Rewrite never_loop as a strict reachability pass

fixes rust-lang#11004

Author: digama0

clippy_lints/src/loops/never_loop.rs

@@ -1,6 +1,5 @@
 use super::utils::make_iterator_snippet;
 use super::NEVER_LOOP;
-use clippy_utils::consts::{constant, Constant};
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::higher::ForLoop;
 use clippy_utils::source::snippet;
@@ -18,7 +17,7 @@ pub(super) fn check<'tcx>(
     for_loop: Option<&ForLoop<'_>>,
 ) {
     match never_loop_block(cx, block, &mut Vec::new(), loop_id) {
-        NeverLoopResult::AlwaysBreak => {
+        NeverLoopResult::Diverging => {
             span_lint_and_then(cx, NEVER_LOOP, span, "this loop never actually loops", |diag| {
                 if let Some(ForLoop {
                     arg: iterator,
@@ -39,83 +38,98 @@ pub(super) fn check<'tcx>(
                 }
             });
         },
-        NeverLoopResult::MayContinueMainLoop | NeverLoopResult::Otherwise => (),
-        NeverLoopResult::IgnoreUntilEnd(_) => unreachable!(),
+        NeverLoopResult::MayContinueMainLoop | NeverLoopResult::Normal => (),
     }
 }
 
+/// The `never_loop` analysis keeps track of three things:
+///
+/// * Has any (reachable) code path hit a `continue` of the main loop?
+/// * Is the current code path diverging (that is, the next expression is not reachable)
+/// * For each block label `'a` inside the main loop, has any (reachable) code path encountered a
+///   `break 'a`?
+///
+/// The first two bits of information are in this enum, and the last part is in the
+/// `local_labels` variable, which contains a list of `(block_id, reachable)` pairs ordered by
+/// scope.
 #[derive(Copy, Clone)]
 enum NeverLoopResult {
-    // A break/return always get triggered but not necessarily for the main loop.
-    AlwaysBreak,
-    // A continue may occur for the main loop.
+    /// A continue may occur for the main loop.
     MayContinueMainLoop,
-    // Ignore everything until the end of the block with this id
-    IgnoreUntilEnd(HirId),
-    Otherwise,
+    /// We have not encountered any main loop continue,
+    /// but we are diverging (subsequent control flow is not reachable)
+    Diverging,
+    /// We have not encountered any main loop continue,
+    /// and subsequent control flow is (possibly) reachable
+    Normal,
 }
 
 #[must_use]
 fn absorb_break(arg: NeverLoopResult) -> NeverLoopResult {
     match arg {
-        NeverLoopResult::AlwaysBreak | NeverLoopResult::Otherwise => NeverLoopResult::Otherwise,
+        NeverLoopResult::Diverging | NeverLoopResult::Normal => NeverLoopResult::Normal,
         NeverLoopResult::MayContinueMainLoop => NeverLoopResult::MayContinueMainLoop,
-        NeverLoopResult::IgnoreUntilEnd(id) => NeverLoopResult::IgnoreUntilEnd(id),
     }
 }
 
 // Combine two results for parts that are called in order.
 #[must_use]
-fn combine_seq(first: NeverLoopResult, second: NeverLoopResult) -> NeverLoopResult {
+fn combine_seq(first: NeverLoopResult, second: impl FnOnce() -> NeverLoopResult) -> NeverLoopResult {
     match first {
-        NeverLoopResult::AlwaysBreak | NeverLoopResult::MayContinueMainLoop | NeverLoopResult::IgnoreUntilEnd(_) => {
-            first
-        },
-        NeverLoopResult::Otherwise => second,
+        NeverLoopResult::Diverging | NeverLoopResult::MayContinueMainLoop => first,
+        NeverLoopResult::Normal => second(),
+    }
+}
+
+// Combine an iterator of results for parts that are called in order.
+#[must_use]
+fn combine_seq_many(iter: impl IntoIterator<Item = NeverLoopResult>) -> NeverLoopResult {
+    for e in iter {
+        if let NeverLoopResult::Diverging | NeverLoopResult::MayContinueMainLoop = e {
+            return e;
+        }
     }
+    NeverLoopResult::Normal
 }
 
