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, snippet, snippet_block, span_help_and_lint, 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) | ast::StmtKind::Expr(ref e) => if let ast::ExprKind::Continue(_) = e.node {
193 /// If `expr` is a loop expression (while/while let/for/loop), calls `func` with
194 /// the AST object representing the loop block of `expr`.
195 fn with_loop_block<F>(expr: &ast::Expr, mut func: F)
197 F: FnMut(&ast::Block),
200 ast::ExprKind::While(_, ref loop_block, _) |
201 ast::ExprKind::WhileLet(_, _, ref loop_block, _) |
202 ast::ExprKind::ForLoop(_, _, ref loop_block, _) |
203 ast::ExprKind::Loop(ref loop_block, _) => func(loop_block),
208 /// If `stmt` is an if expression node with an `else` branch, calls func with
212 /// - The `if` expression itself,
213 /// - The `if` condition expression,
214 /// - The `then` block, and
215 /// - The `else` expression.
217 fn with_if_expr<F>(stmt: &ast::Stmt, mut func: F)
219 F: FnMut(&ast::Expr, &ast::Expr, &ast::Block, &ast::Expr),
222 ast::StmtKind::Semi(ref e) | ast::StmtKind::Expr(ref e) => {
223 if let ast::ExprKind::If(ref cond, ref if_block, Some(ref else_expr)) = e.node {
224 func(e, cond, if_block, else_expr);
231 /// A type to distinguish between the two distinct cases this lint handles.
232 #[derive(Copy, Clone, Debug)]
234 ContinueInsideElseBlock,
235 ContinueInsideThenBlock,
238 /// Data we pass around for construction of help messages.
239 struct LintData<'a> {
240 /// The `if` expression encountered in the above loop.
241 if_expr: &'a ast::Expr,
242 /// The condition expression for the above `if`.
243 if_cond: &'a ast::Expr,
244 /// The `then` block of the `if` statement.
245 if_block: &'a ast::Block,
246 /// The `else` block of the `if` statement.
247 /// Note that we only work with `if` exprs that have an `else` branch.
248 else_expr: &'a ast::Expr,
249 /// The 0-based index of the `if` statement in the containing loop block.
251 /// The statements of the loop block.
252 block_stmts: &'a [ast::Stmt],
255 const MSG_REDUNDANT_ELSE_BLOCK: &str = "This else block is redundant.\n";
257 const MSG_ELSE_BLOCK_NOT_NEEDED: &str = "There is no need for an explicit `else` block for this `if` \
260 const DROP_ELSE_BLOCK_AND_MERGE_MSG: &str = "Consider dropping the else clause and merging the code that \
261 follows (in the loop) with the if block, like so:\n";
263 const DROP_ELSE_BLOCK_MSG: &str = "Consider dropping the else clause, and moving out the code in the else \
267 fn emit_warning<'a>(ctx: &EarlyContext, data: &'a LintData, header: &str, typ: LintType) {
268 // snip is the whole *help* message that appears after the warning.
269 // message is the warning message.
270 // expr is the expression which the lint warning message refers to.
271 let (snip, message, expr) = match typ {
272 LintType::ContinueInsideElseBlock => (
273 suggestion_snippet_for_continue_inside_else(ctx, data, header),
274 MSG_REDUNDANT_ELSE_BLOCK,
277 LintType::ContinueInsideThenBlock => (
278 suggestion_snippet_for_continue_inside_if(ctx, data, header),
279 MSG_ELSE_BLOCK_NOT_NEEDED,
283 span_help_and_lint(ctx, NEEDLESS_CONTINUE, expr.span, message, &snip);
286 fn suggestion_snippet_for_continue_inside_if<'a>(ctx: &EarlyContext, data: &'a LintData, header: &str) -> String {
287 let cond_code = snippet(ctx, data.if_cond.span, "..");
289 let if_code = format!("if {} {{\n continue;\n}}\n", cond_code);
290 /* ^^^^--- Four spaces of indentation. */
292 let else_code = snippet(ctx, data.else_expr.span, "..").into_owned();
293 let else_code = erode_block(&else_code);
294 let else_code = trim_multiline(Cow::from(else_code), false);
296 let mut ret = String::from(header);
297 ret.push_str(&if_code);
298 ret.push_str(&else_code);
299 ret.push_str("\n...");
303 fn suggestion_snippet_for_continue_inside_else<'a>(ctx: &EarlyContext, data: &'a LintData, header: &str) -> String {
304 let cond_code = snippet(ctx, data.if_cond.span, "..");
305 let mut if_code = format!("if {} {{\n", cond_code);
308 let block_code = &snippet(ctx, data.if_block.span, "..").into_owned();
309 let block_code = erode_block(block_code);
310 let block_code = trim_multiline(Cow::from(block_code), false);
312 if_code.push_str(&block_code);
315 // These is the code in the loop block that follows the if/else construction
316 // we are complaining about. We want to pull all of this code into the
317 // `then` block of the `if` statement.
318 let to_annex = data.block_stmts[data.stmt_idx + 1..]
320 .map(|stmt| original_sp(stmt.span, DUMMY_SP))
321 .map(|span| snippet_block(ctx, span, "..").into_owned())
325 let mut ret = String::from(header);
327 ret.push_str(&if_code);
328 ret.push_str("\n// Merged code follows...");
329 ret.push_str(&to_annex);
330 ret.push_str("\n}\n");
334 fn check_and_warn<'a>(ctx: &EarlyContext, expr: &'a ast::Expr) {
335 with_loop_block(expr, |loop_block| {
336 for (i, stmt) in loop_block.stmts.iter().enumerate() {
337 with_if_expr(stmt, |if_expr, cond, then_block, else_expr| {
338 let data = &LintData {
342 if_block: then_block,
344 block_stmts: &loop_block.stmts,
346 if needless_continue_in_else(else_expr) {
347 emit_warning(ctx, data, DROP_ELSE_BLOCK_AND_MERGE_MSG, LintType::ContinueInsideElseBlock);
348 } else if is_first_block_stmt_continue(then_block) {
349 emit_warning(ctx, data, DROP_ELSE_BLOCK_MSG, LintType::ContinueInsideThenBlock);
356 /// Eats at `s` from the end till a closing brace `}` is encountered, and then
357 /// continues eating till a non-whitespace character is found.
366 /// is transformed to
373 /// NOTE: when there is no closing brace in `s`, `s` is _not_ preserved, i.e.,
374 /// an empty string will be returned in that case.
375 pub fn erode_from_back(s: &str) -> String {
376 let mut ret = String::from(s);
377 while ret.pop().map_or(false, |c| c != '}') {}
378 while let Some(c) = ret.pop() {
379 if !c.is_whitespace() {
387 /// Eats at `s` from the front by first skipping all leading whitespace. Then,
388 /// any number of opening braces are eaten, followed by any number of newlines.
394 /// inside_a_block();
398 /// is transformed to
402 /// inside_a_block();
406 pub fn erode_from_front(s: &str) -> String {
408 .skip_while(|c| c.is_whitespace())
409 .skip_while(|c| *c == '{')
410 .skip_while(|c| *c == '\n')
414 /// If `s` contains the code for a block, delimited by braces, this function
415 /// tries to get the contents of the block. If there is no closing brace
417 /// an empty string is returned.
418 pub fn erode_block(s: &str) -> String {
419 erode_from_back(&erode_from_front(s))