//! Checks for continue statements in loops that are redundant.
//!
//! For example, the lint would catch
//!
//! ```
//! while condition() {
//! update_condition();
//! if x {
//! // ...
//! } else {
//! continue;
//! }
//! println!("Hello, world");
//! }
//! ```
//!
//! And suggest something like this:
//!
//! ```
//! while condition() {
//! update_condition();
//! if x {
//! // ...
//! println!("Hello, world");
//! }
//! }
//! ```
//!
//! This lint is **warn** by default.
use rustc::lint::*;
use syntax::ast;
use syntax::codemap::{original_sp, DUMMY_SP};
use std::borrow::Cow;
use utils::{in_macro, snippet, snippet_block, span_help_and_lint, trim_multiline};
/// **What it does:** The lint checks for `if`-statements appearing in loops
/// that contain a `continue` statement in either their main blocks or their
/// `else`-blocks, when omitting the `else`-block possibly with some
/// rearrangement of code can make the code easier to understand.
///
/// **Why is this bad?** Having explicit `else` blocks for `if` statements
/// containing `continue` in their THEN branch adds unnecessary branching and
/// nesting to the code. Having an else block containing just `continue` can
/// also be better written by grouping the statements following the whole `if`
/// statement within the THEN block and omitting the else block completely.
///
/// **Known problems:** None
///
/// **Example:**
/// ```rust
/// while condition() {
/// update_condition();
/// if x {
/// // ...
/// } else {
/// continue;
/// }
/// println!("Hello, world");
/// }
/// ```
///
/// Could be rewritten as
///
/// ```rust
/// while condition() {
/// update_condition();
/// if x {
/// // ...
/// println!("Hello, world");
/// }
/// }
/// ```
///
/// As another example, the following code
///
/// ```rust
/// loop {
/// if waiting() {
/// continue;
/// } else {
/// // Do something useful
/// }
/// }
/// ```
/// Could be rewritten as
///
/// ```rust
/// loop {
/// if waiting() {
/// continue;
/// }
/// // Do something useful
/// }
/// ```
declare_clippy_lint! {
pub NEEDLESS_CONTINUE,
pedantic,
"`continue` statements that can be replaced by a rearrangement of code"
}
#[derive(Copy, Clone)]
pub struct NeedlessContinue;
impl LintPass for NeedlessContinue {
fn get_lints(&self) -> LintArray {
lint_array!(NEEDLESS_CONTINUE)
}
}
impl EarlyLintPass for NeedlessContinue {
fn check_expr(&mut self, ctx: &EarlyContext, expr: &ast::Expr) {
if !in_macro(expr.span) {
check_and_warn(ctx, expr);
}
}
}
/* This lint has to mainly deal with two cases of needless continue
* statements. */
// Case 1 [Continue inside else block]:
//
// loop {
// // region A
// if cond {
// // region B
// } else {
// continue;
// }
// // region C
// }
//
// This code can better be written as follows:
//
// loop {
// // region A
// if cond {
// // region B
// // region C
// }
// }
//
// Case 2 [Continue inside then block]:
//
// loop {
// // region A
// if cond {
// continue;
// // potentially more code here.
// } else {
// // region B
// }
// // region C
// }
//
//
// This snippet can be refactored to:
//
// loop {
// // region A
// if !cond {
// // region B
// // region C
// }
// }
//
/// Given an expression, returns true if either of the following is true
///
/// - The expression is a `continue` node.
/// - The expression node is a block with the first statement being a
/// `continue`.
