rust-clippy/clippy_lints/src/needless_continue.rs
2020-06-01 03:08:51 +07:00

463 lines
13 KiB
Rust

//! Checks for continue statements in loops that are redundant.
//!
//! For example, the lint would catch
//!
//! ```rust
//! let mut a = 1;
//! let x = true;
//!
//! while a < 5 {
//! a = 6;
//! if x {
//! // ...
//! } else {
//! continue;
//! }
//! println!("Hello, world");
//! }
//! ```
//!
//! And suggest something like this:
//!
//! ```rust
//! let mut a = 1;
//! let x = true;
//!
//! while a < 5 {
//! a = 6;
//! if x {
//! // ...
//! println!("Hello, world");
//! }
//! }
//! ```
//!
//! This lint is **warn** by default.
use rustc_ast::ast;
use rustc_lint::{EarlyContext, EarlyLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::{original_sp, DUMMY_SP};
use rustc_span::Span;
use crate::utils::{indent_of, snippet, snippet_block, span_lint_and_help};
declare_clippy_lint! {
/// **What it does:** The lint checks for `if`-statements appearing in loops
/// that contain a `continue` statement in either their main blocks or their
/// `else`-blocks, when omitting the `else`-block possibly with some
/// rearrangement of code can make the code easier to understand.
///
/// **Why is this bad?** Having explicit `else` blocks for `if` statements
/// containing `continue` in their THEN branch adds unnecessary branching and
/// nesting to the code. Having an else block containing just `continue` can
/// also be better written by grouping the statements following the whole `if`
/// statement within the THEN block and omitting the else block completely.
///
/// **Known problems:** None
///
/// **Example:**
/// ```rust
/// # fn condition() -> bool { false }
/// # fn update_condition() {}
/// # let x = false;
/// while condition() {
/// update_condition();
/// if x {
/// // ...
/// } else {
/// continue;
/// }
/// println!("Hello, world");
/// }
/// ```
///
/// Could be rewritten as
///
/// ```rust
/// # fn condition() -> bool { false }
/// # fn update_condition() {}
/// # let x = false;
/// while condition() {
/// update_condition();
/// if x {
/// // ...
/// println!("Hello, world");
/// }
/// }
/// ```
///
/// As another example, the following code
///
/// ```rust
/// # fn waiting() -> bool { false }
/// loop {
/// if waiting() {
/// continue;
/// } else {
/// // Do something useful
/// }
/// # break;
/// }
/// ```
/// Could be rewritten as
///
/// ```rust
/// # fn waiting() -> bool { false }
/// loop {
/// if waiting() {
/// continue;
/// }
/// // Do something useful
/// # break;
/// }
/// ```
pub NEEDLESS_CONTINUE,
pedantic,
"`continue` statements that can be replaced by a rearrangement of code"
}
declare_lint_pass!(NeedlessContinue => [NEEDLESS_CONTINUE]);
impl EarlyLintPass for NeedlessContinue {
fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr) {
if !expr.span.from_expansion() {
check_and_warn(cx, expr);
}
}
}
/* This lint has to mainly deal with two cases of needless continue
* statements. */
// Case 1 [Continue inside else block]:
//
// loop {
// // region A
// if cond {
// // region B
// } else {
// continue;
// }
// // region C
// }
//
// This code can better be written as follows:
//
// loop {
// // region A
// if cond {
// // region B
// // region C
// }
// }
//
// Case 2 [Continue inside then block]:
//
// loop {
// // region A
// if cond {
// continue;
// // potentially more code here.
// } else {
// // region B
// }
// // region C
// }
//
//
// This snippet can be refactored to:
//
// loop {
// // region A
// if !cond {
// // region B
// // region C
// }
// }
//
/// Given an expression, returns true if either of the following is true
///
/// - The expression is a `continue` node.
