rust-clippy/clippy_lints/src/collapsible_if.rs

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// Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! Checks for if expressions that contain only an if expression.
//!
//! For example, the lint would catch:
//!
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//! ```rust,ignore
//! if x {
//! if y {
//! println!("Hello world");
//! }
//! }
//! ```
//!
//! This lint is **warn** by default
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use if_chain::if_chain;
use rustc::lint::{EarlyContext, EarlyLintPass, LintArray, LintPass};
use rustc::{declare_tool_lint, lint_array};
use syntax::ast;
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use crate::utils::sugg::Sugg;
use crate::utils::{in_macro, snippet_block, snippet_block_with_applicability, span_lint_and_sugg, span_lint_and_then};
use rustc_errors::Applicability;
/// **What it does:** Checks for nested `if` statements which can be collapsed
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/// by `&&`-combining their conditions and for `else { if ... }` expressions
/// that
/// can be collapsed to `else if ...`.
///
/// **Why is this bad?** Each `if`-statement adds one level of nesting, which
/// makes code look more complex than it really is.
///
/// **Known problems:** None.
///
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/// **Example:**
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/// ```rust,ignore
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/// if x {
/// if y {
/// …
/// }
/// }
///
/// // or
///
/// if x {
/// …
/// } else {
/// if y {
/// …
/// }
/// }
/// ```
///
/// Should be written:
///
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/// ```rust.ignore
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/// if x && y {
/// …
/// }
///
/// // or
///
/// if x {
/// …
/// } else if y {
/// …
/// }
/// ```
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declare_clippy_lint! {
pub COLLAPSIBLE_IF,
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style,
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"`if`s that can be collapsed (e.g. `if x { if y { ... } }` and `else { if x { ... } }`)"
}
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#[derive(Copy, Clone)]
pub struct CollapsibleIf;
impl LintPass for CollapsibleIf {
fn get_lints(&self) -> LintArray {
lint_array!(COLLAPSIBLE_IF)
}
}
impl EarlyLintPass for CollapsibleIf {
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fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr) {
if !in_macro(expr.span) {
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check_if(cx, expr)
}
}
}
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fn check_if(cx: &EarlyContext<'_>, expr: &ast::Expr) {
match expr.node {
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ast::ExprKind::If(ref check, ref then, ref else_) => {
if let Some(ref else_) = *else_ {
check_collapsible_maybe_if_let(cx, else_);
} else {
check_collapsible_no_if_let(cx, expr, check, then);
}
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},
ast::ExprKind::IfLet(_, _, _, Some(ref else_)) => {
check_collapsible_maybe_if_let(cx, else_);
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},
_ => (),
}
}
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fn block_starts_with_comment(cx: &EarlyContext<'_>, expr: &ast::Block) -> bool {
// We trim all opening braces and whitespaces and then check if the next string is a comment.
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let trimmed_block_text = snippet_block(cx, expr.span, "..")
.trim_start_matches(|c: char| c.is_whitespace() || c == '{')
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.to_owned();
trimmed_block_text.starts_with("//") || trimmed_block_text.starts_with("/*")
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}
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fn check_collapsible_maybe_if_let(cx: &EarlyContext<'_>, else_: &ast::Expr) {
if_chain! {
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if let ast::ExprKind::Block(ref block, _) = else_.node;
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if !block_starts_with_comment(cx, block);
if let Some(else_) = expr_block(block);
if !in_macro(else_.span);
then {
match else_.node {
ast::ExprKind::If(..) | ast::ExprKind::IfLet(..) => {
let mut applicability = Applicability::MachineApplicable;
span_lint_and_sugg(
cx,
COLLAPSIBLE_IF,
block.span,
"this `else { if .. }` block can be collapsed",
"try",
snippet_block_with_applicability(cx, else_.span, "..", &mut applicability).into_owned(),
applicability,
);
}
_ => (),
}
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}
}
}
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fn check_collapsible_no_if_let(cx: &EarlyContext<'_>, expr: &ast::Expr, check: &ast::Expr, then: &ast::Block) {
if_chain! {
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if !block_starts_with_comment(cx, then);
if let Some(inner) = expr_block(then);
if let ast::ExprKind::If(ref check_inner, ref content, None) = inner.node;
then {
if expr.span.ctxt() != inner.span.ctxt() {
return;
}
span_lint_and_then(cx, COLLAPSIBLE_IF, expr.span, "this if statement can be collapsed", |db| {
let lhs = Sugg::ast(cx, check, "..");
let rhs = Sugg::ast(cx, check_inner, "..");
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db.span_suggestion_with_applicability(
expr.span,
"try",
format!(
"if {} {}",
lhs.and(&rhs),
snippet_block(cx, content.span, ".."),
),
Applicability::MachineApplicable, // snippet
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);
});
}
}
}
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/// If the block contains only one expression, return it.
fn expr_block(block: &ast::Block) -> Option<&ast::Expr> {
let mut it = block.stmts.iter();
if let (Some(stmt), None) = (it.next(), it.next()) {
match stmt.node {
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ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => Some(expr),
_ => None,
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}
} else {
None
}
}