///
fn needless_continue_in_else(else_expr: &ast::Expr) -> bool {
match else_expr.node {
ast::ExprKind::Block(ref else_block, _) => is_first_block_stmt_continue(else_block),
ast::ExprKind::Continue(_) => true,
_ => false,
}
}
fn is_first_block_stmt_continue(block: &ast::Block) -> bool {
block.stmts.get(0).map_or(false, |stmt| match stmt.node {
ast::StmtKind::Semi(ref e) | ast::StmtKind::Expr(ref e) => if let ast::ExprKind::Continue(_) = e.node {
true
} else {
false
},
_ => false,
})
}
/// If `expr` is a loop expression (while/while let/for/loop), calls `func` with
/// the AST object representing the loop block of `expr`.
fn with_loop_block<F>(expr: &ast::Expr, mut func: F)
where
F: FnMut(&ast::Block),
{
match expr.node {
ast::ExprKind::While(_, ref loop_block, _) |
ast::ExprKind::WhileLet(_, _, ref loop_block, _) |
ast::ExprKind::ForLoop(_, _, ref loop_block, _) |
ast::ExprKind::Loop(ref loop_block, _) => func(loop_block),
_ => {},
}
}
/// If `stmt` is an if expression node with an `else` branch, calls func with
/// the
/// following:
///
/// - The `if` expression itself,
/// - The `if` condition expression,
/// - The `then` block, and
/// - The `else` expression.
///
fn with_if_expr<F>(stmt: &ast::Stmt, mut func: F)
where
F: FnMut(&ast::Expr, &ast::Expr, &ast::Block, &ast::Expr),
{
match stmt.node {
ast::StmtKind::Semi(ref e) | ast::StmtKind::Expr(ref e) => {
if let ast::ExprKind::If(ref cond, ref if_block, Some(ref else_expr)) = e.node {
func(e, cond, if_block, else_expr);
}
},
_ => {},
}
}
/// A type to distinguish between the two distinct cases this lint handles.
#[derive(Copy, Clone, Debug)]
enum LintType {
ContinueInsideElseBlock,
ContinueInsideThenBlock,
}
/// Data we pass around for construction of help messages.
struct LintData<'a> {
/// The `if` expression encountered in the above loop.
if_expr: &'a ast::Expr,
/// The condition expression for the above `if`.
if_cond: &'a ast::Expr,
/// The `then` block of the `if` statement.
if_block: &'a ast::Block,
/// The `else` block of the `if` statement.
/// Note that we only work with `if` exprs that have an `else` branch.
else_expr: &'a ast::Expr,
/// The 0-based index of the `if` statement in the containing loop block.
stmt_idx: usize,
/// The statements of the loop block.
block_stmts: &'a [ast::Stmt],
}
const MSG_REDUNDANT_ELSE_BLOCK: &str = "This else block is redundant.\n";
const MSG_ELSE_BLOCK_NOT_NEEDED: &str = "There is no need for an explicit `else` block for this `if` \
expression\n";
const DROP_ELSE_BLOCK_AND_MERGE_MSG: &str = "Consider dropping the else clause and merging the code that \
follows (in the loop) with the if block, like so:\n";
const DROP_ELSE_BLOCK_MSG: &str = "Consider dropping the else clause, and moving out the code in the else \
block, like so:\n";
fn emit_warning<'a>(ctx: &EarlyContext, data: &'a LintData, header: &str, typ: LintType) {
// snip is the whole *help* message that appears after the warning.
// message is the warning message.
// expr is the expression which the lint warning message refers to.
