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.
32 use syntax::codemap::{original_sp, DUMMY_SP};
35 use utils::{in_macro, span_help_and_lint, snippet_block, snippet, trim_multiline};
37 /// **What it does:** The lint checks for `if`-statements appearing in loops
38 /// that contain a `continue` statement in either their main blocks or their
39 /// `else`-blocks, when omitting the `else`-block possibly with some
40 /// rearrangement of code can make the code easier to understand.
42 /// **Why is this bad?** Having explicit `else` blocks for `if` statements
43 /// containing `continue` in their THEN branch adds unnecessary branching and
44 /// nesting to the code. Having an else block containing just `continue` can
45 /// also be better written by grouping the statements following the whole `if`
46 /// statement within the THEN block and omitting the else block completely.
48 /// **Known problems:** None
52 /// while condition() {
53 /// update_condition();
59 /// println!("Hello, world");
63 /// Could be rewritten as
66 /// while condition() {
67 /// update_condition();
70 /// println!("Hello, world");
75 /// As another example, the following code
82 /// // Do something useful
86 /// Could be rewritten as
93 /// // Do something useful
97 pub NEEDLESS_CONTINUE,
99 "`continue` statements that can be replaced by a rearrangement of code"
102 #[derive(Copy, Clone)]
103 pub struct NeedlessContinue;
105 impl LintPass for NeedlessContinue {
106 fn get_lints(&self) -> LintArray {
107 lint_array!(NEEDLESS_CONTINUE)
111 impl EarlyLintPass for NeedlessContinue {
112 fn check_expr(&mut self, ctx: &EarlyContext, expr: &ast::Expr) {
113 if !in_macro(expr.span) {
114 check_and_warn(ctx, expr);
119 /* This lint has to mainly deal with two cases of needless continue
121 // Case 1 [Continue inside else block]:
133 // This code can better be written as follows:
143 // Case 2 [Continue inside then block]:
149 // // potentially more code here.
157 // This snippet can be refactored to:
168 /// Given an expression, returns true if either of the following is true
170 /// - The expression is a `continue` node.
171 /// - The expression node is a block with the first statement being a
174 fn needless_continue_in_else(else_expr: &ast::Expr) -> bool {
175 match else_expr.node {
176 ast::ExprKind::Block(ref else_block) => is_first_block_stmt_continue(else_block),
177 ast::ExprKind::Continue(_) => true,
182 fn is_first_block_stmt_continue(block: &ast::Block) -> bool {
183 block.stmts.get(0).map_or(false, |stmt| match stmt.node {
184 ast::StmtKind::Semi(ref e) |
185 ast::StmtKind::Expr(ref e) => {
186 if let ast::ExprKind::Continue(_) = e.node {
196 /// If `expr` is a loop expression (while/while let/for/loop), calls `func` with
197 /// the AST object representing the loop block of `expr`.
198 fn with_loop_block<F>(expr: &ast::Expr, mut func: F)
200 F: FnMut(&ast::Block),
203 ast::ExprKind::While(_, ref loop_block, _) |
204 ast::ExprKind::WhileLet(_, _, ref loop_block, _) |
205 ast::ExprKind::ForLoop(_, _, ref loop_block, _) |
206 ast::ExprKind::Loop(ref loop_block, _) => func(loop_block),
211 /// If `stmt` is an if expression node with an `else` branch, calls func with
215 /// - The `if` expression itself,
216 /// - The `if` condition expression,
217 /// - The `then` block, and
218 /// - The `else` expression.
220 fn with_if_expr<F>(stmt: &ast::Stmt, mut func: F)
222 F: FnMut(&ast::Expr, &ast::Expr, &ast::Block, &ast::Expr),
225 ast::StmtKind::Semi(ref e) |
226 ast::StmtKind::Expr(ref e) => {
227 if let ast::ExprKind::If(ref cond, ref if_block, Some(ref else_expr)) = e.node {
228 func(e, cond, if_block, else_expr);
235 /// A type to distinguish between the two distinct cases this lint handles.
236 #[derive(Copy, Clone, Debug)]
238 ContinueInsideElseBlock,
239 ContinueInsideThenBlock,
242 /// Data we pass around for construction of help messages.
243 struct LintData<'a> {
244 /// The `if` expression encountered in the above loop.
245 if_expr: &'a ast::Expr,
246 /// The condition expression for the above `if`.
247 if_cond: &'a ast::Expr,
248 /// The `then` block of the `if` statement.
249 if_block: &'a ast::Block,
250 /// The `else` block of the `if` statement.
251 /// Note that we only work with `if` exprs that have an `else` branch.
252 else_expr: &'a ast::Expr,
253 /// The 0-based index of the `if` statement in the containing loop block.
255 /// The statements of the loop block.
