Ignore instructions following a break from block in never_loop lint

It is not sufficient to ignore break from a block inside the loop.
Instructions after the break must be ignored, as they are unreachable.
This is also true for all instructions in outer blocks and loops
until the right block is reached.

3 files changed, 78 insertions, 10 deletions

diff --git a/clippy_lints/src/loops/never_loop.rs b/clippy_lints/src/loops/never_loop.rs
index 3cb5b1ffc7b..ea7630ce56d 100644
--- a/clippy_lints/src/loops/never_loop.rs
+++ b/clippy_lints/src/loops/never_loop.rs
@@ -39,6 +39,7 @@ pub(super) fn check(
             });
         },
         NeverLoopResult::MayContinueMainLoop | NeverLoopResult::Otherwise => (),
+        NeverLoopResult::IgnoreUntilEnd(_) => unreachable!(),
     }
 }
 
@@ -48,6 +49,8 @@ enum NeverLoopResult {
     AlwaysBreak,
     // A continue may occur for the main loop.
     MayContinueMainLoop,
+    // Ignore everything until the end of the block with this id
+    IgnoreUntilEnd(HirId),
     Otherwise,
 }
 
@@ -56,6 +59,7 @@ fn absorb_break(arg: NeverLoopResult) -> NeverLoopResult {
     match arg {
         NeverLoopResult::AlwaysBreak | NeverLoopResult::Otherwise => NeverLoopResult::Otherwise,
         NeverLoopResult::MayContinueMainLoop => NeverLoopResult::MayContinueMainLoop,
+        NeverLoopResult::IgnoreUntilEnd(id) => NeverLoopResult::IgnoreUntilEnd(id),
     }
 }
 
@@ -63,15 +67,26 @@ fn absorb_break(arg: NeverLoopResult) -> NeverLoopResult {
 #[must_use]
 fn combine_seq(first: NeverLoopResult, second: NeverLoopResult) -> NeverLoopResult {
     match first {
-        NeverLoopResult::AlwaysBreak | NeverLoopResult::MayContinueMainLoop => first,
+        NeverLoopResult::AlwaysBreak | NeverLoopResult::MayContinueMainLoop | NeverLoopResult::IgnoreUntilEnd(_) => {
+            first
+        },
         NeverLoopResult::Otherwise => second,
     }
 }
 
 // Combine two results where only one of the part may have been executed.
 #[must_use]
-fn combine_branches(b1: NeverLoopResult, b2: NeverLoopResult) -> NeverLoopResult {
+fn combine_branches(b1: NeverLoopResult, b2: NeverLoopResult, ignore_ids: &[HirId]) -> 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
@@ -91,7 +106,7 @@ fn never_loop_block(block: &Block<'_>, ignore_ids: &mut Vec<HirId>, main_loop_id
         let e = never_loop_expr(e, ignore_ids, main_loop_id);
         // els is an else block in a let...else binding
         els.map_or(e, |els| {
-            combine_branches(e, never_loop_block(els, ignore_ids, main_loop_id))
+            combine_branches(e, never_loop_block(els, ignore_ids, main_loop_id), ignore_ids)
         })
     })
     .fold(NeverLoopResult::Otherwise, combine_seq)
@@ -147,7 +162,7 @@ fn never_loop_expr(expr: &Expr<'_>, ignore_ids: &mut Vec<HirId>, main_loop_id: H
             let e3 = e3.as_ref().map_or(NeverLoopResult::Otherwise, |e| {
                 never_loop_expr(e, ignore_ids, main_loop_id)
             });
-            combine_seq(e1, combine_branches(e2, e3))
+            combine_seq(e1, combine_branches(e2, e3, ignore_ids))
         },
         ExprKind::Match(e, arms, _) => {
             let e = never_loop_expr(e, ignore_ids, main_loop_id);
@@ -166,7 +181,10 @@ fn never_loop_expr(expr: &Expr<'_>, ignore_ids: &mut Vec<HirId>, main_loop_id: H
             if l.is_some() {
                 ignore_ids.pop();
             }
-            ret
+            match ret {
+                NeverLoopResult::IgnoreUntilEnd(a) if a == b.hir_id => NeverLoopResult::Otherwise,
+                _ => ret,
+            }
         },
         ExprKind::Continue(d) => {
             let id = d
@@ -180,7 +198,7 @@ fn never_loop_expr(expr: &Expr<'_>, ignore_ids: &mut Vec<HirId>, main_loop_id: H
         },
         // 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::Otherwise, |e| {
+            .map_or(NeverLoopResult::IgnoreUntilEnd(t), |e| {
                 never_loop_expr(e, ignore_ids, main_loop_id)
             }),
         ExprKind::Break(_, e) | ExprKind::Ret(e) => e.as_ref().map_or(NeverLoopResult::AlwaysBreak, |e| {
@@ -232,8 +250,9 @@ fn never_loop_expr_branch<'a, T: Iterator<Item = &'a Expr<'a>>>(
     ignore_ids: &mut Vec<HirId>,
     main_loop_id: HirId,
 ) -> NeverLoopResult {
-    e.map(|e| never_loop_expr(e, ignore_ids, main_loop_id))
-        .fold(NeverLoopResult::AlwaysBreak, combine_branches)
+    e.fold(NeverLoopResult::AlwaysBreak, |a, b| {
+        combine_branches(a, never_loop_expr(b, ignore_ids, main_loop_id), ignore_ids)
+    })
 }
 
 fn for_to_if_let_sugg(cx: &LateContext<'_>, iterator: &Expr<'_>, pat: &Pat<'_>) -> String {
diff --git a/tests/ui/never_loop.rs b/tests/ui/never_loop.rs
index a177d5e06aa..29821ff96fc 100644
--- a/tests/ui/never_loop.rs
+++ b/tests/ui/never_loop.rs
@@ -253,12 +253,48 @@ pub fn test20() {
 pub fn test21() {
     loop {
         'a: {
-            { }
+            {}
             break 'a;
         }
     }
 }
 
+// Issue 10304: code after break from block was not considered
+// unreachable code and was considered for further analysis of
+// whether the loop would ever be executed or not.
+pub fn test22() {
+    for _ in 0..10 {
+        'block: {
+            break 'block;
+            return;
+        }
+        println!("looped");
+    }
+}
+
+pub fn test23() {
+    for _ in 0..10 {
+        'block: {
+            for _ in 0..20 {
+                break 'block;
+            }
+        }
+        println!("looped");
+    }
+}
+
+pub fn test24() {
+    'a: for _ in 0..10 {
+        'b: {
+            let x = Some(1);
+            match x {
+                None => break 'a,
+                Some(_) => break 'b,
+            }
+        }
+    }
+}
+
 fn main() {
     test1();
     test2();
diff --git a/tests/ui/never_loop.stderr b/tests/ui/never_loop.stderr
index b7029bf8bed..704d448644e 100644
--- a/tests/ui/never_loop.stderr
+++ b/tests/ui/never_loop.stderr
@@ -126,5 +126,18 @@ LL | |         }
 LL | |     }
    | |_____^
 
-error: aborting due to 11 previous errors
+error: this loop never actually loops
+  --> $DIR/never_loop.rs:278:13
+   |
+LL | /             for _ in 0..20 {
+LL | |                 break 'block;
+LL | |             }
+   | |_____________^
+   |
+help: if you need the first element of the iterator, try writing
+   |
+LL |             if let Some(_) = (0..20).next() {
+   |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 12 previous errors