 // Combine two results where only one of the part may have been executed.
 #[must_use]
-fn combine_branches(b1: NeverLoopResult, b2: NeverLoopResult, ignore_ids: &[HirId]) -> NeverLoopResult {
+fn combine_branches(b1: NeverLoopResult, b2: NeverLoopResult) -> NeverLoopResult {
     match (b1, b2) {
-        (NeverLoopResult::IgnoreUntilEnd(a), NeverLoopResult::IgnoreUntilEnd(b)) => {
-            if ignore_ids.iter().find(|&e| e == &a || e == &b).unwrap() == &a {
-                NeverLoopResult::IgnoreUntilEnd(b)
-            } else {
-                NeverLoopResult::IgnoreUntilEnd(a)
-            }
-        },
-        (i @ NeverLoopResult::IgnoreUntilEnd(_), NeverLoopResult::AlwaysBreak)
-        | (NeverLoopResult::AlwaysBreak, i @ NeverLoopResult::IgnoreUntilEnd(_)) => i,
-        (NeverLoopResult::AlwaysBreak, NeverLoopResult::AlwaysBreak) => NeverLoopResult::AlwaysBreak,
         (NeverLoopResult::MayContinueMainLoop, _) | (_, NeverLoopResult::MayContinueMainLoop) => {
             NeverLoopResult::MayContinueMainLoop
         },
-        (NeverLoopResult::Otherwise, _) | (_, NeverLoopResult::Otherwise) => NeverLoopResult::Otherwise,
+        (NeverLoopResult::Normal, _) | (_, NeverLoopResult::Normal) => NeverLoopResult::Normal,
+        (NeverLoopResult::Diverging, NeverLoopResult::Diverging) => NeverLoopResult::Diverging,
     }
 }
 
 fn never_loop_block<'tcx>(
     cx: &LateContext<'tcx>,
     block: &Block<'tcx>,
-    ignore_ids: &mut Vec<HirId>,
+    local_labels: &mut Vec<(HirId, bool)>,
     main_loop_id: HirId,
 ) -> NeverLoopResult {
     let iter = block
         .stmts
         .iter()
         .filter_map(stmt_to_expr)
         .chain(block.expr.map(|expr| (expr, None)));
-
-    iter.map(|(e, els)| {
-        let e = never_loop_expr(cx, e, ignore_ids, main_loop_id);
+    combine_seq_many(iter.map(|(e, els)| {
+        let e = never_loop_expr(cx, e, local_labels, main_loop_id);
         // els is an else block in a let...else binding
         els.map_or(e, |els| {
-            combine_branches(e, never_loop_block(cx, els, ignore_ids, main_loop_id), ignore_ids)
+            combine_seq(e, || match never_loop_block(cx, els, local_labels, main_loop_id) {
+                // Returning MayContinueMainLoop here means that
+                // we will not evaluate the rest of the body
+                NeverLoopResult::MayContinueMainLoop => NeverLoopResult::MayContinueMainLoop,
+                // An else block always diverges, so the Normal case should not happen,
+                // but the analysis is approximate so it might return Normal anyway.
+                // Returning Normal here says that nothing more happens on the main path
+                NeverLoopResult::Diverging | NeverLoopResult::Normal => NeverLoopResult::Normal,
+            })
         })
-    })
-    .fold(NeverLoopResult::Otherwise, combine_seq)
+    }))
 }
 
