469 lines
15 KiB
Rust
469 lines
15 KiB
Rust
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//! 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 std;
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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 utils::{in_macro, span_help_and_lint, snippet_block, snippet};
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use self::LintType::*;
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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_lint! {
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pub NEEDLESS_CONTINUE,
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Warn,
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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(ctx, 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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/// Given an Expr, returns true if either of the following is true
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///
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/// - The Expr is a `continue` node.
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/// - The Expr node is a block with the first statement being a `continue`.
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///
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fn needless_continue_in_else(else_expr: &ast::Expr) -> bool {
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let mut found = false;
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match else_expr.node {
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ast::ExprKind::Block(ref else_block) => {
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found = is_first_block_stmt_continue(else_block);
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},
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ast::ExprKind::Continue(_) => { found = true },
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_ => { },
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};
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found
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}
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fn is_first_block_stmt_continue(block: &ast::Block) -> bool {
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let mut ret = false;
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block.stmts.get(0).map(|stmt| {
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if_let_chain! {[
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let ast::StmtKind::Semi(ref e) = stmt.node,
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let ast::ExprKind::Continue(_) = e.node,
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], {
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ret = true;
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}}
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}).unwrap_or(());
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ret
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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) where F: FnMut(&ast::Block) {
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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 the
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/// following:
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///
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/// - The if Expr,
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/// - The if condition Expr,
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/// - The then block of this if Expr, and
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/// - The else expr.
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///
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fn with_if_expr<F>(stmt: &ast::Stmt, mut func: F)
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where F: FnMut(&ast::Expr, &ast::Expr, &ast::Block, &ast::Expr) {
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match stmt.node {
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ast::StmtKind::Semi(ref e) |
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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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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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if_expr: &'a ast::Expr, // The `if` expr encountered in the above loop.
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if_cond: &'a ast::Expr, // The condition expression for the above `if`.
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if_block: &'a ast::Block, // The `then` block of the `if` statement.
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else_expr: &'a ast::Expr, /* The `else` block of the `if` statement.
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Note that we only work with `if` exprs that
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have an `else` branch. */
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stmt_idx: usize, /* The 0-based index of the `if` statement in
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the containing loop block. */
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block_stmts: &'a [ast::Stmt], // The statements of the loop block.
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}
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const MSG_REDUNDANT_ELSE_BLOCK: &'static str = "This else block is redundant.\n";
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const MSG_ELSE_BLOCK_NOT_NEEDED: &'static str = "There is no need for an explicit `else` block for this `if` expression\n";
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const DROP_ELSE_BLOCK_AND_MERGE_MSG: &'static str =
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"Consider dropping the else clause and merging the code that follows (in the loop) with the if block, like so:\n";
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const DROP_ELSE_BLOCK_MSG: &'static str =
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"Consider dropping the else clause, and moving out the code in the else block, like so:\n";
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fn emit_warning<'a>(ctx: &EarlyContext,
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data: &'a LintData,
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header: &str,
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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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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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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,
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&format!("{}", snip));
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}
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fn suggestion_snippet_for_continue_inside_if<'a>(ctx: &EarlyContext,
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data: &'a LintData,
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header: &str) -> String {
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let cond_code = &snippet(ctx, data.if_cond.span, "..").into_owned();
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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_indent(&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,
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data: &'a LintData,
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header: &str) -> String
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{
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let cond_code = &snippet(ctx, data.if_cond.span, "..").into_owned();
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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_indent(&block_code, false);
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let block_code = left_pad_lines_with_spaces(&block_code, 4usize);
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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| {
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original_sp(ctx.sess().codemap(), stmt.span, DUMMY_SP)
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})
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.map(|span| snippet_block(ctx, span, "..").into_owned())
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.collect::<Vec<_>>().join("\n");
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let mut ret = String::from(header);
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ret.push_str(&align_snippets(&[&if_code,
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"\n// Merged code follows...",
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&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: if_expr,
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if_cond: cond,
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if_block: then_block,
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else_expr: 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, 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, 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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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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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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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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fn is_all_whitespace(s: &str) -> bool { s.chars().all(|c| c.is_whitespace()) }
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/// Returns true if a string is empty or just spaces.
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fn is_null(s: &str) -> bool { s.is_empty() || is_all_whitespace(s) }
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/// Returns the indentation level of a string. It just returns the count of
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/// whitespace characters in the string before a non-whitespace character
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/// is encountered.
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fn indent_level(s: &str) -> usize {
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s.chars()
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.enumerate()
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.find(|&(_, c)| !c.is_whitespace())
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.map_or(0usize, |(i, _)| i)
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}
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/// Trims indentation from a snippet such that the line with the minimum
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/// indentation has no indentation after the trasformation.