/// - The expression node is a block with the first statement being a
/// `continue`.
fn needless_continue_in_else(else_expr: &ast::Expr, label: Option<&ast::Label>) -> bool {
match else_expr.kind {
ast::ExprKind::Block(ref else_block, _) => is_first_block_stmt_continue(else_block, label),
ast::ExprKind::Continue(l) => compare_labels(label, l.as_ref()),
_ => false,
}
}
fn is_first_block_stmt_continue(block: &ast::Block, label: Option<&ast::Label>) -> bool {
block.stmts.get(0).map_or(false, |stmt| match stmt.kind {
ast::StmtKind::Semi(ref e) | ast::StmtKind::Expr(ref e) => {
if let ast::ExprKind::Continue(ref l) = e.kind {
compare_labels(label, l.as_ref())
} else {
false
}
},
_ => false,
})
}
/// If the `continue` has a label, check it matches the label of the loop.
fn compare_labels(loop_label: Option<&ast::Label>, continue_label: Option<&ast::Label>) -> bool {
match (loop_label, continue_label) {
// `loop { continue; }` or `'a loop { continue; }`
(_, None) => true,
// `loop { continue 'a; }`
(None, _) => false,
// `'a loop { continue 'a; }` or `'a loop { continue 'b; }`
(Some(x), Some(y)) => x.ident == y.ident,
}
}
/// If `expr` is a loop expression (while/while let/for/loop), calls `func` with
/// the AST object representing the loop block of `expr`.
fn with_loop_block<F>(expr: &ast::Expr, mut func: F)
where
F: FnMut(&ast::Block, Option<&ast::Label>),
{
if let ast::ExprKind::While(_, loop_block, label)
| ast::ExprKind::ForLoop(_, _, loop_block, label)
| ast::ExprKind::Loop(loop_block, label) = &expr.kind
{
func(loop_block, label.as_ref());
}
}
/// If `stmt` is an if expression node with an `else` branch, calls func with
/// the
/// following:
///
/// - The `if` expression itself,
/// - The `if` condition expression,
/// - The `then` block, and
/// - The `else` expression.
fn with_if_expr<F>(stmt: &ast::Stmt, mut func: F)
where
F: FnMut(&ast::Expr, &ast::Expr, &ast::Block, &ast::Expr),
{
match stmt.kind {
ast::StmtKind::Semi(ref e) | ast::StmtKind::Expr(ref e) => {
if let ast::ExprKind::If(ref cond, ref if_block, Some(ref else_expr)) = e.kind {
func(e, cond, if_block, else_expr);
}
},
_ => {},
}
}
/// A type to distinguish between the two distinct cases this lint handles.
#[derive(Copy, Clone, Debug)]
enum LintType {
ContinueInsideElseBlock,
ContinueInsideThenBlock,
}
/// Data we pass around for construction of help messages.
struct LintData<'a> {
/// The `if` expression encountered in the above loop.
if_expr: &'a ast::Expr,
/// The condition expression for the above `if`.
if_cond: &'a ast::Expr,
/// The `then` block of the `if` statement.
if_block: &'a ast::Block,
/// The `else` block of the `if` statement.
/// Note that we only work with `if` exprs that have an `else` branch.
else_expr: &'a ast::Expr,
/// The 0-based index of the `if` statement in the containing loop block.
stmt_idx: usize,
/// The statements of the loop block.
block_stmts: &'a [ast::Stmt],
}
const MSG_REDUNDANT_ELSE_BLOCK: &str = "this `else` block is redundant";
const MSG_ELSE_BLOCK_NOT_NEEDED: &str = "there is no need for an explicit `else` block for this `if` \
expression";
const DROP_ELSE_BLOCK_AND_MERGE_MSG: &str = "consider dropping the `else` clause and merging the code that \
follows (in the loop) with the `if` block";
const DROP_ELSE_BLOCK_MSG: &str = "consider dropping the `else` clause";
fn emit_warning<'a>(cx: &EarlyContext<'_>, data: &'a LintData<'_>, header: &str, typ: LintType) {
// snip is the whole *help* message that appears after the warning.
// message is the warning message.
// expr is the expression which the lint warning message refers to.
