rust/clippy_lints/src/let_if_seq.rs

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use crate::utils::{higher, qpath_res, snippet, span_lint_and_then};
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use if_chain::if_chain;
use rustc_errors::Applicability;
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use rustc_hir as hir;
use rustc_hir::def::Res;
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use rustc_hir::intravisit;
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use rustc_hir::BindingAnnotation;
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use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::hir::map::Map;
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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declare_clippy_lint! {
/// **What it does:** Checks for variable declarations immediately followed by a
/// conditional affectation.
///
/// **Why is this bad?** This is not idiomatic Rust.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust,ignore
/// let foo;
///
/// if bar() {
/// foo = 42;
/// } else {
/// foo = 0;
/// }
///
/// let mut baz = None;
///
/// if bar() {
/// baz = Some(42);
/// }
/// ```
///
/// should be written
///
/// ```rust,ignore
/// let foo = if bar() {
/// 42
/// } else {
/// 0
/// };
///
/// let baz = if bar() {
/// Some(42)
/// } else {
/// None
/// };
/// ```
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pub USELESS_LET_IF_SEQ,
nursery,
"unidiomatic `let mut` declaration followed by initialization in `if`"
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}
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declare_lint_pass!(LetIfSeq => [USELESS_LET_IF_SEQ]);
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetIfSeq {
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fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx hir::Block<'_>) {
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let mut it = block.stmts.iter().peekable();
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while let Some(stmt) = it.next() {
if_chain! {
if let Some(expr) = it.peek();
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if let hir::StmtKind::Local(ref local) = stmt.kind;
if let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind;
if let hir::StmtKind::Expr(ref if_) = expr.kind;
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if let Some((ref cond, ref then, ref else_)) = higher::if_block(&if_);
if !used_in_expr(cx, canonical_id, cond);
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if let hir::ExprKind::Block(ref then, _) = then.kind;
if let Some(value) = check_assign(cx, canonical_id, &*then);
if !used_in_expr(cx, canonical_id, value);
then {
let span = stmt.span.to(if_.span);
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let has_interior_mutability = !cx.tables.node_type(canonical_id).is_freeze(
cx.tcx.at(span),
cx.param_env,
);
if has_interior_mutability { return; }
let (default_multi_stmts, default) = if let Some(ref else_) = *else_ {
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if let hir::ExprKind::Block(ref else_, _) = else_.kind {
if let Some(default) = check_assign(cx, canonical_id, else_) {
(else_.stmts.len() > 1, default)
} else if let Some(ref default) = local.init {
(true, &**default)
} else {
continue;
}
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} else {
continue;
}
} else if let Some(ref default) = local.init {
(false, &**default)
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} else {
continue;
};
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let mutability = match mode {
BindingAnnotation::RefMut | BindingAnnotation::Mutable => "<mut> ",
_ => "",
};
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// FIXME: this should not suggest `mut` if we can detect that the variable is not
// use mutably after the `if`
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let sug = format!(
"let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
mut=mutability,
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name=ident.name,
cond=snippet(cx, cond.span, "_"),
then=if then.stmts.len() > 1 { " ..;" } else { "" },
else=if default_multi_stmts { " ..;" } else { "" },
value=snippet(cx, value.span, "<value>"),
default=snippet(cx, default.span, "<default>"),
);
span_lint_and_then(cx,
USELESS_LET_IF_SEQ,
span,
"`if _ { .. } else { .. }` is an expression",
|diag| {
diag.span_suggestion(
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span,
"it is more idiomatic to write",
sug,
Applicability::HasPlaceholders,
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);
if !mutability.is_empty() {
diag.note("you might not need `mut` at all");
}
});
}
}
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}
}
}
struct UsedVisitor<'a, 'tcx> {
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cx: &'a LateContext<'a, 'tcx>,
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id: hir::HirId,
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used: bool,
}
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impl<'a, 'tcx> intravisit::Visitor<'tcx> for UsedVisitor<'a, 'tcx> {
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type Map = Map<'tcx>;
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fn visit_expr(&mut self, expr: &'tcx hir::Expr<'_>) {
if_chain! {
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if let hir::ExprKind::Path(ref qpath) = expr.kind;
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if let Res::Local(local_id) = qpath_res(self.cx, qpath, expr.hir_id);
if self.id == local_id;
then {
self.used = true;
return;
}
}
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intravisit::walk_expr(self, expr);
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}
fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
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intravisit::NestedVisitorMap::None
}
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}
fn check_assign<'a, 'tcx>(
cx: &LateContext<'a, 'tcx>,
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decl: hir::HirId,
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block: &'tcx hir::Block<'_>,
) -> Option<&'tcx hir::Expr<'tcx>> {
if_chain! {
if block.expr.is_none();
if let Some(expr) = block.stmts.iter().last();
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if let hir::StmtKind::Semi(ref expr) = expr.kind;
if let hir::ExprKind::Assign(ref var, ref value, _) = expr.kind;
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if let hir::ExprKind::Path(ref qpath) = var.kind;
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if let Res::Local(local_id) = qpath_res(cx, qpath, var.hir_id);
if decl == local_id;
then {
let mut v = UsedVisitor {
cx,
id: decl,
used: false,
};
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for s in block.stmts.iter().take(block.stmts.len()-1) {
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intravisit::walk_stmt(&mut v, s);
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if v.used {
return None;
}
}
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return Some(value);
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}
}
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None
}
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fn used_in_expr<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, id: hir::HirId, expr: &'tcx hir::Expr<'_>) -> bool {
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let mut v = UsedVisitor { cx, id, used: false };
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intravisit::walk_expr(&mut v, expr);
v.used
}