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Needless continue: This is a complete rewrite of this lint as an early-pass lint.

Browse Source 
The previous version had troubles computing suggestions without macro expansions
creeping in. This fixes it by using original_sp, which works on AST nodes.
This commit is contained in:
Yati Sagade 2016-08-21 23:18:58 +02:00
parent 0432f1efd3
commit 240471f926
3 changed files with 472 additions and 0 deletions
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1
CHANGELOG.md
View File

@ -405,6 +405,7 @@ All notable changes to this project will be documented in this file.
[`mutex_integer`]: https://github.com/Manishearth/rust-clippy/wiki#mutex_integer
[`needless_bool`]: https://github.com/Manishearth/rust-clippy/wiki#needless_bool
[`needless_borrow`]: https://github.com/Manishearth/rust-clippy/wiki#needless_borrow
[`needless_continue`]: https://github.com/Manishearth/rust-clippy/wiki#needless_continue
[`needless_lifetimes`]: https://github.com/Manishearth/rust-clippy/wiki#needless_lifetimes
[`needless_pass_by_value`]: https://github.com/Manishearth/rust-clippy/wiki#needless_pass_by_value
[`needless_range_loop`]: https://github.com/Manishearth/rust-clippy/wiki#needless_range_loop

3
clippy_lints/src/lib.rs
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@ -107,6 +107,7 @@ pub mod mutex_atomic;
pub mod needless_bool;
pub mod needless_borrow;
pub mod needless_pass_by_value;
pub mod needless_continue;
pub mod needless_update;
pub mod neg_multiply;
pub mod new_without_default;
@ -219,6 +220,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
reg.register_late_lint_pass(box approx_const::Pass);
reg.register_late_lint_pass(box misc::Pass);
reg.register_early_lint_pass(box precedence::Precedence);
reg.register_early_lint_pass(box needless_continue::NeedlessContinue);
reg.register_late_lint_pass(box eta_reduction::EtaPass);
reg.register_late_lint_pass(box identity_op::IdentityOp);
reg.register_early_lint_pass(box items_after_statements::ItemsAfterStatements);
@ -461,6 +463,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
needless_bool::NEEDLESS_BOOL,
needless_borrow::NEEDLESS_BORROW,
needless_pass_by_value::NEEDLESS_PASS_BY_VALUE,
needless_continue::NEEDLESS_CONTINUE,
needless_update::NEEDLESS_UPDATE,
neg_multiply::NEG_MULTIPLY,
new_without_default::NEW_WITHOUT_DEFAULT,

468
clippy_lints/src/needless_continue.rs Normal file
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@ -0,0 +1,468 @@
//! Checks for continue statements in loops that are redundant.
//!
//! For example, the lint would catch
//!
//! ```
//! while condition() {
//! update_condition();
//! if x {
//! // ...
//! } else {
//! continue;
//! }
//! println!("Hello, world");
//! }
//! ```
//!
//! And suggest something like this:
//!
//! ```
//! while condition() {
//! update_condition();
//! if x {
//! // ...
//! println!("Hello, world");
//! }
//! }
//! ```
//!
//! This lint is **warn** by default.
use std;
use rustc::lint::*;
use syntax::ast;
use syntax::codemap::{original_sp,DUMMY_SP};
use utils::{in_macro, span_help_and_lint, snippet_block, snippet};
use self::LintType::*;
/// **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
/// while condition() {
/// update_condition();
/// if x {
/// // ...
/// } else {
/// continue;
/// }
/// println!("Hello, world");
/// }
/// ```
///
/// Could be rewritten as
///
/// ```rust
/// while condition() {
/// update_condition();
/// if x {
/// // ...