let (snip, message, expr) = match typ {
LintType::ContinueInsideElseBlock => (
suggestion_snippet_for_continue_inside_else(ctx, data, header),
MSG_REDUNDANT_ELSE_BLOCK,
data.else_expr,
),
LintType::ContinueInsideThenBlock => (
suggestion_snippet_for_continue_inside_if(ctx, data, header),
MSG_ELSE_BLOCK_NOT_NEEDED,
data.if_expr,
),
};
span_help_and_lint(ctx, NEEDLESS_CONTINUE, expr.span, message, &snip);
}
fn suggestion_snippet_for_continue_inside_if<'a>(ctx: &EarlyContext, data: &'a LintData, header: &str) -> String {
let cond_code = snippet(ctx, data.if_cond.span, "..");
let if_code = format!("if {} {{\n continue;\n}}\n", cond_code);
/* ^^^^--- Four spaces of indentation. */
// region B
let else_code = snippet(ctx, data.else_expr.span, "..").into_owned();
let else_code = erode_block(&else_code);
let else_code = trim_multiline(Cow::from(else_code), false);
let mut ret = String::from(header);
ret.push_str(&if_code);
ret.push_str(&else_code);
ret.push_str("\n...");
ret
}
fn suggestion_snippet_for_continue_inside_else<'a>(ctx: &EarlyContext, data: &'a LintData, header: &str) -> String {
let cond_code = snippet(ctx, data.if_cond.span, "..");
let mut if_code = format!("if {} {{\n", cond_code);
// Region B
let block_code = &snippet(ctx, data.if_block.span, "..").into_owned();
let block_code = erode_block(block_code);
let block_code = trim_multiline(Cow::from(block_code), false);
if_code.push_str(&block_code);
// Region C
// These is the code in the loop block that follows the if/else construction
// we are complaining about. We want to pull all of this code into the
// `then` block of the `if` statement.
let to_annex = data.block_stmts[data.stmt_idx + 1..]
.iter()
.map(|stmt| original_sp(stmt.span, DUMMY_SP))
.map(|span| snippet_block(ctx, span, "..").into_owned())
.collect::<Vec<_>>()
.join("\n");
let mut ret = String::from(header);
ret.push_str(&if_code);
ret.push_str("\n// Merged code follows...");
ret.push_str(&to_annex);
ret.push_str("\n}\n");
ret
}
fn check_and_warn<'a>(ctx: &EarlyContext, expr: &'a ast::Expr) {
with_loop_block(expr, |loop_block| {
for (i, stmt) in loop_block.stmts.iter().enumerate() {
with_if_expr(stmt, |if_expr, cond, then_block, else_expr| {
let data = &LintData {
stmt_idx: i,
if_expr,
if_cond: cond,
if_block: then_block,
else_expr,
block_stmts: &loop_block.stmts,
};
if needless_continue_in_else(else_expr) {
emit_warning(ctx, data, DROP_ELSE_BLOCK_AND_MERGE_MSG, LintType::ContinueInsideElseBlock);
} else if is_first_block_stmt_continue(then_block) {
emit_warning(ctx, data, DROP_ELSE_BLOCK_MSG, LintType::ContinueInsideThenBlock);
}
});
}
});
}
/// Eats at `s` from the end till a closing brace `}` is encountered, and then
/// continues eating till a non-whitespace character is found.
/// e.g., the string
///
/// ```
/// {
/// let x = 5;
/// }
/// ```
///
/// is transformed to
///
/// ```
/// {
/// let x = 5;
/// ```
///
/// NOTE: when there is no closing brace in `s`, `s` is _not_ preserved, i.e.,
/// an empty string will be returned in that case.
pub fn erode_from_back(s: &str) -> String {
let mut ret = String::from(s);
while ret.pop().map_or(false, |c| c != '}') {}
while let Some(c) = ret.pop() {
if !c.is_whitespace() {
ret.push(c);
break;
}
}
ret
}
/// Eats at `s` from the front by first skipping all leading whitespace. Then,
/// any number of opening braces are eaten, followed by any number of newlines.
/// e.g., the string
///
/// ```
/// {
/// something();
/// inside_a_block();
/// }
/// ```
///
/// is transformed to
///
/// ```
/// something();
/// inside_a_block();
/// }
/// ```
///
pub fn erode_from_front(s: &str) -> String {
s.chars()
.skip_while(|c| c.is_whitespace())
.skip_while(|c| *c == '{')
.skip_while(|c| *c == '\n')
.collect::<String>()
}
/// If `s` contains the code for a block, delimited by braces, this function
/// tries to get the contents of the block. If there is no closing brace
/// present,
/// an empty string is returned.
pub fn erode_block(s: &str) -> String {
erode_from_back(&erode_from_front(s))
}