Ignore unwinding edges when checking for unconditional recursion

The unconditional recursion lint determines if all execution paths
eventually lead to a self-recursive call.

The implementation always follows unwinding edges which limits its
practical utility. For example, it would not lint function `f` because a
call to `g` might unwind. It also wouldn't lint function `h` because an
overflow check preceding the self-recursive call might unwind:

```rust
pub fn f() {
    g();
    f();
}

pub fn g() { /* ... */ }

pub fn h(a: usize) {
  h(a + 1);
}
```

To avoid the issue, assume that terminators that might continue
execution along non-unwinding edges do so.

compiler/rustc_mir_build/src/lints.rs

@@ -33,6 +33,9 @@
         if let Some(NonRecursive) = TriColorDepthFirstSearch::new(&body).run_from_start(&mut vis) {
             return;
         }
+        if vis.reachable_recursive_calls.is_empty() {
+            return;
+        }
 
         vis.reachable_recursive_calls.sort();
 
@@ -148,13 +151,14 @@ fn node_settled(&mut self, bb: BasicBlock) -> ControlFlow<Self::BreakVal> {
     }
 
     fn ignore_edge(&mut self, bb: BasicBlock, target: BasicBlock) -> bool {
+        let terminator = self.body[bb].terminator();
+        if terminator.unwind() == Some(&Some(target)) && terminator.successors().count() > 1 {
+            return true;
+        }
         // Don't traverse successors of recursive calls or false CFG edges.
         match self.body[bb].terminator().kind {
             TerminatorKind::Call { ref func, .. } => self.is_recursive_call(func),
-
-            TerminatorKind::FalseUnwind { unwind: Some(imaginary_target), .. }
-            | TerminatorKind::FalseEdge { imaginary_target, .. } => imaginary_target == target,
-
+            TerminatorKind::FalseEdge { imaginary_target, .. } => imaginary_target == target,
             _ => false,
         }
     }

src/test/ui/lint/lint-unconditional-recursion.rs

@@ -149,4 +149,46 @@ pub fn panics(x: bool) {
     }
 }
 
+pub fn unreachable1() {
+    panic!();
+    unreachable1(); // WARN unreachable statement
+}
+
+pub fn unreachable2() {
+    loop {}
+    unreachable2(); // WARN unreachable statement
+}
+
+pub fn drop_and_replace(mut a: Option<String>) { //~ ERROR function cannot return without recursing
+    a = None;
+    drop_and_replace(a);
+}
+
+// Calls are assumed to return normally.
+pub fn call() -> String { //~ ERROR function cannot return without recursing
+    let s = String::new();
+    call();
+    s
+}
+
+// Arithmetic operations are assumed not to overflow.
+pub fn overflow_check(a: i32, b: i32) { //~ ERROR function cannot return without recursing
+    let _ = a + b;
+    overflow_check(a, b);
+}
+
+pub struct Point {
+    pub x: f32,
+    pub y: f32,
+}
+
+impl Default for Point {
+    fn default() -> Self { //~ ERROR function cannot return without recursing
+        Point {
+            x: Default::default(),
+            ..Default::default()
+        }
+    }
+}
+
 fn main() {}

src/test/ui/lint/lint-unconditional-recursion.stderr

@@ -153,5 +153,49 @@ LL |         self.as_ref()
    |
    = help: a `loop` may express intention better if this is on purpose
 
-error: aborting due to 14 previous errors
+error: function cannot return without recursing
+  --> $DIR/lint-unconditional-recursion.rs:162:1
+   |
+LL | pub fn drop_and_replace(mut a: Option<String>) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
+LL |     a = None;
+LL |     drop_and_replace(a);
+   |     ------------------- recursive call site
+   |
+   = help: a `loop` may express intention better if this is on purpose
+
+error: function cannot return without recursing
+  --> $DIR/lint-unconditional-recursion.rs:168:1
+   |
+LL | pub fn call() -> String {
+   | ^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
+LL |     let s = String::new();
+LL |     call();
+   |     ------ recursive call site
+   |
+   = help: a `loop` may express intention better if this is on purpose
+
+error: function cannot return without recursing
+  --> $DIR/lint-unconditional-recursion.rs:175:1
+   |
+LL | pub fn overflow_check(a: i32, b: i32) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
+LL |     let _ = a + b;
+LL |     overflow_check(a, b);
+   |     -------------------- recursive call site
+   |
+   = help: a `loop` may express intention better if this is on purpose
+
+error: function cannot return without recursing
+  --> $DIR/lint-unconditional-recursion.rs:186:5
+   |
+LL |     fn default() -> Self {
+   |     ^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
+...
+LL |             ..Default::default()
+   |               ------------------ recursive call site
+   |
+   = help: a `loop` may express intention better if this is on purpose
+
+error: aborting due to 18 previous errors
 

src/test/ui/recursion/issue-83150.rs

@@ -6,6 +6,6 @@ fn main() {
     func(&mut iter)
 }
 
-fn func<T: Iterator<Item = u8>>(iter: &mut T) {
+fn func<T: Iterator<Item = u8>>(iter: &mut T) { //~ WARN function cannot return without recursing
     func(&mut iter.map(|x| x + 1))
 }

src/test/ui/recursion/issue-83150.stderr

+warning: function cannot return without recursing
+  --> $DIR/issue-83150.rs:9:1
+   |
+LL | fn func<T: Iterator<Item = u8>>(iter: &mut T) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
+LL |     func(&mut iter.map(|x| x + 1))
+   |     ------------------------------ recursive call site
+   |
+   = note: `#[warn(unconditional_recursion)]` on by default
+   = help: a `loop` may express intention better if this is on purpose
+
 error[E0275]: overflow evaluating the requirement `Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut std::ops::Range<u8>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>: Iterator`
    |
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`issue_83150`)
    = note: required because of the requirements on the impl of `Iterator` for `&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut Map<&mut std::ops::Range<u8>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>, [closure@$DIR/issue-83150.rs:10:24: 10:33]>`
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0275`.