1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution.
4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
7 // option. This file may not be copied, modified, or distributed
8 // except according to those terms.
10 //! Checks for continue statements in loops that are redundant.
12 //! For example, the lint would catch
15 //! while condition() {
16 //! update_condition();
22 //! println!("Hello, world");
26 //! And suggest something like this:
29 //! while condition() {
30 //! update_condition();
33 //! println!("Hello, world");
38 //! This lint is **warn** by default.
39 use crate::rustc::lint::{EarlyContext, EarlyLintPass, LintArray, LintPass};
40 use crate::rustc::{declare_tool_lint, lint_array};
41 use crate::syntax::ast;
42 use crate::syntax::source_map::{original_sp, DUMMY_SP};
45 use crate::utils::{in_macro, snippet, snippet_block, span_help_and_lint, trim_multiline};
47 /// **What it does:** The lint checks for `if`-statements appearing in loops
48 /// that contain a `continue` statement in either their main blocks or their
49 /// `else`-blocks, when omitting the `else`-block possibly with some
50 /// rearrangement of code can make the code easier to understand.
52 /// **Why is this bad?** Having explicit `else` blocks for `if` statements
53 /// containing `continue` in their THEN branch adds unnecessary branching and
54 /// nesting to the code. Having an else block containing just `continue` can
55 /// also be better written by grouping the statements following the whole `if`
56 /// statement within the THEN block and omitting the else block completely.
58 /// **Known problems:** None
62 /// while condition() {
63 /// update_condition();
69 /// println!("Hello, world");
73 /// Could be rewritten as
76 /// while condition() {
77 /// update_condition();
80 /// println!("Hello, world");
85 /// As another example, the following code
92 /// // Do something useful
96 /// Could be rewritten as
103 /// // Do something useful
106 declare_clippy_lint! {
107 pub NEEDLESS_CONTINUE,
109 "`continue` statements that can be replaced by a rearrangement of code"
112 #[derive(Copy, Clone)]
113 pub struct NeedlessContinue;
115 impl LintPass for NeedlessContinue {
116 fn get_lints(&self) -> LintArray {
117 lint_array!(NEEDLESS_CONTINUE)
121 impl EarlyLintPass for NeedlessContinue {
122 fn check_expr(&mut self, ctx: &EarlyContext<'_>, expr: &ast::Expr) {
123 if !in_macro(expr.span) {
124 check_and_warn(ctx, expr);
129 /* This lint has to mainly deal with two cases of needless continue
131 // Case 1 [Continue inside else block]:
143 // This code can better be written as follows:
153 // Case 2 [Continue inside then block]:
159 // // potentially more code here.
167 // This snippet can be refactored to:
178 /// Given an expression, returns true if either of the following is true
180 /// - The expression is a `continue` node.
181 /// - The expression node is a block with the first statement being a
183 fn needless_continue_in_else(else_expr: &ast::Expr) -> bool {
184 match else_expr.node {
185 ast::ExprKind::Block(ref else_block, _) => is_first_block_stmt_continue(else_block),
186 ast::ExprKind::Continue(_) => true,
191 fn is_first_block_stmt_continue(block: &ast::Block) -> bool {
192 block.stmts.get(0).map_or(false, |stmt| match stmt.node {
193 ast::StmtKind::Semi(ref e) | ast::StmtKind::Expr(ref e) => {
194 if let ast::ExprKind::Continue(_) = e.node {
204 /// If `expr` is a loop expression (while/while let/for/loop), calls `func` with
205 /// the AST object representing the loop block of `expr`.
206 fn with_loop_block<F>(expr: &ast::Expr, mut func: F)
208 F: FnMut(&ast::Block),
211 ast::ExprKind::While(_, ref loop_block, _)
212 | ast::ExprKind::WhileLet(_, _, ref loop_block, _)
213 | ast::ExprKind::ForLoop(_, _, ref loop_block, _)
214 | ast::ExprKind::Loop(ref loop_block, _) => func(loop_block),
219 /// If `stmt` is an if expression node with an `else` branch, calls func with
223 /// - The `if` expression itself,
224 /// - The `if` condition expression,
225 /// - The `then` block, and
226 /// - The `else` expression.
227 fn with_if_expr<F>(stmt: &ast::Stmt, mut func: F)
229 F: FnMut(&ast::Expr, &ast::Expr, &ast::Block, &ast::Expr),
232 ast::StmtKind::Semi(ref e) | ast::StmtKind::Expr(ref e) => {
233 if let ast::ExprKind::If(ref cond, ref if_block, Some(ref else_expr)) = e.node {
234 func(e, cond, if_block, else_expr);
241 /// A type to distinguish between the two distinct cases this lint handles.
242 #[derive(Copy, Clone, Debug)]
244 ContinueInsideElseBlock,
245 ContinueInsideThenBlock,
248 /// Data we pass around for construction of help messages.
249 struct LintData<'a> {
250 /// The `if` expression encountered in the above loop.
251 if_expr: &'a ast::Expr,
252 /// The condition expression for the above `if`.
253 if_cond: &'a ast::Expr,
254 /// The `then` block of the `if` statement.
255 if_block: &'a ast::Block,
256 /// The `else` block of the `if` statement.
257 /// Note that we only work with `if` exprs that have an `else` branch.
258 else_expr: &'a ast::Expr,
259 /// The 0-based index of the `if` statement in the containing loop block.