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|
|
fn trim_indent(s: &str, skip_first_line: bool) -> String {
|
||
|
|
let min_indent_level = s.lines()
|
||
|
|
.filter(|line| !is_null(line))
|
||
|
|
.skip(skip_first_line as usize)
|
||
|
|
.map(indent_level)
|
||
|
|
.min()
|
||
|
|
.unwrap_or(0usize);
|
||
|
|
let ret = s.lines().map(|line| {
|
||
|
|
if is_null(line) {
|
||
|
|
String::from(line)
|
||
|
|
} else {
|
||
|
|
line.chars()
|
||
|
|
.enumerate()
|
||
|
|
.skip_while(|&(i, c)| c.is_whitespace() && i < min_indent_level)
|
||
|
|
.map(|pair| pair.1)
|
||
|
|
.collect::<String>()
|
||
|
|
}
|
||
|
|
}).collect::<Vec<String>>();
|
||
|
|
ret.join("\n")
|
||
|
|
}
|
||
|
|
|
||
|
|
/// Add `n` spaces to the left of `s`.
|
||
|
|
fn left_pad_with_spaces(s: &str, n: usize) -> String {
|
||
|
|
let mut new_s = std::iter::repeat(' '/* <-space */).take(n).collect::<String>();
|
||
|
|
new_s.push_str(s);
|
||
|
|
new_s
|
||
|
|
}
|
||
|
|
|
||
|
|
/// Add `n` spaces to the left of each line in `s` and return the result
|
||
|
|
/// in a new String.
|
||
|
|
fn left_pad_lines_with_spaces(s: &str, n: usize) -> String {
|
||
|
|
s.lines()
|
||
|
|
.map(|line| left_pad_with_spaces(line, n))
|
||
|
|
.collect::<Vec<_>>()
|
||
|
|
.join("\n")
|
||
|
|
}
|
||
|
|
|
||
|
|
/// Remove upto `n` whitespace characters from the beginning of `s`.
|
||
|
|
fn remove_whitespace_from_left(s: &str, n: usize) -> String {
|
||
|
|
s.chars()
|
||
|
|
.enumerate()
|
||
|
|
.skip_while(|&(i, c)| i < n && c.is_whitespace())
|
||
|
|
.map(|(_, c)| c)
|
||
|
|
.collect::<String>()
|
||
|
|
}
|
||
|
|
|
||
|
|
/// Aligns two snippets such that the indentation level of the last non-empty,
|
||
|
|
/// non-space line of the first snippet matches the first non-empty, non-space
|
||
|
|
/// line of the second.
|
||
|
|
fn align_two_snippets(s: &str, t: &str) -> String {
|
||
|
|
// indent level of the last nonempty, non-whitespace line of s.
|
||
|
|
let target_ilevel = s.lines()
|
||
|
|
.rev()
|
||
|
|
.skip_while(|line| line.is_empty() || is_all_whitespace(line))
|
||
|
|
.next()
|
||
|
|
.map_or(0usize, indent_level);
|
||
|
|
|
||
|
|
// We want to align the first nonempty, non-all-whitespace line of t to
|
||
|
|
// have the same indent level as target_ilevel
|
||
|
|
let level = t.lines()
|
||
|
|
.skip_while(|line| line.is_empty() || is_all_whitespace(line))
|
||
|
|
.next()
|
||
|
|
.map_or(0usize, indent_level);
|
||
|
|
|
||
|
|
let add_or_not_remove = target_ilevel > level; /* when true, we add spaces,
|
||
|
|
otherwise eat. */
|
||
|
|
|
||
|
|
let delta = if add_or_not_remove {
|
||
|
|
target_ilevel - level
|
||
|
|
} else {
|
||
|
|
level - target_ilevel
|
||
|
|
};
|
||
|
|
|
||
|
|
let new_t = t.lines()
|
||
|
|
.filter(|line| !is_null(line))
|
||
|
|
.map(|line| if add_or_not_remove {
|
||
|
|
left_pad_with_spaces(line, delta)
|
||
|
|
} else {
|
||
|
|
remove_whitespace_from_left(line, delta)
|
||
|
|
})
|
||
|
|
.collect::<Vec<_>>().join("\n");
|
||
|
|
|
||
|
|
format!("{}\n{}", s, new_t)
|
||
|
|
}
|
||
|
|
|
||
|
|
fn align_snippets(xs: &[&str]) -> String {
|
||
|
|
match xs.len() {
|
||
|
|
0 => String::from(""),
|
||
|
|
_ => {
|
||
|
|
let mut ret = String::new();
|
||
|
|
ret.push_str(xs[0]);
|
||
|
|
for x in xs.iter().skip(1usize) {
|
||
|
|
ret = align_two_snippets(&ret, x);
|
||
|
|
}
|
||
|
|
ret
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|