src/tools/clippy/clippy_lints/src/loops/never_loop.rs

   1 use super::utils::make_iterator_snippet;
   2 use super::NEVER_LOOP;
   3 use clippy_utils::diagnostics::span_lint_and_then;
   4 use clippy_utils::higher::ForLoop;
   5 use clippy_utils::source::snippet;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{Block, Expr, ExprKind, HirId, InlineAsmOperand, Pat, Stmt, StmtKind};
   8 use rustc_lint::LateContext;
   9 use rustc_span::Span;
  10 use std::iter::{once, Iterator};
  11
  12 pub(super) fn check(
  13     cx: &LateContext<'_>,
  14     block: &Block<'_>,
  15     loop_id: HirId,
  16     span: Span,
  17     for_loop: Option<&ForLoop<'_>>,
  18 ) {
  19     match never_loop_block(block, loop_id) {
  20         NeverLoopResult::AlwaysBreak => {
  21             span_lint_and_then(cx, NEVER_LOOP, span, "this loop never actually loops", |diag| {
  22                 if let Some(ForLoop {
  23                     arg: iterator,
  24                     pat,
  25                     span: for_span,
  26                     ..
  27                 }) = for_loop
  28                 {
  29                     // Suggests using an `if let` instead. This is `Unspecified` because the
  30                     // loop may (probably) contain `break` statements which would be invalid
  31                     // in an `if let`.
  32                     diag.span_suggestion_verbose(
  33                         for_span.with_hi(iterator.span.hi()),
  34                         "if you need the first element of the iterator, try writing",
  35                         for_to_if_let_sugg(cx, iterator, pat),
  36                         Applicability::Unspecified,
  37                     );
  38                 }
  39             });
  40         },
  41         NeverLoopResult::MayContinueMainLoop | NeverLoopResult::Otherwise => (),
  42     }
  43 }
  44
  45 #[derive(Copy, Clone)]
  46 enum NeverLoopResult {
  47     // A break/return always get triggered but not necessarily for the main loop.
  48     AlwaysBreak,
  49     // A continue may occur for the main loop.
  50     MayContinueMainLoop,
  51     Otherwise,
  52 }
  53
  54 #[must_use]
  55 fn absorb_break(arg: NeverLoopResult) -> NeverLoopResult {
  56     match arg {
  57         NeverLoopResult::AlwaysBreak | NeverLoopResult::Otherwise => NeverLoopResult::Otherwise,
  58         NeverLoopResult::MayContinueMainLoop => NeverLoopResult::MayContinueMainLoop,
  59     }
  60 }
  61
  62 // Combine two results for parts that are called in order.
  63 #[must_use]
  64 fn combine_seq(first: NeverLoopResult, second: NeverLoopResult) -> NeverLoopResult {
  65     match first {
  66         NeverLoopResult::AlwaysBreak | NeverLoopResult::MayContinueMainLoop => first,
  67         NeverLoopResult::Otherwise => second,
  68     }
  69 }
  70
  71 // Combine two results where both parts are called but not necessarily in order.
  72 #[must_use]
  73 fn combine_both(left: NeverLoopResult, right: NeverLoopResult) -> NeverLoopResult {
  74     match (left, right) {
  75         (NeverLoopResult::MayContinueMainLoop, _) | (_, NeverLoopResult::MayContinueMainLoop) => {
  76             NeverLoopResult::MayContinueMainLoop
  77         },
  78         (NeverLoopResult::AlwaysBreak, _) | (_, NeverLoopResult::AlwaysBreak) => NeverLoopResult::AlwaysBreak,
  79         (NeverLoopResult::Otherwise, NeverLoopResult::Otherwise) => NeverLoopResult::Otherwise,
  80     }
  81 }
  82
  83 // Combine two results where only one of the part may have been executed.