261 /// The statements of the loop block.
262 block_stmts: &'a [ast::Stmt],
265 const MSG_REDUNDANT_ELSE_BLOCK: &str = "This else block is redundant.\n";
267 const MSG_ELSE_BLOCK_NOT_NEEDED: &str = "There is no need for an explicit `else` block for this `if` \
270 const DROP_ELSE_BLOCK_AND_MERGE_MSG: &str = "Consider dropping the else clause and merging the code that \
271 follows (in the loop) with the if block, like so:\n";
273 const DROP_ELSE_BLOCK_MSG: &str = "Consider dropping the else clause, and moving out the code in the else \
276 fn emit_warning<'a>(ctx: &EarlyContext<'_>, data: &'a LintData<'_>, header: &str, typ: LintType) {
277 // snip is the whole *help* message that appears after the warning.
278 // message is the warning message.
279 // expr is the expression which the lint warning message refers to.
280 let (snip, message, expr) = match typ {
281 LintType::ContinueInsideElseBlock => (
282 suggestion_snippet_for_continue_inside_else(ctx, data, header),
283 MSG_REDUNDANT_ELSE_BLOCK,
286 LintType::ContinueInsideThenBlock => (
287 suggestion_snippet_for_continue_inside_if(ctx, data, header),
288 MSG_ELSE_BLOCK_NOT_NEEDED,
292 span_help_and_lint(ctx, NEEDLESS_CONTINUE, expr.span, message, &snip);
295 fn suggestion_snippet_for_continue_inside_if<'a>(
296 ctx: &EarlyContext<'_>,
297 data: &'a LintData<'_>,
300 let cond_code = snippet(ctx, data.if_cond.span, "..");
302 let if_code = format!("if {} {{\n continue;\n}}\n", cond_code);
303 /* ^^^^--- Four spaces of indentation. */
305 let else_code = snippet(ctx, data.else_expr.span, "..").into_owned();
306 let else_code = erode_block(&else_code);
307 let else_code = trim_multiline(Cow::from(else_code), false);
309 let mut ret = String::from(header);
310 ret.push_str(&if_code);
311 ret.push_str(&else_code);
312 ret.push_str("\n...");
316 fn suggestion_snippet_for_continue_inside_else<'a>(
317 ctx: &EarlyContext<'_>,
318 data: &'a LintData<'_>,
321 let cond_code = snippet(ctx, data.if_cond.span, "..");
322 let mut if_code = format!("if {} {{\n", cond_code);
325 let block_code = &snippet(ctx, data.if_block.span, "..").into_owned();
326 let block_code = erode_block(block_code);
327 let block_code = trim_multiline(Cow::from(block_code), false);
329 if_code.push_str(&block_code);
332 // These is the code in the loop block that follows the if/else construction
333 // we are complaining about. We want to pull all of this code into the
334 // `then` block of the `if` statement.
335 let to_annex = data.block_stmts[data.stmt_idx + 1..]
337 .map(|stmt| original_sp(stmt.span, DUMMY_SP))
338 .map(|span| snippet_block(ctx, span, "..").into_owned())
342 let mut ret = String::from(header);
344 ret.push_str(&if_code);
345 ret.push_str("\n// Merged code follows...");
346 ret.push_str(&to_annex);
347 ret.push_str("\n}\n");
351 fn check_and_warn<'a>(ctx: &EarlyContext<'_>, expr: &'a ast::Expr) {
352 with_loop_block(expr, |loop_block| {
353 for (i, stmt) in loop_block.stmts.iter().enumerate() {
354 with_if_expr(stmt, |if_expr, cond, then_block, else_expr| {
355 let data = &LintData {
359 if_block: then_block,
361 block_stmts: &loop_block.stmts,
363 if needless_continue_in_else(else_expr) {
367 DROP_ELSE_BLOCK_AND_MERGE_MSG,
368 LintType::ContinueInsideElseBlock,
370 } else if is_first_block_stmt_continue(then_block) {
371 emit_warning(ctx, data, DROP_ELSE_BLOCK_MSG, LintType::ContinueInsideThenBlock);
378 /// Eats at `s` from the end till a closing brace `}` is encountered, and then
379 /// continues eating till a non-whitespace character is found.
388 /// is transformed to
395 /// NOTE: when there is no closing brace in `s`, `s` is _not_ preserved, i.e.,
396 /// an empty string will be returned in that case.
397 pub fn erode_from_back(s: &str) -> String {
398 let mut ret = String::from(s);
399 while ret.pop().map_or(false, |c| c != '}') {}
400 while let Some(c) = ret.pop() {
401 if !c.is_whitespace() {
409 /// Eats at `s` from the front by first skipping all leading whitespace. Then,
410 /// any number of opening braces are eaten, followed by any number of newlines.
416 /// inside_a_block();
420 /// is transformed to
424 /// inside_a_block();
427 pub fn erode_from_front(s: &str) -> String {
429 .skip_while(|c| c.is_whitespace())
430 .skip_while(|c| *c == '{')
431 .skip_while(|c| *c == '\n')
435 /// If `s` contains the code for a block, delimited by braces, this function
436 /// tries to get the contents of the block. If there is no closing brace
438 /// an empty string is returned.
439 pub fn erode_block(s: &str) -> String {
440 erode_from_back(&erode_from_front(s))