 fn stmt_to_expr<'tcx>(stmt: &Stmt<'tcx>) -> Option<(&'tcx Expr<'tcx>, Option<&'tcx Block<'tcx>>)> {
@@ -131,7 +145,7 @@ fn stmt_to_expr<'tcx>(stmt: &Stmt<'tcx>) -> Option<(&'tcx Expr<'tcx>, Option<&'t
 fn never_loop_expr<'tcx>(
     cx: &LateContext<'tcx>,
     expr: &Expr<'tcx>,
-    ignore_ids: &mut Vec<HirId>,
+    local_labels: &mut Vec<(HirId, bool)>,
     main_loop_id: HirId,
 ) -> NeverLoopResult {
     match expr.kind {
@@ -141,66 +155,61 @@ fn never_loop_expr<'tcx>(
         | ExprKind::Field(e, _)
         | ExprKind::AddrOf(_, _, e)
         | ExprKind::Repeat(e, _)
-        | ExprKind::DropTemps(e) => never_loop_expr(cx, e, ignore_ids, main_loop_id),
-        ExprKind::Let(let_expr) => never_loop_expr(cx, let_expr.init, ignore_ids, main_loop_id),
-        ExprKind::Array(es) | ExprKind::Tup(es) => never_loop_expr_all(cx, &mut es.iter(), ignore_ids, main_loop_id),
+        | ExprKind::DropTemps(e) => never_loop_expr(cx, e, local_labels, main_loop_id),
+        ExprKind::Let(let_expr) => never_loop_expr(cx, let_expr.init, local_labels, main_loop_id),
+        ExprKind::Array(es) | ExprKind::Tup(es) => never_loop_expr_all(cx, es.iter(), local_labels, main_loop_id),
         ExprKind::MethodCall(_, receiver, es, _) => never_loop_expr_all(
             cx,
-            &mut std::iter::once(receiver).chain(es.iter()),
-            ignore_ids,
+            std::iter::once(receiver).chain(es.iter()),
+            local_labels,
             main_loop_id,
         ),
         ExprKind::Struct(_, fields, base) => {
-            let fields = never_loop_expr_all(cx, &mut fields.iter().map(|f| f.expr), ignore_ids, main_loop_id);
+            let fields = never_loop_expr_all(cx, fields.iter().map(|f| f.expr), local_labels, main_loop_id);
             if let Some(base) = base {
-                combine_seq(fields, never_loop_expr(cx, base, ignore_ids, main_loop_id))
+                combine_seq(fields, || never_loop_expr(cx, base, local_labels, main_loop_id))
             } else {
                 fields
             }
         },
-        ExprKind::Call(e, es) => never_loop_expr_all(cx, &mut once(e).chain(es.iter()), ignore_ids, main_loop_id),
+        ExprKind::Call(e, es) => never_loop_expr_all(cx, once(e).chain(es.iter()), local_labels, main_loop_id),
         ExprKind::Binary(_, e1, e2)
         | ExprKind::Assign(e1, e2, _)
         | ExprKind::AssignOp(_, e1, e2)
-        | ExprKind::Index(e1, e2, _) => {
-            never_loop_expr_all(cx, &mut [e1, e2].iter().copied(), ignore_ids, main_loop_id)
-        },
+        | ExprKind::Index(e1, e2, _) => never_loop_expr_all(cx, [e1, e2].iter().copied(), local_labels, main_loop_id),
         ExprKind::Loop(b, _, _, _) => {
-            // Break can come from the inner loop so remove them.
-            absorb_break(never_loop_block(cx, b, ignore_ids, main_loop_id))
+            // We don't attempt to track reachability after a loop,
+            // just assume there may have been a break somewhere
+            absorb_break(never_loop_block(cx, b, local_labels, main_loop_id))
         },
         ExprKind::If(e, e2, e3) => {
-            let e1 = never_loop_expr(cx, e, ignore_ids, main_loop_id);
-            let e2 = never_loop_expr(cx, e2, ignore_ids, main_loop_id);
-            // If we know the `if` condition evaluates to `true`, don't check everything past it; it
-            // should just return whatever's evaluated for `e1` and `e2` since `e3` is unreachable
-            if let Some(Constant::Bool(true)) = constant(cx, cx.typeck_results(), e) {
-                return combine_seq(e1, e2);
-            }
-            let e3 = e3.as_ref().map_or(NeverLoopResult::Otherwise, |e| {
-                never_loop_expr(cx, e, ignore_ids, main_loop_id)
-            });
-            combine_seq(e1, combine_branches(e2, e3, ignore_ids))
+            let e1 = never_loop_expr(cx, e, local_labels, main_loop_id);
+            combine_seq(e1, || {
+                let e2 = never_loop_expr(cx, e2, local_labels, main_loop_id);
+                let e3 = e3.as_ref().map_or(NeverLoopResult::Normal, |e| {
+                    never_loop_expr(cx, e, local_labels, main_loop_id)
+                });
+                combine_branches(e2, e3)
+            })
         },
         ExprKind::Match(e, arms, _) => {
-            let e = never_loop_expr(cx, e, ignore_ids, main_loop_id);
+            let e = never_loop_expr(cx, e, local_labels, main_loop_id);
             if arms.is_empty() {
                 e
             } else {
-                let arms = never_loop_expr_branch(cx, &mut arms.iter().map(|a| a.body), ignore_ids, main_loop_id);
-                combine_seq(e, arms)
+                combine_seq(e, || {
+                    never_loop_expr_branch(cx, &mut arms.iter().map(|a| a.body), local_labels, main_loop_id)
+                })