let (snip, message, expr) = match typ {
LintType::ContinueInsideElseBlock => (
suggestion_snippet_for_continue_inside_else(cx, data),
MSG_REDUNDANT_ELSE_BLOCK,
data.else_expr,
),
LintType::ContinueInsideThenBlock => (
suggestion_snippet_for_continue_inside_if(cx, data),
MSG_ELSE_BLOCK_NOT_NEEDED,
data.if_expr,
),
};
span_lint_and_help(
cx,
NEEDLESS_CONTINUE,
expr.span,
message,
None,
&format!("{}\n{}", header, snip),
);
}
fn suggestion_snippet_for_continue_inside_if<'a>(cx: &EarlyContext<'_>, data: &'a LintData<'_>) -> String {
let cond_code = snippet(cx, data.if_cond.span, "..");
let continue_code = snippet_block(cx, data.if_block.span, "..", Some(data.if_expr.span));
let else_code = snippet_block(cx, data.else_expr.span, "..", Some(data.if_expr.span));
let indent_if = indent_of(cx, data.if_expr.span).unwrap_or(0);
format!(
"{indent}if {} {}\n{indent}{}",
cond_code,
continue_code,
else_code,
indent = " ".repeat(indent_if),
)
}
fn suggestion_snippet_for_continue_inside_else<'a>(cx: &EarlyContext<'_>, data: &'a LintData<'_>) -> String {
let cond_code = snippet(cx, data.if_cond.span, "..");
// Region B
let block_code = erode_from_back(&snippet_block(cx, data.if_block.span, "..", Some(data.if_expr.span)));
// Region C
// These is the code in the loop block that follows the if/else construction
// we are complaining about. We want to pull all of this code into the
// `then` block of the `if` statement.
let indent = span_of_first_expr_in_block(data.if_block)
.and_then(|span| indent_of(cx, span))
.unwrap_or(0);
let to_annex = data.block_stmts[data.stmt_idx + 1..]
.iter()
.map(|stmt| original_sp(stmt.span, DUMMY_SP))
.map(|span| {
let snip = snippet_block(cx, span, "..", None).into_owned();
snip.lines()
.map(|line| format!("{}{}", " ".repeat(indent), line))
.collect::<Vec<_>>()
.join("\n")
})
.collect::<Vec<_>>()
.join("\n");
let indent_if = indent_of(cx, data.if_expr.span).unwrap_or(0);
format!(
"{indent_if}if {} {}\n{indent}// merged code follows:\n{}\n{indent_if}}}",
cond_code,
block_code,
to_annex,
indent = " ".repeat(indent),
indent_if = " ".repeat(indent_if),
)
}
fn check_and_warn<'a>(cx: &EarlyContext<'_>, expr: &'a ast::Expr) {
with_loop_block(expr, |loop_block, label| {
for (i, stmt) in loop_block.stmts.iter().enumerate() {
with_if_expr(stmt, |if_expr, cond, then_block, else_expr| {
let data = &LintData {
stmt_idx: i,
if_expr,
if_cond: cond,
if_block: then_block,
else_expr,
block_stmts: &loop_block.stmts,
};
if needless_continue_in_else(else_expr, label) {
emit_warning(
cx,
data,
DROP_ELSE_BLOCK_AND_MERGE_MSG,
LintType::ContinueInsideElseBlock,
);
} else if is_first_block_stmt_continue(then_block, label) {
emit_warning(cx, data, DROP_ELSE_BLOCK_MSG, LintType::ContinueInsideThenBlock);
}
});
}
});
}
/// Eats at `s` from the end till a closing brace `}` is encountered, and then continues eating
/// till a non-whitespace character is found. e.g., the string. If no closing `}` is present, the
/// string will be preserved.
///
/// ```rust
/// {
/// let x = 5;
/// }
/// ```
///
/// is transformed to
///
/// ```ignore
/// {
/// let x = 5;
/// ```
#[must_use]
fn erode_from_back(s: &str) -> String {
let mut ret = s.to_string();
while ret.pop().map_or(false, |c| c != '}') {}
while let Some(c) = ret.pop() {
if !c.is_whitespace() {
ret.push(c);
break;
}
}
if ret.is_empty() {
s.to_string()
} else {
ret
}
}
fn span_of_first_expr_in_block(block: &ast::Block) -> Option<Span> {
block.stmts.get(0).map(|stmt| stmt.span)
}
#[cfg(test)]
mod test {
use super::erode_from_back;
#[test]
#[rustfmt::skip]
fn test_erode_from_back() {
let input = "\
{
let x = 5;
let y = format!(\"{}\", 42);
}";
let expected = "\
{
let x = 5;
let y = format!(\"{}\", 42);";
let got = erode_from_back(input);
assert_eq!(expected, got);
}
#[test]
#[rustfmt::skip]
fn test_erode_from_back_no_brace() {
let input = "\
let x = 5;
let y = something();
";
let expected = input;
let got = erode_from_back(input);
assert_eq!(expected, got);
}
}