/// println!("Hello, world");
/// }
/// }
/// ```
///
/// As another example, the following code
///
/// ```rust
/// loop {
/// if waiting() {
/// continue;
/// } else {
/// // Do something useful
/// }
/// }
/// ```
/// Could be rewritten as
///
/// ```rust
/// loop {
/// if waiting() {
/// continue;
/// }
/// // Do something useful
/// }
/// ```
declare_lint! {
pub NEEDLESS_CONTINUE,
Warn,
"`continue` statements that can be replaced by a rearrangement of code"
}
#[derive(Copy,Clone)]
pub struct NeedlessContinue;
impl LintPass for NeedlessContinue {
fn get_lints(&self) -> LintArray {
lint_array!(NEEDLESS_CONTINUE)
}
}
impl EarlyLintPass for NeedlessContinue {
fn check_expr(&mut self, ctx: &EarlyContext, expr: &ast::Expr) {
if !in_macro(ctx, expr.span) {
check_and_warn(ctx, expr);
}
}
}
/// Given an Expr, returns true if either of the following is true
///
/// - The Expr is a `continue` node.
/// - The Expr node is a block with the first statement being a `continue`.
///
fn needless_continue_in_else(else_expr: &ast::Expr) -> bool {
let mut found = false;
match else_expr.node {
ast::ExprKind::Block(ref else_block) => {
found = is_first_block_stmt_continue(else_block);
},
ast::ExprKind::Continue(_) => { found = true },
_ => { },
};
found
}
fn is_first_block_stmt_continue(block: &ast::Block) -> bool {
let mut ret = false;
block.stmts.get(0).map(|stmt| {
if_let_chain! {[
let ast::StmtKind::Semi(ref e) = stmt.node,
let ast::ExprKind::Continue(_) = e.node,
], {
ret = true;
}}
}).unwrap_or(());
ret
}
/// 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) {
match expr.node {
ast::ExprKind::While(_, ref loop_block, _) |
ast::ExprKind::WhileLet(_, _, ref loop_block, _) |
ast::ExprKind::ForLoop( _, _, ref loop_block, _) |
ast::ExprKind::Loop(ref loop_block, _) => func(loop_block),
_ => {},
}
}
/// If `stmt` is an if expression node with an else branch, calls func with the
/// following:
///
/// - The if Expr,
/// - The if condition Expr,
/// - The then block of this if Expr, and
/// - The else expr.
///
fn with_if_expr<F>(stmt: &ast::Stmt, mut func: F)
where F: FnMut(&ast::Expr, &ast::Expr, &ast::Block, &ast::Expr) {
match stmt.node {
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.node {
func(e, cond, if_block, else_expr);
}
},
_ => { },
}
}
/// A type to distinguish between the two distinct cases this lint handles.
enum LintType {
ContinueInsideElseBlock,
ContinueInsideThenBlock,
}
/// Data we pass around for construction of help messages.
struct LintData<'a> {
if_expr: &'a ast::Expr, // The `if` expr encountered in the above loop.
if_cond: &'a ast::Expr, // The condition expression for the above `if`.
if_block: &'a ast::Block, // The `then` block of the `if` statement.
else_expr: &'a ast::Expr, /* The `else` block of the `if` statement.
Note that we only work with `if` exprs that
have an `else` branch. */
stmt_idx: usize, /* The 0-based index of the `if` statement in
the containing loop block. */
block_stmts: &'a [ast::Stmt], // The statements of the loop block.