             }
         },
         ExprKind::Block(b, l) => {
             if l.is_some() {
-                ignore_ids.push(b.hir_id);
-            }
-            let ret = never_loop_block(cx, b, ignore_ids, main_loop_id);
-            if l.is_some() {
-                ignore_ids.pop();
+                local_labels.push((b.hir_id, false));
             }
+            let ret = never_loop_block(cx, b, local_labels, main_loop_id);
+            let jumped_to = l.is_some() && local_labels.pop().unwrap().1;
             match ret {
-                NeverLoopResult::IgnoreUntilEnd(a) if a == b.hir_id => NeverLoopResult::Otherwise,
+                NeverLoopResult::Diverging if jumped_to => NeverLoopResult::Normal,
                 _ => ret,
             }
         },
@@ -211,73 +220,70 @@ fn never_loop_expr<'tcx>(
             if id == main_loop_id {
                 NeverLoopResult::MayContinueMainLoop
             } else {
-                NeverLoopResult::AlwaysBreak
+                NeverLoopResult::Diverging
             }
         },
-        // checks if break targets a block instead of a loop
-        ExprKind::Break(Destination { target_id: Ok(t), .. }, e) if ignore_ids.contains(&t) => e
-            .map_or(NeverLoopResult::IgnoreUntilEnd(t), |e| {
-                never_loop_expr(cx, e, ignore_ids, main_loop_id)
-            }),
-        ExprKind::Break(_, e) | ExprKind::Ret(e) => e.as_ref().map_or(NeverLoopResult::AlwaysBreak, |e| {
-            combine_seq(
-                never_loop_expr(cx, e, ignore_ids, main_loop_id),
-                NeverLoopResult::AlwaysBreak,
-            )
-        }),
-        ExprKind::Become(e) => combine_seq(
-            never_loop_expr(cx, e, ignore_ids, main_loop_id),
-            NeverLoopResult::AlwaysBreak,
-        ),
-        ExprKind::InlineAsm(asm) => asm
-            .operands
-            .iter()
-            .map(|(o, _)| match o {
-                InlineAsmOperand::In { expr, .. } | InlineAsmOperand::InOut { expr, .. } => {
-                    never_loop_expr(cx, expr, ignore_ids, main_loop_id)
-                },
-                InlineAsmOperand::Out { expr, .. } => {
-                    never_loop_expr_all(cx, &mut expr.iter().copied(), ignore_ids, main_loop_id)
-                },
-                InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => never_loop_expr_all(
-                    cx,
-                    &mut once(*in_expr).chain(out_expr.iter().copied()),
-                    ignore_ids,
-                    main_loop_id,
-                ),
-                InlineAsmOperand::Const { .. }
-                | InlineAsmOperand::SymFn { .. }
-                | InlineAsmOperand::SymStatic { .. } => NeverLoopResult::Otherwise,
+        ExprKind::Break(_, e) | ExprKind::Ret(e) => {
+            let first = e.as_ref().map_or(NeverLoopResult::Normal, |e| {
+                never_loop_expr(cx, e, local_labels, main_loop_id)
+            });
+            combine_seq(first, || {
+                // checks if break targets a block instead of a loop
+                if let ExprKind::Break(Destination { target_id: Ok(t), .. }, _) = expr.kind {
+                    if let Some((_, reachable)) = local_labels.iter_mut().find(|(label, _)| *label == t) {
+                        *reachable = true;
+                    }
+                }
+                NeverLoopResult::Diverging
             })
-            .fold(NeverLoopResult::Otherwise, combine_seq),
+        },
+        ExprKind::Become(e) => combine_seq(never_loop_expr(cx, e, local_labels, main_loop_id), || {
+            NeverLoopResult::Diverging
+        }),
+        ExprKind::InlineAsm(asm) => combine_seq_many(asm.operands.iter().map(|(o, _)| match o {
+            InlineAsmOperand::In { expr, .. } | InlineAsmOperand::InOut { expr, .. } => {
+                never_loop_expr(cx, expr, local_labels, main_loop_id)
+            },
+            InlineAsmOperand::Out { expr, .. } => {
+                never_loop_expr_all(cx, expr.iter().copied(), local_labels, main_loop_id)
+            },
+            InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => never_loop_expr_all(
+                cx,
+                once(*in_expr).chain(out_expr.iter().copied()),
+                local_labels,
+                main_loop_id,
+            ),
+            InlineAsmOperand::Const { .. } | InlineAsmOperand::SymFn { .. } | InlineAsmOperand::SymStatic { .. } => {
+                NeverLoopResult::Normal
+            },
+        })),
         ExprKind::OffsetOf(_, _)
         | ExprKind::Yield(_, _)
         | ExprKind::Closure { .. }
         | ExprKind::Path(_)
         | ExprKind::ConstBlock(_)
         | ExprKind::Lit(_)
-        | ExprKind::Err(_) => NeverLoopResult::Otherwise,
+        | ExprKind::Err(_) => NeverLoopResult::Normal,
     }
 }
 
