1 //! Checks for continue statements in loops that are redundant.
3 //! For example, the lint would catch
6 //! while condition() {
7 //! update_condition();
13 //! println!("Hello, world");
17 //! And suggest something like this:
20 //! while condition() {
21 //! update_condition();
24 //! println!("Hello, world");
29 //! This lint is **warn** by default.
30 use rustc::lint::{EarlyContext, EarlyLintPass, LintArray, LintPass};
31 use rustc::{declare_tool_lint, lint_array};
34 use syntax::source_map::{original_sp, DUMMY_SP};
36 use crate::utils::{in_macro, snippet, snippet_block, span_help_and_lint, trim_multiline};
38 declare_clippy_lint! {
39 /// **What it does:** The lint checks for `if`-statements appearing in loops
40 /// that contain a `continue` statement in either their main blocks or their
41 /// `else`-blocks, when omitting the `else`-block possibly with some
42 /// rearrangement of code can make the code easier to understand.
44 /// **Why is this bad?** Having explicit `else` blocks for `if` statements
45 /// containing `continue` in their THEN branch adds unnecessary branching and
46 /// nesting to the code. Having an else block containing just `continue` can
47 /// also be better written by grouping the statements following the whole `if`
48 /// statement within the THEN block and omitting the else block completely.
50 /// **Known problems:** None
54 /// while condition() {
55 /// update_condition();
61 /// println!("Hello, world");
65 /// Could be rewritten as
68 /// while condition() {
69 /// update_condition();
72 /// println!("Hello, world");
77 /// As another example, the following code
84 /// // Do something useful
88 /// Could be rewritten as
95 /// // Do something useful
98 pub NEEDLESS_CONTINUE,
100 "`continue` statements that can be replaced by a rearrangement of code"
103 #[derive(Copy, Clone)]
104 pub struct NeedlessContinue;
106 impl LintPass for NeedlessContinue {
107 fn get_lints(&self) -> LintArray {
108 lint_array!(NEEDLESS_CONTINUE)
111 fn name(&self) -> &'static str {
116 impl EarlyLintPass for NeedlessContinue {
117 fn check_expr(&mut self, ctx: &EarlyContext<'_>, expr: &ast::Expr) {
118 if !in_macro(expr.span) {
119 check_and_warn(ctx, expr);
124 /* This lint has to mainly deal with two cases of needless continue
126 // Case 1 [Continue inside else block]:
138 // This code can better be written as follows:
148 // Case 2 [Continue inside then block]:
154 // // potentially more code here.
162 // This snippet can be refactored to:
173 /// Given an expression, returns true if either of the following is true
175 /// - The expression is a `continue` node.
176 /// - The expression node is a block with the first statement being a
178 fn needless_continue_in_else(else_expr: &ast::Expr, label: Option<&ast::Label>) -> bool {
179 match else_expr.node {
180 ast::ExprKind::Block(ref else_block, _) => is_first_block_stmt_continue(else_block, label),
181 ast::ExprKind::Continue(l) => compare_labels(label, l.as_ref()),
186 fn is_first_block_stmt_continue(block: &ast::Block, label: Option<&ast::Label>) -> bool {
187 block.stmts.get(0).map_or(false, |stmt| match stmt.node {
188 ast::StmtKind::Semi(ref e) | ast::StmtKind::Expr(ref e) => {
189 if let ast::ExprKind::Continue(ref l) = e.node {
190 compare_labels(label, l.as_ref())
199 /// If the `continue` has a label, check it matches the label of the loop.
200 fn compare_labels(loop_label: Option<&ast::Label>, continue_label: Option<&ast::Label>) -> bool {
201 match (loop_label, continue_label) {
202 // `loop { continue; }` or `'a loop { continue; }`
204 // `loop { continue 'a; }`
206 // `'a loop { continue 'a; }` or `'a loop { continue 'b; }`
207 (Some(x), Some(y)) => x.ident == y.ident,
211 /// If `expr` is a loop expression (while/while let/for/loop), calls `func` with
212 /// the AST object representing the loop block of `expr`.
213 fn with_loop_block<F>(expr: &ast::Expr, mut func: F)
215 F: FnMut(&ast::Block, Option<&ast::Label>),
218 ast::ExprKind::While(_, ref loop_block, ref label)
219 | ast::ExprKind::WhileLet(_, _, ref loop_block, ref label)
220 | ast::ExprKind::ForLoop(_, _, ref loop_block, ref label)
221 | ast::ExprKind::Loop(ref loop_block, ref label) => func(loop_block, label.as_ref()),
226 /// If `stmt` is an if expression node with an `else` branch, calls func with
230 /// - The `if` expression itself,
231 /// - The `if` condition expression,
232 /// - The `then` block, and
233 /// - The `else` expression.
234 fn with_if_expr<F>(stmt: &ast::Stmt, mut func: F)
236 F: FnMut(&ast::Expr, &ast::Expr, &ast::Block, &ast::Expr),
239 ast::StmtKind::Semi(ref e) | ast::StmtKind::Expr(ref e) => {
240 if let ast::ExprKind::If(ref cond, ref if_block, Some(ref else_expr)) = e.node {
241 func(e, cond, if_block, else_expr);
248 /// A type to distinguish between the two distinct cases this lint handles.
249 #[derive(Copy, Clone, Debug)]
251 ContinueInsideElseBlock,
252 ContinueInsideThenBlock,
255 /// Data we pass around for construction of help messages.
256 struct LintData<'a> {
257 /// The `if` expression encountered in the above loop.
258 if_expr: &'a ast::Expr,
259 /// The condition expression for the above `if`.
