Auto merge of #84333 - tmiasko:liveness-yield, r=tmandry

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. Fixes #84292.
rust-lang · Aug 25, 2021 · 1a9ac38 · 1a9ac38
2 parents e5484ce + 9f6f862
commit 1a9ac38
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Showing 3 changed files with 107 additions and 23 deletions.
diff --git a/compiler/rustc_passes/src/liveness.rs b/compiler/rustc_passes/src/liveness.rs
@@ -428,7 +428,10 @@ 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);
             }
@@ -462,7 +465,6 @@ 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(..))
@@ -856,6 +858,13 @@ 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)
@@ -1015,7 +1024,6 @@ 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) => {

diff --git a/src/test/ui/liveness/liveness-upvars.rs b/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() {}
diff --git a/src/test/ui/liveness/liveness-upvars.stderr b/src/test/ui/liveness/liveness-upvars.stderr
@@ -1,150 +1,190 @@
 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)]
    |         ^^^^^^
    = note: `#[warn(unused_assignments)]` implied by `#[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)]
    |         ^^^^^^
    = note: `#[warn(unused_variables)]` implied by `#[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;
    |         ^^^
    |
    = 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;
    |         ^
    |
    = 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;
    |         ^
    |
    = 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;
    |         ^
    |
    = 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;
    |         ^
    |
    = 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;
    |         ^
    |
    = 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;
    |         ^
    |
    = 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;
    |         ^
    |
    = 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");
    |             ^
    |
    = 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");
    |             ^
    |
    = 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");
    |             ^
    |
    = 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");
    |             ^
    |
    = 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();
    |             ^
    |
    = 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();
    |                 ^
    |
    = 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