 fn never_loop_expr_all<'tcx, T: Iterator<Item = &'tcx Expr<'tcx>>>(
     cx: &LateContext<'tcx>,
-    es: &mut T,
-    ignore_ids: &mut Vec<HirId>,
+    es: T,
+    local_labels: &mut Vec<(HirId, bool)>,
     main_loop_id: HirId,
 ) -> NeverLoopResult {
-    es.map(|e| never_loop_expr(cx, e, ignore_ids, main_loop_id))
-        .fold(NeverLoopResult::Otherwise, combine_seq)
+    combine_seq_many(es.map(|e| never_loop_expr(cx, e, local_labels, main_loop_id)))
 }
 
 fn never_loop_expr_branch<'tcx, T: Iterator<Item = &'tcx Expr<'tcx>>>(
     cx: &LateContext<'tcx>,
     e: &mut T,
-    ignore_ids: &mut Vec<HirId>,
+    local_labels: &mut Vec<(HirId, bool)>,
     main_loop_id: HirId,
 ) -> NeverLoopResult {
-    e.fold(NeverLoopResult::AlwaysBreak, |a, b| {
-        combine_branches(a, never_loop_expr(cx, b, ignore_ids, main_loop_id), ignore_ids)
+    e.fold(NeverLoopResult::Diverging, |a, b| {
+        combine_branches(a, never_loop_expr(cx, b, local_labels, main_loop_id))
     })
 }