2021-12-06 11:33:31 +00:00
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use clippy_utils::diagnostics::span_lint_and_then;
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use clippy_utils::path_to_local;
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use clippy_utils::source::snippet_opt;
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use clippy_utils::visitors::{expr_visitor, is_local_used};
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use rustc_errors::Applicability;
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use rustc_hir::intravisit::Visitor;
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use rustc_hir::{Block, Expr, ExprKind, HirId, Local, LocalSource, MatchSource, Node, Pat, PatKind, Stmt, StmtKind};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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use rustc_span::Span;
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declare_clippy_lint! {
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/// ### What it does
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/// Checks for late initializations that can be replaced by a `let` statement
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/// with an initializer.
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///
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/// ### Why is this bad?
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/// Assigning in the `let` statement is less repetitive.
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///
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/// ### Example
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/// ```rust
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/// let a;
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/// a = 1;
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///
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/// let b;
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/// match 3 {
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/// 0 => b = "zero",
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/// 1 => b = "one",
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/// _ => b = "many",
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/// }
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///
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/// let c;
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/// if true {
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/// c = 1;
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/// } else {
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/// c = -1;
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/// }
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/// ```
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/// Use instead:
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/// ```rust
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/// let a = 1;
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///
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/// let b = match 3 {
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/// 0 => "zero",
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/// 1 => "one",
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/// _ => "many",
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/// };
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///
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/// let c = if true {
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/// 1
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/// } else {
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/// -1
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/// };
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/// ```
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#[clippy::version = "1.58.0"]
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pub NEEDLESS_LATE_INIT,
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style,
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"late initializations that can be replaced by a `let` statement with an initializer"
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}
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declare_lint_pass!(NeedlessLateInit => [NEEDLESS_LATE_INIT]);
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fn contains_assign_expr<'tcx>(cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'tcx>) -> bool {
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let mut seen = false;
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expr_visitor(cx, |expr| {
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if let ExprKind::Assign(..) = expr.kind {
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seen = true;
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}
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!seen
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})
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.visit_stmt(stmt);
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seen
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}
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2021-11-18 13:54:49 +00:00
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#[derive(Debug)]
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2021-12-06 11:33:31 +00:00
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struct LocalAssign {
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lhs_id: HirId,
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lhs_span: Span,
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rhs_span: Span,
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span: Span,
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}
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impl LocalAssign {
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fn from_expr(expr: &Expr<'_>, span: Span) -> Option<Self> {
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if let ExprKind::Assign(lhs, rhs, _) = expr.kind {
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if lhs.span.from_expansion() {
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return None;
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}
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Some(Self {
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lhs_id: path_to_local(lhs)?,
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lhs_span: lhs.span,
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rhs_span: rhs.span.source_callsite(),
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span,
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})
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} else {
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None
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}
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}
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fn new<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>, binding_id: HirId) -> Option<LocalAssign> {
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let assign = match expr.kind {
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ExprKind::Block(Block { expr: Some(expr), .. }, _) => Self::from_expr(expr, expr.span),
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ExprKind::Block(block, _) => {
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if_chain! {
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if let Some((last, other_stmts)) = block.stmts.split_last();
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if let StmtKind::Expr(expr) | StmtKind::Semi(expr) = last.kind;
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let assign = Self::from_expr(expr, last.span)?;
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// avoid visiting if not needed
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if assign.lhs_id == binding_id;
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if other_stmts.iter().all(|stmt| !contains_assign_expr(cx, stmt));
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then {
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Some(assign)
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} else {
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None
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}
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}
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},
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ExprKind::Assign(..) => Self::from_expr(expr, expr.span),
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_ => None,
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}?;
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if assign.lhs_id == binding_id {
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Some(assign)
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} else {
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None
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}
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}
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}
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fn assignment_suggestions<'tcx>(
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cx: &LateContext<'tcx>,
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binding_id: HirId,
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exprs: impl IntoIterator<Item = &'tcx Expr<'tcx>>,
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) -> Option<(Applicability, Vec<(Span, String)>)> {
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let mut assignments = Vec::new();
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for expr in exprs {
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let ty = cx.typeck_results().expr_ty(expr);
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if ty.is_never() {
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continue;
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}
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if !ty.is_unit() {
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return None;
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}
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let assign = LocalAssign::new(cx, expr, binding_id)?;
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assignments.push(assign);
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}
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2021-11-18 13:54:49 +00:00
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let suggestions = assignments
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2021-12-17 11:22:41 +00:00
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.iter()
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2021-11-16 22:06:25 +00:00
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.map(|assignment| Some((assignment.span.until(assignment.rhs_span), String::new())))
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2021-12-17 11:22:41 +00:00
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.chain(assignments.iter().map(|assignment| {
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Some((
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assignment.rhs_span.shrink_to_hi().with_hi(assignment.span.hi()),
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String::new(),
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))
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}))
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2021-12-06 11:33:31 +00:00
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.collect::<Option<Vec<(Span, String)>>>()?;
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let applicability = if suggestions.len() > 1 {
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// multiple suggestions don't work with rustfix in multipart_suggest
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// https://github.com/rust-lang/rustfix/issues/141
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Applicability::Unspecified
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} else {
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Applicability::MachineApplicable
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};
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Some((applicability, suggestions))