  84 #[must_use]
  85 fn combine_branches(b1: NeverLoopResult, b2: NeverLoopResult) -> NeverLoopResult {
  86     match (b1, b2) {
  87         (NeverLoopResult::AlwaysBreak, NeverLoopResult::AlwaysBreak) => NeverLoopResult::AlwaysBreak,
  88         (NeverLoopResult::MayContinueMainLoop, _) | (_, NeverLoopResult::MayContinueMainLoop) => {
  89             NeverLoopResult::MayContinueMainLoop
  90         },
  91         (NeverLoopResult::Otherwise, _) | (_, NeverLoopResult::Otherwise) => NeverLoopResult::Otherwise,
  92     }
  93 }
  94
  95 fn never_loop_block(block: &Block<'_>, main_loop_id: HirId) -> NeverLoopResult {
  96     let mut iter = block
  97         .stmts
  98         .iter()
  99         .filter_map(stmt_to_expr)
 100         .chain(block.expr.map(|expr| (expr, None)));
 101     never_loop_expr_seq(&mut iter, main_loop_id)
 102 }
 103
 104 fn never_loop_expr_seq<'a, T: Iterator<Item = (&'a Expr<'a>, Option<&'a Block<'a>>)>>(
 105     es: &mut T,
 106     main_loop_id: HirId,
 107 ) -> NeverLoopResult {
 108     es.map(|(e, els)| {
 109         let e = never_loop_expr(e, main_loop_id);
 110         els.map_or(e, |els| combine_branches(e, never_loop_block(els, main_loop_id)))
 111     })
 112     .fold(NeverLoopResult::Otherwise, combine_seq)
 113 }
 114
 115 fn stmt_to_expr<'tcx>(stmt: &Stmt<'tcx>) -> Option<(&'tcx Expr<'tcx>, Option<&'tcx Block<'tcx>>)> {
 116     match stmt.kind {
 117         StmtKind::Semi(e, ..) | StmtKind::Expr(e, ..) => Some((e, None)),
 118         StmtKind::Local(local) => local.init.map(|init| (init, local.els)),
 119         StmtKind::Item(..) => None,
 120     }
 121 }
 122
 123 fn never_loop_expr(expr: &Expr<'_>, main_loop_id: HirId) -> NeverLoopResult {
 124     match expr.kind {
 125         ExprKind::Box(e)
 126         | ExprKind::Unary(_, e)
 127         | ExprKind::Cast(e, _)
 128         | ExprKind::Type(e, _)
 129         | ExprKind::Field(e, _)
 130         | ExprKind::AddrOf(_, _, e)
 131         | ExprKind::Repeat(e, _)
 132         | ExprKind::DropTemps(e) => never_loop_expr(e, main_loop_id),
 133         ExprKind::Let(let_expr) => never_loop_expr(let_expr.init, main_loop_id),
 134         ExprKind::Array(es) | ExprKind::Tup(es) => never_loop_expr_all(&mut es.iter(), main_loop_id),
 135         ExprKind::MethodCall(_, receiver, es, _) => {
 136             never_loop_expr_all(&mut std::iter::once(receiver).chain(es.iter()), main_loop_id)
 137         },
 138         ExprKind::Struct(_, fields, base) => {
 139             let fields = never_loop_expr_all(&mut fields.iter().map(|f| f.expr), main_loop_id);
 140             if let Some(base) = base {
 141                 combine_both(fields, never_loop_expr(base, main_loop_id))
 142             } else {
 143                 fields
 144             }
 145         },
 146         ExprKind::Call(e, es) => never_loop_expr_all(&mut once(e).chain(es.iter()), main_loop_id),
 147         ExprKind::Binary(_, e1, e2)
 148         | ExprKind::Assign(e1, e2, _)
 149         | ExprKind::AssignOp(_, e1, e2)
 150         | ExprKind::Index(e1, e2) => never_loop_expr_all(&mut [e1, e2].iter().copied(), main_loop_id),
 151         ExprKind::Loop(b, _, _, _) => {
 152             // Break can come from the inner loop so remove them.
