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