256 block_stmts: &'a [ast::Stmt],
259 const MSG_REDUNDANT_ELSE_BLOCK: &'static str = "This else block is redundant.\n";
261 const MSG_ELSE_BLOCK_NOT_NEEDED: &'static str = "There is no need for an explicit `else` block for this `if` \
264 const DROP_ELSE_BLOCK_AND_MERGE_MSG: &'static str = "Consider dropping the else clause and merging the code that \
265 follows (in the loop) with the if block, like so:\n";
267 const DROP_ELSE_BLOCK_MSG: &'static str = "Consider dropping the else clause, and moving out the code in the else \
271 fn emit_warning<'a>(ctx: &EarlyContext, data: &'a LintData, header: &str, typ: LintType) {
273 // snip is the whole *help* message that appears after the warning.
274 // message is the warning message.
275 // expr is the expression which the lint warning message refers to.
276 let (snip, message, expr) = match typ {
277 LintType::ContinueInsideElseBlock => {
279 suggestion_snippet_for_continue_inside_else(ctx, data, header),
280 MSG_REDUNDANT_ELSE_BLOCK,
284 LintType::ContinueInsideThenBlock => {
286 suggestion_snippet_for_continue_inside_if(ctx, data, header),
287 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>(ctx: &EarlyContext, data: &'a LintData, header: &str) -> String {
296 let cond_code = snippet(ctx, data.if_cond.span, "..");
298 let if_code = format!("if {} {{\n continue;\n}}\n", cond_code);
299 /* ^^^^--- Four spaces of indentation. */
301 let else_code = snippet(ctx, data.else_expr.span, "..").into_owned();
302 let else_code = erode_block(&else_code);
303 let else_code = trim_multiline(Cow::from(else_code), false);
305 let mut ret = String::from(header);
306 ret.push_str(&if_code);
307 ret.push_str(&else_code);
308 ret.push_str("\n...");
312 fn suggestion_snippet_for_continue_inside_else<'a>(ctx: &EarlyContext, data: &'a LintData, header: &str) -> String {
313 let cond_code = snippet(ctx, data.if_cond.span, "..");
314 let mut if_code = format!("if {} {{\n", cond_code);
317 let block_code = &snippet(ctx, data.if_block.span, "..").into_owned();
318 let block_code = erode_block(block_code);
319 let block_code = trim_multiline(Cow::from(block_code), false);
321 if_code.push_str(&block_code);
324 // These is the code in the loop block that follows the if/else construction
325 // we are complaining about. We want to pull all of this code into the
326 // `then` block of the `if` statement.
327 let to_annex = data.block_stmts[data.stmt_idx + 1..]
329 .map(|stmt| original_sp(stmt.span, DUMMY_SP))
330 .map(|span| snippet_block(ctx, span, "..").into_owned())
334 let mut ret = String::from(header);
336 ret.push_str(&if_code);
337 ret.push_str("\n// Merged code follows...");
338 ret.push_str(&to_annex);
339 ret.push_str("\n}\n");
343 fn check_and_warn<'a>(ctx: &EarlyContext, expr: &'a ast::Expr) {
344 with_loop_block(expr, |loop_block| for (i, stmt) in loop_block.stmts.iter().enumerate() {
345 with_if_expr(stmt, |if_expr, cond, then_block, else_expr| {
346 let data = &LintData {
350 if_block: then_block,
351 else_expr: else_expr,
352 block_stmts: &loop_block.stmts,
354 if needless_continue_in_else(else_expr) {
355 emit_warning(ctx, data, DROP_ELSE_BLOCK_AND_MERGE_MSG, LintType::ContinueInsideElseBlock);
356 } else if is_first_block_stmt_continue(then_block) {
357 emit_warning(ctx, data, DROP_ELSE_BLOCK_MSG, LintType::ContinueInsideThenBlock);
363 /// Eats at `s` from the end till a closing brace `}` is encountered, and then
364 /// continues eating till a non-whitespace character is found.
373 /// is transformed to
380 /// NOTE: when there is no closing brace in `s`, `s` is _not_ preserved, i.e.,
381 /// an empty string will be returned in that case.
382 pub fn erode_from_back(s: &str) -> String {
383 let mut ret = String::from(s);
384 while ret.pop().map_or(false, |c| c != '}') {}
385 while let Some(c) = ret.pop() {
386 if !c.is_whitespace() {
394 /// Eats at `s` from the front by first skipping all leading whitespace. Then,
395 /// any number of opening braces are eaten, followed by any number of newlines.
401 /// inside_a_block();
405 /// is transformed to
409 /// inside_a_block();
413 pub fn erode_from_front(s: &str) -> String {
415 .skip_while(|c| c.is_whitespace())
416 .skip_while(|c| *c == '{')
417 .skip_while(|c| *c == '\n')
421 /// If `s` contains the code for a block, delimited by braces, this function
422 /// tries to get the contents of the block. If there is no closing brace
424 /// an empty string is returned.
425 pub fn erode_block(s: &str) -> String {
426 erode_from_back(&erode_from_front(s))