 153             absorb_break(never_loop_block(b, main_loop_id))
 154         },
 155         ExprKind::If(e, e2, e3) => {
 156             let e1 = never_loop_expr(e, main_loop_id);
 157             let e2 = never_loop_expr(e2, main_loop_id);
 158             let e3 = e3
 159                 .as_ref()
 160                 .map_or(NeverLoopResult::Otherwise, |e| never_loop_expr(e, main_loop_id));
 161             combine_seq(e1, combine_branches(e2, e3))
 162         },
 163         ExprKind::Match(e, arms, _) => {
 164             let e = never_loop_expr(e, main_loop_id);
 165             if arms.is_empty() {
 166                 e
 167             } else {
 168                 let arms = never_loop_expr_branch(&mut arms.iter().map(|a| a.body), main_loop_id);
 169                 combine_seq(e, arms)
 170             }
 171         },
 172         ExprKind::Block(b, _) => never_loop_block(b, main_loop_id),
 173         ExprKind::Continue(d) => {
 174             let id = d
 175                 .target_id
 176                 .expect("target ID can only be missing in the presence of compilation errors");
 177             if id == main_loop_id {
 178                 NeverLoopResult::MayContinueMainLoop
 179             } else {
 180                 NeverLoopResult::AlwaysBreak
 181             }
 182         },
 183         ExprKind::Break(_, e) | ExprKind::Ret(e) => e.as_ref().map_or(NeverLoopResult::AlwaysBreak, |e| {
 184             combine_seq(never_loop_expr(e, main_loop_id), NeverLoopResult::AlwaysBreak)
 185         }),
 186         ExprKind::InlineAsm(asm) => asm
 187             .operands
 188             .iter()
 189             .map(|(o, _)| match o {
 190                 InlineAsmOperand::In { expr, .. } | InlineAsmOperand::InOut { expr, .. } => {
 191                     never_loop_expr(expr, main_loop_id)
 192                 },
 193                 InlineAsmOperand::Out { expr, .. } => never_loop_expr_all(&mut expr.iter().copied(), main_loop_id),
 194                 InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
 195                     never_loop_expr_all(&mut once(*in_expr).chain(out_expr.iter().copied()), main_loop_id)
 196                 },
 197                 InlineAsmOperand::Const { .. }
 198                 | InlineAsmOperand::SymFn { .. }
 199                 | InlineAsmOperand::SymStatic { .. } => NeverLoopResult::Otherwise,
 200             })
 201             .fold(NeverLoopResult::Otherwise, combine_both),
 202         ExprKind::Yield(_, _)
 203         | ExprKind::Closure { .. }
 204         | ExprKind::Path(_)
 205         | ExprKind::ConstBlock(_)
 206         | ExprKind::Lit(_)
 207         | ExprKind::Err => NeverLoopResult::Otherwise,
 208     }
 209 }
 210
 211 fn never_loop_expr_all<'a, T: Iterator<Item = &'a Expr<'a>>>(es: &mut T, main_loop_id: HirId) -> NeverLoopResult {
 212     es.map(|e| never_loop_expr(e, main_loop_id))
 213         .fold(NeverLoopResult::Otherwise, combine_both)
 214 }
 215
 216 fn never_loop_expr_branch<'a, T: Iterator<Item = &'a Expr<'a>>>(e: &mut T, main_loop_id: HirId) -> NeverLoopResult {
 217     e.map(|e| never_loop_expr(e, main_loop_id))
 218         .fold(NeverLoopResult::AlwaysBreak, combine_branches)
 219 }
 220
 221 fn for_to_if_let_sugg(cx: &LateContext<'_>, iterator: &Expr<'_>, pat: &Pat<'_>) -> String {
 222     let pat_snippet = snippet(cx, pat.span, "_");
 223     let iter_snippet = make_iterator_snippet(cx, iterator, &mut Applicability::Unspecified);
 224
 225     format!("if let Some({pat_snippet}) = {iter_snippet}.next()")
 226 }