}
const MSG_REDUNDANT_ELSE_BLOCK: &'static str = "This else block is redundant.\n";
const MSG_ELSE_BLOCK_NOT_NEEDED: &'static str = "There is no need for an explicit `else` block for this `if` expression\n";
const DROP_ELSE_BLOCK_AND_MERGE_MSG: &'static str =
"Consider dropping the else clause and merging the code that follows (in the loop) with the if block, like so:\n";
const DROP_ELSE_BLOCK_MSG: &'static str =
"Consider dropping the else clause, and moving out the code in the else block, like so:\n";
fn emit_warning<'a>(ctx: &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 {
ContinueInsideElseBlock => {
(suggestion_snippet_for_continue_inside_else(ctx, data, header),
MSG_REDUNDANT_ELSE_BLOCK,
data.else_expr)
},
ContinueInsideThenBlock => {
(suggestion_snippet_for_continue_inside_if(ctx, data, header),
MSG_ELSE_BLOCK_NOT_NEEDED,
data.if_expr)
}
};
span_help_and_lint(ctx, NEEDLESS_CONTINUE, expr.span, message,
&format!("{}", snip));
}
fn suggestion_snippet_for_continue_inside_if<'a>(ctx: &EarlyContext,
data: &'a LintData,
header: &str) -> String {
let cond_code = &snippet(ctx, data.if_cond.span, "..").into_owned();
let if_code = &format!("if {} {{\n continue;\n}}\n", cond_code);
/* ^^^^--- Four spaces of indentation. */
// region B
let else_code = &snippet(ctx, data.else_expr.span, "..").into_owned();
let else_code = erode_block(&else_code);
let else_code = trim_indent(&else_code, false);
let mut ret = String::from(header);
ret.push_str(&if_code);
ret.push_str(&else_code);
ret.push_str("\n...");
ret
}
fn suggestion_snippet_for_continue_inside_else<'a>(ctx: &EarlyContext,
data: &'a LintData,
header: &str) -> String
{
let cond_code = &snippet(ctx, data.if_cond.span, "..").into_owned();
let mut if_code = format!("if {} {{\n", cond_code);
// Region B
let block_code = &snippet(ctx, data.if_block.span, "..").into_owned();
let block_code = erode_block(block_code);
let block_code = trim_indent(&block_code, false);
let block_code = left_pad_lines_with_spaces(&block_code, 4usize);
if_code.push_str(&block_code);
// 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 to_annex = data.block_stmts[data.stmt_idx+1..]
.iter()
.map(|stmt| {
original_sp(ctx.sess().codemap(), stmt.span, DUMMY_SP)
})
.map(|span| snippet_block(ctx, span, "..").into_owned())
.collect::<Vec<_>>().join("\n");
let mut ret = String::from(header);
ret.push_str(&align_snippets(&[&if_code,
"\n// Merged code follows...",
&to_annex]));
ret.push_str("\n}\n");
ret
}
fn check_and_warn<'a>(ctx: &EarlyContext, expr: &'a ast::Expr) {
with_loop_block(expr, |loop_block| {
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_expr,
if_cond: cond,
if_block: then_block,
else_expr: else_expr,
block_stmts: &loop_block.stmts,
};
if needless_continue_in_else(else_expr) {
emit_warning(ctx, data, DROP_ELSE_BLOCK_AND_MERGE_MSG, ContinueInsideElseBlock);
} else if is_first_block_stmt_continue(then_block) {
emit_warning(ctx, data, DROP_ELSE_BLOCK_MSG, 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
///
/// "
/// {
/// let x = 5;
/// }
/// "
///
/// is transformed to
///
/// "
/// {
/// let x = 5;"
///
fn erode_from_back(s: &str) -> String {
let mut ret = String::from(s);
while ret.pop().map_or(false, |c| c != '}') { }
while let Some(c) = ret.pop() {
if !c.is_whitespace() {
ret.push(c);
break;
}
}
ret
}
fn erode_from_front(s: &str) -> String {
s.chars()
.skip_while(|c| c.is_whitespace())
.skip_while(|c| *c == '{')
.skip_while(|c| *c == '\n')
.collect::<String>()
}
fn erode_block(s: &str) -> String {
erode_from_back(&erode_from_front(s))
}
fn is_all_whitespace(s: &str) -> bool { s.chars().all(|c| c.is_whitespace()) }
/// Returns true if a string is empty or just spaces.
fn is_null(s: &str) -> bool { s.is_empty() || is_all_whitespace(s) }
/// Returns the indentation level of a string. It just returns the count of
/// whitespace characters in the string before a non-whitespace character
/// is encountered.
fn indent_level(s: &str) -> usize {
s.chars()
.enumerate()
.find(|&(_, c)| !c.is_whitespace())
.map_or(0usize, |(i, _)| i)
}
/// Trims indentation from a snippet such that the line with the minimum
/// indentation has no indentation after the trasformation.
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
}
}
}