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}
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struct Usage<'tcx> {
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stmt: &'tcx Stmt<'tcx>,
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expr: &'tcx Expr<'tcx>,
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needs_semi: bool,
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}
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fn first_usage<'tcx>(
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cx: &LateContext<'tcx>,
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binding_id: HirId,
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local_stmt_id: HirId,
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block: &'tcx Block<'tcx>,
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) -> Option<Usage<'tcx>> {
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block
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.stmts
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.iter()
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.skip_while(|stmt| stmt.hir_id != local_stmt_id)
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.skip(1)
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.find(|&stmt| is_local_used(cx, stmt, binding_id))
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.and_then(|stmt| match stmt.kind {
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StmtKind::Expr(expr) => Some(Usage {
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stmt,
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expr,
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needs_semi: true,
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}),
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StmtKind::Semi(expr) => Some(Usage {
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stmt,
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expr,
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needs_semi: false,
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}),
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_ => None,
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})
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}
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fn local_snippet_without_semicolon(cx: &LateContext<'_>, local: &Local<'_>) -> Option<String> {
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let span = local.span.with_hi(match local.ty {
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// let <pat>: <ty>;
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// ~~~~~~~~~~~~~~~
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Some(ty) => ty.span.hi(),
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// let <pat>;
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// ~~~~~~~~~
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None => local.pat.span.hi(),
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});
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snippet_opt(cx, span)
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}
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fn check<'tcx>(
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cx: &LateContext<'tcx>,
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local: &'tcx Local<'tcx>,
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local_stmt: &'tcx Stmt<'tcx>,
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block: &'tcx Block<'tcx>,
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binding_id: HirId,
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) -> Option<()> {
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let usage = first_usage(cx, binding_id, local_stmt.hir_id, block)?;
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let binding_name = cx.tcx.hir().opt_name(binding_id)?;
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let let_snippet = local_snippet_without_semicolon(cx, local)?;
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match usage.expr.kind {
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ExprKind::Assign(..) => {
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let assign = LocalAssign::new(cx, usage.expr, binding_id)?;
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span_lint_and_then(
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cx,
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NEEDLESS_LATE_INIT,
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local_stmt.span,
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"unneeded late initalization",
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|diag| {
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diag.tool_only_span_suggestion(
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local_stmt.span,
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"remove the local",
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String::new(),
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Applicability::MachineApplicable,
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);
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diag.span_suggestion(
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assign.lhs_span,
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&format!("declare `{}` here", binding_name),
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let_snippet,
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Applicability::MachineApplicable,
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);
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},
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);
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},
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ExprKind::If(_, then_expr, Some(else_expr)) => {
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let (applicability, suggestions) = assignment_suggestions(cx, binding_id, [then_expr, else_expr])?;
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span_lint_and_then(
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cx,
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NEEDLESS_LATE_INIT,
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local_stmt.span,
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"unneeded late initalization",
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|diag| {
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diag.tool_only_span_suggestion(local_stmt.span, "remove the local", String::new(), applicability);
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diag.span_suggestion_verbose(
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usage.stmt.span.shrink_to_lo(),
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&format!("declare `{}` here", binding_name),
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format!("{} = ", let_snippet),
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applicability,
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);
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diag.multipart_suggestion("remove the assignments from the branches", suggestions, applicability);
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if usage.needs_semi {
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diag.span_suggestion(
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usage.stmt.span.shrink_to_hi(),
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"add a semicolon after the `if` expression",
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";".to_string(),
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applicability,
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);
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}
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},
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);
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},
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ExprKind::Match(_, arms, MatchSource::Normal) => {
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let (applicability, suggestions) = assignment_suggestions(cx, binding_id, arms.iter().map(|arm| arm.body))?;
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span_lint_and_then(
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cx,
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NEEDLESS_LATE_INIT,
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local_stmt.span,
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"unneeded late initalization",
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|diag| {
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diag.tool_only_span_suggestion(local_stmt.span, "remove the local", String::new(), applicability);
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diag.span_suggestion_verbose(
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usage.stmt.span.shrink_to_lo(),
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&format!("declare `{}` here", binding_name),
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format!("{} = ", let_snippet),
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applicability,
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);
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diag.multipart_suggestion(
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"remove the assignments from the `match` arms",
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suggestions,
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applicability,
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);
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if usage.needs_semi {
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diag.span_suggestion(
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usage.stmt.span.shrink_to_hi(),
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"add a semicolon after the `match` expression",
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";".to_string(),
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applicability,
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);
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}
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},
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);
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},
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_ => {},
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};
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Some(())
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}
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2022-01-11 15:52:23 +00:00
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impl<'tcx> LateLintPass<'tcx> for NeedlessLateInit {
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2021-12-06 11:33:31 +00:00
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fn check_local(&mut self, cx: &LateContext<'tcx>, local: &'tcx Local<'tcx>) {
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let mut parents = cx.tcx.hir().parent_iter(local.hir_id);
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if_chain! {
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if let Local {
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init: None,
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pat: &Pat {
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kind: PatKind::Binding(_, binding_id, _, None),
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..
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},
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source: LocalSource::Normal,
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..
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} = local;
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if let Some((_, Node::Stmt(local_stmt))) = parents.next();
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if let Some((_, Node::Block(block))) = parents.next();
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then {
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check(cx, local, local_stmt, block, binding_id);
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}
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}
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}
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}
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