260 if_cond: &'a ast::Expr,
261 /// The `then` block of the `if` statement.
262 if_block: &'a ast::Block,
263 /// The `else` block of the `if` statement.
264 /// Note that we only work with `if` exprs that have an `else` branch.
265 else_expr: &'a ast::Expr,
266 /// The 0-based index of the `if` statement in the containing loop block.
268 /// The statements of the loop block.
269 block_stmts: &'a [ast::Stmt],
272 const MSG_REDUNDANT_ELSE_BLOCK: &str = "This else block is redundant.\n";
274 const MSG_ELSE_BLOCK_NOT_NEEDED: &str = "There is no need for an explicit `else` block for this `if` \
277 const DROP_ELSE_BLOCK_AND_MERGE_MSG: &str = "Consider dropping the else clause and merging the code that \
278 follows (in the loop) with the if block, like so:\n";
280 const DROP_ELSE_BLOCK_MSG: &str = "Consider dropping the else clause, and moving out the code in the else \
283 fn emit_warning<'a>(ctx: &EarlyContext<'_>, data: &'a LintData<'_>, header: &str, typ: LintType) {
284 // snip is the whole *help* message that appears after the warning.
285 // message is the warning message.
286 // expr is the expression which the lint warning message refers to.
287 let (snip, message, expr) = match typ {
288 LintType::ContinueInsideElseBlock => (
289 suggestion_snippet_for_continue_inside_else(ctx, data, header),
290 MSG_REDUNDANT_ELSE_BLOCK,
293 LintType::ContinueInsideThenBlock => (
294 suggestion_snippet_for_continue_inside_if(ctx, data, header),
295 MSG_ELSE_BLOCK_NOT_NEEDED,
299 span_help_and_lint(ctx, NEEDLESS_CONTINUE, expr.span, message, &snip);
302 fn suggestion_snippet_for_continue_inside_if<'a>(
303 ctx: &EarlyContext<'_>,
304 data: &'a LintData<'_>,
307 let cond_code = snippet(ctx, data.if_cond.span, "..");
309 let if_code = format!("if {} {{\n continue;\n}}\n", cond_code);
310 /* ^^^^--- Four spaces of indentation. */
312 let else_code = snippet(ctx, data.else_expr.span, "..").into_owned();
313 let else_code = erode_block(&else_code);
314 let else_code = trim_multiline(Cow::from(else_code), false);
316 let mut ret = String::from(header);
317 ret.push_str(&if_code);
318 ret.push_str(&else_code);
319 ret.push_str("\n...");
323 fn suggestion_snippet_for_continue_inside_else<'a>(
324 ctx: &EarlyContext<'_>,
325 data: &'a LintData<'_>,
328 let cond_code = snippet(ctx, data.if_cond.span, "..");
329 let mut if_code = format!("if {} {{\n", cond_code);
332 let block_code = &snippet(ctx, data.if_block.span, "..").into_owned();
333 let block_code = erode_block(block_code);
334 let block_code = trim_multiline(Cow::from(block_code), false);
336 if_code.push_str(&block_code);
339 // These is the code in the loop block that follows the if/else construction
340 // we are complaining about. We want to pull all of this code into the
341 // `then` block of the `if` statement.
342 let to_annex = data.block_stmts[data.stmt_idx + 1..]
344 .map(|stmt| original_sp(stmt.span, DUMMY_SP))
345 .map(|span| snippet_block(ctx, span, "..").into_owned())
349 let mut ret = String::from(header);
351 ret.push_str(&if_code);
352 ret.push_str("\n// Merged code follows...");
353 ret.push_str(&to_annex);
354 ret.push_str("\n}\n");
358 fn check_and_warn<'a>(ctx: &EarlyContext<'_>, expr: &'a ast::Expr) {
359 with_loop_block(expr, |loop_block, label| {
360 for (i, stmt) in loop_block.stmts.iter().enumerate() {
361 with_if_expr(stmt, |if_expr, cond, then_block, else_expr| {
362 let data = &LintData {
366 if_block: then_block,
368 block_stmts: &loop_block.stmts,
370 if needless_continue_in_else(else_expr, label) {
374 DROP_ELSE_BLOCK_AND_MERGE_MSG,
375 LintType::ContinueInsideElseBlock,
377 } else if is_first_block_stmt_continue(then_block, label) {
378 emit_warning(ctx, data, DROP_ELSE_BLOCK_MSG, LintType::ContinueInsideThenBlock);
385 /// Eats at `s` from the end till a closing brace `}` is encountered, and then
386 /// continues eating till a non-whitespace character is found.
395 /// is transformed to
402 /// NOTE: when there is no closing brace in `s`, `s` is _not_ preserved, i.e.,
403 /// an empty string will be returned in that case.
404 pub fn erode_from_back(s: &str) -> String {
405 let mut ret = String::from(s);
406 while ret.pop().map_or(false, |c| c != '}') {}
407 while let Some(c) = ret.pop() {
408 if !c.is_whitespace() {
416 /// Eats at `s` from the front by first skipping all leading whitespace. Then,
417 /// any number of opening braces are eaten, followed by any number of newlines.
423 /// inside_a_block();
427 /// is transformed to
431 /// inside_a_block();
434 pub fn erode_from_front(s: &str) -> String {
436 .skip_while(|c| c.is_whitespace())
437 .skip_while(|c| *c == '{')
438 .skip_while(|c| *c == '\n')
442 /// If `s` contains the code for a block, delimited by braces, this function
443 /// tries to get the contents of the block. If there is no closing brace
445 /// an empty string is returned.
446 pub fn erode_block(s: &str) -> String {
447 erode_from_back(&erode_from_front(s))