ptrs_ty: calling iter() on a reference can make sense because it
    // will allow further borrows afterwards
    let ty = cx.tcx.expr_ty(e);
    is_iterable_array(ty) || match_type(cx, ty, &VEC_PATH) || match_type(cx, ty, &LL_PATH) ||
    match_type(cx, ty, &HASHMAP_PATH) || match_type(cx, ty, &["std", "collections", "hash", "set", "HashSet"]) ||
    match_type(cx, ty, &["collections", "vec_deque", "VecDeque"]) ||
    match_type(cx, ty, &["collections", "binary_heap", "BinaryHeap"]) ||
    match_type(cx, ty, &["collections", "btree", "map", "BTreeMap"]) ||
    match_type(cx, ty, &["collections", "btree", "set", "BTreeSet"])
}

fn is_iterable_array(ty: ty::Ty) -> bool {
    // IntoIterator is currently only implemented for array sizes <= 32 in rustc
    match ty.sty {
        ty::TyArray(_, 0...32) => true,
        _ => false,
    }
}

/// If a block begins with a statement (possibly a `let` binding) and has an expression, return it.
fn extract_expr_from_first_stmt(block: &Block) -> Option<&Expr> {
    if block.stmts.is_empty() {
        return None;
    }
    if let StmtDecl(ref decl, _) = block.stmts[0].node {
        if let DeclLocal(ref local) = decl.node {
            if let Some(ref expr) = local.init {
                Some(expr)
            } else {
                None
            }
        } else {
            None
        }
    } else {
        None
    }
}

/// If a block begins with an expression (with or without semicolon), return it.
fn extract_first_expr(block: &Block) -> Option<&Expr> {
    match block.expr {
        Some(ref expr) => Some(expr),
        None if !block.stmts.is_empty() => {
            match block.stmts[0].node {
                StmtExpr(ref expr, _) | StmtSemi(ref expr, _) => Some(expr),
                _ => None,
            }
        }
        _ => None,
    }
}

/// Return true if expr contains a single break expr (maybe within a block).
fn is_break_expr(expr: &Expr) -> bool {
    match expr.node {
        ExprBreak(None) => true,
        // there won't be a `let <pat> = break` and so we can safely ignore the StmtDecl case
        ExprBlock(ref b) => {
            match extract_first_expr(b) {
                Some(ref subexpr) => is_break_expr(subexpr),
                None => false,
            }
        }
        _ => false,
    }
}

// To trigger the EXPLICIT_COUNTER_LOOP lint, a variable must be
// incremented exactly once in the loop body, and initialized to zero
// at the start of the loop.
#[derive(PartialEq)]
enum VarState {
    Initial, // Not examined yet
    IncrOnce, // Incremented exactly once, may be a loop counter
    Declared, // Declared but not (yet) initialized to zero
    Warn,
    DontWarn,
}

// Scan a for loop for variables that are incremented exactly once.
struct IncrementVisitor<'v, 't: 'v> {
    cx: &'v LateContext<'v, 't>, // context reference
    states: HashMap<NodeId, VarState>, // incremented variables
    depth: u32, // depth of conditional expressions
    done: bool,
}

impl<'v, 't> Visitor<'v> for IncrementVisitor<'v, 't> {
    fn visit_expr(&mut self, expr: &'v Expr) {
        if self.done {
            return;
        }

        // If node is a variable
        if let Some(def_id) = var_def_id(self.cx, expr) {
            if let Some(parent) = get_parent_expr(self.cx, expr) {
                let state = self.states.entry(def_id).or_insert(VarState::Initial);

                match parent.node {
                    ExprAssignOp(op, ref lhs, ref rhs) => {
                        if lhs.id == expr.id {
                            if op.node == BiAdd && is_integer_literal(rhs, 1) {
                                *state = match *state {
                                    VarState::Initial if self.depth == 0 => VarState::IncrOnce,
                                    _ => VarState::DontWarn,
                                };
                            } else {
                                // Assigned some other value
                                *state = VarState::DontWarn;
                            }
                        }
                    }
                    ExprAssign(ref lhs, _) if lhs.id == expr.id => *state = VarState::DontWarn,
                    ExprAddrOf(mutability, _) if mutability == MutMutable => *state = VarState::DontWarn,
                    _ => (),
                }
            }
        } else if is_loop(expr) {
            self.states.clear();
            self.done = true;
            return;
        } else if is_conditional(expr) {
            self.depth += 1;
            walk_expr(self, expr);
            self.depth -= 1;
            return;
        }
        walk_expr(self, expr);
    }
}

// Check whether a variable is initialized to zero at the start of a loop.
struct InitializeVisitor<'v, 't: 'v> {
    cx: &'v LateContext<'v, 't>, // context reference
    end_expr: &'v Expr, // the for loop. Stop scanning here.
    var_id: NodeId,
    state: VarState,
    name: Option<Name>,
    depth: u32, // depth of conditional expressions
    past_loop: bool,
}

impl<'v, 't> Visitor<'v> for InitializeVisitor<'v, 't> {
    fn visit_decl(&mut self, decl: &'v Decl) {
        // Look for declarations of the variable
        if let DeclLocal(ref local) = decl.node {
            if local.pat.id == self.var_id {
                if let PatIdent(_, ref ident, _) = local.pat.node {
                    self.name = Some(ident.node.name);

                    self.state = if let Some(ref init) = local.init {
                        if is_integer_literal(init, 0) {
                            VarState::Warn
                        } else {
                            VarState::Declared
                        }
                    } else {
                        VarState::Declared
                    }
                }
            }
        }
        walk_decl(self, decl);
    }

    fn visit_expr(&mut self, expr: &'v Expr) {
        if self.state == VarState::DontWarn {
            return;
        }
        if expr == self.end_expr {
            self.past_loop = true;
            return;
        }
        // No need to visit expressions before the variable is
        // declared
        if self.state == VarState::IncrOnce {
            return;
        }

        // If node is the desired variable, see how it's used
        if var_def_id(self.cx, expr) == Some(self.var_id) {
            if let Some(parent) = get_parent_expr(self.cx, expr) {
                match parent.node {
                    ExprAssignOp(_, ref lhs, _) if lhs.id == expr.id => {
                        self.state = VarState::DontWarn;
                    }
                    ExprAssign(ref lhs, ref rhs) if lhs.id == expr.id => {
                        self.state = if is_integer_literal(rhs, 0) && self.depth == 0 {
                            VarState::Warn
                        } else {
                            VarState::DontWarn
                        }
                    }
                    ExprAddrOf(mutability, _) if mutability == MutMutable => self.state = VarState::DontWarn,
                    _ => (),
                }
            }

            if self.past_loop {
                self.state = VarState::DontWarn;
                return;
            }
        } else if !self.past_loop && is_loop(expr) {
            self.state = VarState::DontWarn;
            return;
        } else if is_conditional(expr) {
            self.depth += 1;
            walk_expr(self, expr);
            self.depth -= 1;
            return;
        }
        walk_expr(self, expr);
    }
}

fn var_def_id(cx: &LateContext, expr: &Expr) -> Option<NodeId> {
    if let Some(path_res) = cx.tcx.def_map.borrow().get(&expr.id) {
        if let Def::Local(_, node_id) = path_res.base_def {
            return Some(node_id);
        }
    }
    None
}

fn is_loop(expr: &Expr) -> bool {
    match expr.node {
        ExprLoop(..) | ExprWhile(..) => true,
        _ => false,
    }
}

fn is_conditional(expr: &Expr) -> bool {
    match expr.node {
        ExprIf(..) | ExprMatch(..) => true,
        _ => false,
    }
}