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//! Checks for needless boolean results of if-else expressions
//!
//! This lint is **warn** by default
use rustc::lint::*;
use rustc_front::hir::*;
use syntax::ast::LitKind;
use syntax::codemap::Spanned;
use utils::{span_lint, span_lint_and_then, snippet};
/// **What it does:** This lint checks for expressions of the form `if c { true } else { false }` (or vice versa) and suggest using the condition directly.
///
/// **Why is this bad?** Redundant code.
///
/// **Known problems:** Maybe false positives: Sometimes, the two branches are painstakingly documented (which we of course do not detect), so they *may* have some value. Even then, the documentation can be rewritten to match the shorter code.
///
/// **Example:** `if x { false } else { true }`
declare_lint! {
pub NEEDLESS_BOOL,
Warn,
"if-statements with plain booleans in the then- and else-clause, e.g. \
`if p { true } else { false }`"
}
/// **What it does:** This lint checks for expressions of the form `x == true` (or vice versa) and suggest using the variable directly.
///
/// **Why is this bad?** Unnecessary code.
///
/// **Known problems:** None.
///
/// **Example:** `if x == true { }` could be `if x { }`
declare_lint! {
pub BOOL_COMPARISON,
Warn,
"comparing a variable to a boolean, e.g. \
`if x == true`"
}
#[derive(Copy,Clone)]
pub struct NeedlessBool;
impl LintPass for NeedlessBool {
fn get_lints(&self) -> LintArray {
lint_array!(NEEDLESS_BOOL)
}
}
impl LateLintPass for NeedlessBool {
fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
if let ExprIf(ref pred, ref then_block, Some(ref else_expr)) = e.node {
match (fetch_bool_block(then_block), fetch_bool_expr(else_expr)) {
(Some(true), Some(true)) => {
span_lint(cx,
NEEDLESS_BOOL,
e.span,
"this if-then-else expression will always return true");
}
(Some(false), Some(false)) => {
span_lint(cx,
NEEDLESS_BOOL,
e.span,
"this if-then-else expression will always return false");
}
(Some(true), Some(false)) => {
let pred_snip = snippet(cx, pred.span, "..");
let hint = if pred_snip == ".." {
"its predicate".into()
} else {
format!("`{}`", pred_snip)
};
span_lint(cx,
NEEDLESS_BOOL,
e.span,
&format!("you can reduce this if-then-else expression to just {}", hint));
}
(Some(false), Some(true)) => {
let pred_snip = snippet(cx, pred.span, "..");
let hint = if pred_snip == ".." {
"`!` and its predicate".into()
} else {
format!("`!{}`", pred_snip)
};
span_lint(cx,
NEEDLESS_BOOL,
e.span,
&format!("you can reduce this if-then-else expression to just {}", hint));
}
_ => (),
}
}
}
}
#[derive(Copy,Clone)]
pub struct BoolComparison;
impl LintPass for BoolComparison {
fn get_lints(&self) -> LintArray {
lint_array!(BOOL_COMPARISON)
}
}
impl LateLintPass for BoolComparison {
fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
if let ExprBinary(Spanned{ node: BiEq, .. }, ref left_side, ref right_side) = e.node {
match (fetch_bool_expr(left_side), fetch_bool_expr(right_side)) {
(Some(true), None) => {
let hint = snippet(cx, right_side.span, "..").into_owned();
span_lint_and_then(cx,
BOOL_COMPARISON,
e.span,
"equality checks against true are unnecesary",
|db| {
db.span_suggestion(e.span, "try simplifying it as shown:", hint);
});
}
(None, Some(true)) => {
let hint = snippet(cx, left_side.span, "..").into_owned();
span_lint_and_then(cx,
BOOL_COMPARISON,
e.span,
"equality checks against true are unnecesary",
|db| {
db.span_suggestion(e.span, "try simplifying it as shown:", hint);
});
}
(Some(false), None) => {
let hint = format!("!{}", snippet(cx, right_side.span, ".."));
span_lint_and_then(cx,
BOOL_COMPARISON,
e.span,
"equality checks against false can be replaced by a negation",
|db| {
db.span_suggestion(e.span, "try simplifying it as shown:", hint);
});
}
(None, Some(false)) => {
let hint = format!("!{}", snippet(cx, left_side.span, ".."));
span_lint_and_then(cx,
BOOL_COMPARISON,
e.span,
"equality checks against false can be replaced by a negation",
|db| {
db.span_suggestion(e.span, "try simplifying it as shown:", hint);
});
}
_ => (),
}
}
}
}
fn fetch_bool_block(block: &Block) -> Option<bool> {
if block.stmts.is_empty() {
block.expr.as_ref().and_then(|e| fetch_bool_expr(e))
} else {
None
}
}
fn fetch_bool_expr(expr: &Expr) -> Option<bool> {
match expr.node {
ExprBlock(ref block) => fetch_bool_block(block),
ExprLit(ref lit_ptr) => {
if let LitKind::Bool(value) = lit_ptr.node {
Some(value)
} else {
None
}
}
_ => None,
}
}