Improve liveness analysis for generators

Liveness analysis for generators assumes that execution always continues
normally after a yield point, not accounting for the fact that generator
could be dropped before completion.

If generators captures any variables by reference, those variables could
be used within a generator, or when the generator completes, but also
after each yield point in the case the generator is dropped.

Account for the case when generator is dropped after yielding, but
before running to the completion. This effectively considers all
variables captured by reference to be used after a yield point.

compiler/rustc_passes/src/liveness.rs

@@ -435,7 +435,10 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
             }
 
             // live nodes required for interesting control flow:
-            hir::ExprKind::If(..) | hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => {
+            hir::ExprKind::If(..)
+            | hir::ExprKind::Match(..)
+            | hir::ExprKind::Loop(..)
+            | hir::ExprKind::Yield(..) => {
                 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
                 intravisit::walk_expr(self, expr);
             }
@@ -469,7 +472,6 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
             | hir::ExprKind::InlineAsm(..)
             | hir::ExprKind::LlvmInlineAsm(..)
             | hir::ExprKind::Box(..)
-            | hir::ExprKind::Yield(..)
             | hir::ExprKind::Type(..)
             | hir::ExprKind::Err
             | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
@@ -866,6 +868,13 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             // at the label ident
             hir::ExprKind::Loop(ref blk, ..) => self.propagate_through_loop(expr, &blk, succ),
 
+            hir::ExprKind::Yield(ref e, ..) => {
+                let yield_ln = self.live_node(expr.hir_id, expr.span);
+                self.init_from_succ(yield_ln, succ);
+                self.merge_from_succ(yield_ln, self.exit_ln);
+                self.propagate_through_expr(e, yield_ln)
+            }
+
             hir::ExprKind::If(ref cond, ref then, ref else_opt) => {
                 //
                 //     (cond)
@@ -1025,7 +1034,6 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             | hir::ExprKind::Type(ref e, _)
             | hir::ExprKind::DropTemps(ref e)
             | hir::ExprKind::Unary(_, ref e)
-            | hir::ExprKind::Yield(ref e, _)
             | hir::ExprKind::Repeat(ref e, _) => self.propagate_through_expr(&e, succ),
 
             hir::ExprKind::InlineAsm(ref asm) => {

src/test/ui/liveness/liveness-upvars.rs

@@ -1,5 +1,6 @@
 // edition:2018
 // check-pass
+#![feature(generators)]
 #![warn(unused)]
 #![allow(unreachable_code)]
 
@@ -105,4 +106,39 @@ pub fn h<T: Copy + Default + std::fmt::Debug>() {
     };
 }
 
+async fn yield_now() {
+    todo!();
+}
+
+pub fn async_generator() {
+    let mut state: u32 = 0;
+
+    let _ = async {
+        state = 1;
+        yield_now().await;
+        state = 2;
+        yield_now().await;
+        state = 3;
+    };
+
+    let _ = async move {
+        state = 4;  //~  WARN value assigned to `state` is never read
+                    //~| WARN unused variable: `state`
+        yield_now().await;
+        state = 5;  //~ WARN value assigned to `state` is never read
+    };
+}
+
+pub fn generator() {
+    let mut s: u32 = 0;
+    let _ = |_| {
+        s = 0;
+        yield ();
+        s = 1; //~ WARN value assigned to `s` is never read
+        yield (s = 2);
+        s = yield (); //~ WARN value assigned to `s` is never read
+        s = 3;
+    };
+}
+
 fn main() {}

src/test/ui/liveness/liveness-upvars.stderr

@@ -1,11 +1,11 @@
 warning: value assigned to `last` is never read
-  --> $DIR/liveness-upvars.rs:9:9
+  --> $DIR/liveness-upvars.rs:10:9
    |
 LL |         last = Some(s);
    |         ^^^^
    |
 note: the lint level is defined here
-  --> $DIR/liveness-upvars.rs:3:9
+  --> $DIR/liveness-upvars.rs:4:9
    |
 LL | #![warn(unused)]
    |         ^^^^^^
@@ -13,13 +13,13 @@ LL | #![warn(unused)]
    = help: maybe it is overwritten before being read?
 
 warning: unused variable: `last`
-  --> $DIR/liveness-upvars.rs:9:9
+  --> $DIR/liveness-upvars.rs:10:9
    |
 LL |         last = Some(s);
    |         ^^^^
    |
 note: the lint level is defined here
-  --> $DIR/liveness-upvars.rs:3:9
+  --> $DIR/liveness-upvars.rs:4:9
    |
 LL | #![warn(unused)]
    |         ^^^^^^
@@ -27,7 +27,7 @@ LL | #![warn(unused)]
    = help: did you mean to capture by reference instead?
 
 warning: unused variable: `sum`
-  --> $DIR/liveness-upvars.rs:21:9
+  --> $DIR/liveness-upvars.rs:22:9
    |
 LL |         sum += x;
    |         ^^^
@@ -35,7 +35,7 @@ LL |         sum += x;
    = help: did you mean to capture by reference instead?
 
 warning: value captured by `c` is never read
-  --> $DIR/liveness-upvars.rs:31:9
+  --> $DIR/liveness-upvars.rs:32:9
    |
 LL |         c = 1;
    |         ^
@@ -43,7 +43,7 @@ LL |         c = 1;
    = help: did you mean to capture by reference instead?
 
 warning: value captured by `c` is never read
-  --> $DIR/liveness-upvars.rs:35:9
+  --> $DIR/liveness-upvars.rs:36:9
    |
 LL |         c = 1;
    |         ^
@@ -51,7 +51,7 @@ LL |         c = 1;
    = help: did you mean to capture by reference instead?
 
 warning: unused variable: `c`
-  --> $DIR/liveness-upvars.rs:41:9
+  --> $DIR/liveness-upvars.rs:42:9
    |
 LL |         c += 1;
    |         ^
@@ -59,7 +59,7 @@ LL |         c += 1;
    = help: did you mean to capture by reference instead?
 
 warning: value assigned to `c` is never read
-  --> $DIR/liveness-upvars.rs:44:9
+  --> $DIR/liveness-upvars.rs:45:9
    |
 LL |         c += 1;
    |         ^
@@ -67,7 +67,7 @@ LL |         c += 1;
    = help: maybe it is overwritten before being read?
 
 warning: unused variable: `c`
-  --> $DIR/liveness-upvars.rs:44:9
+  --> $DIR/liveness-upvars.rs:45:9
    |
 LL |         c += 1;
    |         ^
@@ -75,7 +75,7 @@ LL |         c += 1;
    = help: did you mean to capture by reference instead?
 
 warning: value assigned to `c` is never read
-  --> $DIR/liveness-upvars.rs:57:9
+  --> $DIR/liveness-upvars.rs:58:9
    |
 LL |         c += 1;
    |         ^
@@ -83,7 +83,7 @@ LL |         c += 1;
    = help: maybe it is overwritten before being read?
 
 warning: value assigned to `c` is never read
-  --> $DIR/liveness-upvars.rs:63:9
+  --> $DIR/liveness-upvars.rs:64:9
    |
 LL |         c += 1;
    |         ^
@@ -91,7 +91,7 @@ LL |         c += 1;
    = help: maybe it is overwritten before being read?
 
 warning: value assigned to `d` is never read
-  --> $DIR/liveness-upvars.rs:72:13
+  --> $DIR/liveness-upvars.rs:73:13
    |
 LL |             d = Some("d1");
    |             ^
@@ -99,7 +99,7 @@ LL |             d = Some("d1");
    = help: maybe it is overwritten before being read?
 
 warning: value assigned to `e` is never read
-  --> $DIR/liveness-upvars.rs:76:13
+  --> $DIR/liveness-upvars.rs:77:13
    |
 LL |             e = Some("e1");
    |             ^
@@ -107,7 +107,7 @@ LL |             e = Some("e1");
    = help: maybe it is overwritten before being read?
 
 warning: value assigned to `e` is never read
-  --> $DIR/liveness-upvars.rs:78:13
+  --> $DIR/liveness-upvars.rs:79:13
    |
 LL |             e = Some("e2");
    |             ^
@@ -115,7 +115,7 @@ LL |             e = Some("e2");
    = help: maybe it is overwritten before being read?
 
 warning: unused variable: `e`
-  --> $DIR/liveness-upvars.rs:76:13
+  --> $DIR/liveness-upvars.rs:77:13
    |
 LL |             e = Some("e1");
    |             ^
@@ -123,7 +123,7 @@ LL |             e = Some("e1");
    = help: did you mean to capture by reference instead?
 
 warning: value assigned to `v` is never read
-  --> $DIR/liveness-upvars.rs:86:13
+  --> $DIR/liveness-upvars.rs:87:13
    |
 LL |             v = T::default();
    |             ^
@@ -131,7 +131,7 @@ LL |             v = T::default();
    = help: maybe it is overwritten before being read?
 
 warning: value assigned to `z` is never read
-  --> $DIR/liveness-upvars.rs:98:17
+  --> $DIR/liveness-upvars.rs:99:17
    |
 LL |                 z = T::default();
    |                 ^
@@ -139,12 +139,52 @@ LL |                 z = T::default();
    = help: maybe it is overwritten before being read?
 
 warning: unused variable: `z`
-  --> $DIR/liveness-upvars.rs:98:17
+  --> $DIR/liveness-upvars.rs:99:17
    |
 LL |                 z = T::default();
    |                 ^
    |
    = help: did you mean to capture by reference instead?
 
-warning: 17 warnings emitted
+warning: value assigned to `state` is never read
+  --> $DIR/liveness-upvars.rs:125:9
+   |
+LL |         state = 4;
+   |         ^^^^^
+   |
+   = help: maybe it is overwritten before being read?
+
+warning: value assigned to `state` is never read
+  --> $DIR/liveness-upvars.rs:128:9
+   |
+LL |         state = 5;
+   |         ^^^^^
+   |
+   = help: maybe it is overwritten before being read?
+
+warning: unused variable: `state`
+  --> $DIR/liveness-upvars.rs:125:9
+   |
+LL |         state = 4;
+   |         ^^^^^
+   |
+   = help: did you mean to capture by reference instead?
+
+warning: value assigned to `s` is never read
+  --> $DIR/liveness-upvars.rs:137:9
+   |
+LL |         s = 1;
+   |         ^
+   |
+   = help: maybe it is overwritten before being read?
+
+warning: value assigned to `s` is never read
+  --> $DIR/liveness-upvars.rs:139:9
+   |
+LL |         s = yield ();
+   |         ^
+   |
+   = help: maybe it is overwritten before being read?
+
+warning: 22 warnings emitted