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Auto merge of #70015 - jonas-schievink:gen-needs-drop, r=matthewjasper
Make `needs_drop` less pessimistic on generators Generators only have non-trivial drop logic when they may store (in upvars or across yields) a type that does. This prevents generation of some unnecessary MIR in simple generators. There might be some impact on compile times, but this is probably limited in real-world applications. ~~This builds off of #69814 since that contains some fixes that are made relevant by *this* PR (see #69814 (comment) (this has been merged)
2 parents e7497a8 + ae53315 commit 36b1a92

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6 files changed

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-44
lines changed

6 files changed

+85
-44
lines changed

‎src/librustc_middle/ty/util.rs

+3-5
Original file line numberDiff line numberDiff line change
@@ -1047,10 +1047,7 @@ pub fn needs_drop_components(
10471047
// Foreign types can never have destructors.
10481048
ty::Foreign(..) => Ok(SmallVec::new()),
10491049

1050-
// Pessimistically assume that all generators will require destructors
1051-
// as we don't know if a destructor is a noop or not until after the MIR
1052-
// state transformation pass.
1053-
ty::Generator(..) | ty::Dynamic(..) | ty::Error => Err(AlwaysRequiresDrop),
1050+
ty::Dynamic(..) | ty::Error => Err(AlwaysRequiresDrop),
10541051

10551052
ty::Slice(ty) => needs_drop_components(ty, target_layout),
10561053
ty::Array(elem_ty, size) => {
@@ -1083,7 +1080,8 @@ pub fn needs_drop_components(
10831080
| ty::Placeholder(..)
10841081
| ty::Opaque(..)
10851082
| ty::Infer(_)
1086-
| ty::Closure(..) => Ok(smallvec![ty]),
1083+
| ty::Closure(..)
1084+
| ty::Generator(..) => Ok(smallvec![ty]),
10871085
}
10881086
}
10891087

‎src/librustc_ty/needs_drop.rs

+17
Original file line numberDiff line numberDiff line change
@@ -99,6 +99,23 @@ where
9999
}
100100
}
101101

102+
ty::Generator(_, substs, _) => {
103+
let substs = substs.as_generator();
104+
for upvar_ty in substs.upvar_tys() {
105+
queue_type(self, upvar_ty);
106+
}
107+
108+
let witness = substs.witness();
109+
let interior_tys = match &witness.kind {
110+
ty::GeneratorWitness(tys) => tcx.erase_late_bound_regions(tys),
111+
_ => bug!(),
112+
};
113+
114+
for interior_ty in interior_tys {
115+
queue_type(self, interior_ty);
116+
}
117+
}
118+
102119
// Check for a `Drop` impl and whether this is a union or
103120
// `ManuallyDrop`. If it's a struct or enum without a `Drop`
104121
// impl then check whether the field types need `Drop`.

‎src/test/mir-opt/generator-drop-cleanup.rs

+3
Original file line numberDiff line numberDiff line change
@@ -1,11 +1,14 @@
11
#![feature(generators, generator_trait)]
22

3+
// ignore-wasm32-bare compiled with panic=abort by default
4+
35
// Regression test for #58892, generator drop shims should not have blocks
46
// spuriously marked as cleanup
57

68
// EMIT_MIR rustc.main-{{closure}}.generator_drop.0.mir
79
fn main() {
810
let gen = || {
11+
let _s = String::new();
912
yield;
1013
};
1114
}

‎src/test/mir-opt/generator-drop-cleanup/rustc.main-{{closure}}.generator_drop.0.mir

+53-26
Original file line numberDiff line numberDiff line change
@@ -1,53 +1,80 @@
11
// MIR for `main::{{closure}}#0` 0 generator_drop
2-
// generator_layout = GeneratorLayout { field_tys: [], variant_fields: [[], [], [], []], storage_conflicts: BitMatrix { num_rows: 0, num_columns: 0, words: [], marker: PhantomData } }
2+
// generator_layout = GeneratorLayout { field_tys: [std::string::String], variant_fields: [[], [], [], [_0]], storage_conflicts: BitMatrix { num_rows: 1, num_columns: 1, words: [1], marker: PhantomData } }
33

4-
fn main::{{closure}}#0(_1: *mut [generator@$DIR/generator-drop-cleanup.rs:8:15: 10:6 {()}]) -> () {
5-
let mut _0: (); // return place in scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
6-
let mut _2: (); // in scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
7-
let _3: (); // in scope 0 at $DIR/generator-drop-cleanup.rs:9:9: 9:14
8-
let mut _4: (); // in scope 0 at $DIR/generator-drop-cleanup.rs:9:9: 9:14
9-
let mut _5: (); // in scope 0 at $DIR/generator-drop-cleanup.rs:8:18: 8:18
10-
let mut _6: (); // in scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
11-
let mut _7: isize; // in scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
4+
fn main::{{closure}}#0(_1: *mut [generator@$DIR/generator-drop-cleanup.rs:10:15: 13:6 {std::string::String, ()}]) -> () {
5+
let mut _0: (); // return place in scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
6+
let mut _2: (); // in scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
7+
let _3: std::string::String; // in scope 0 at $DIR/generator-drop-cleanup.rs:11:13: 11:15
8+
let _4: (); // in scope 0 at $DIR/generator-drop-cleanup.rs:12:9: 12:14
9+
let mut _5: (); // in scope 0 at $DIR/generator-drop-cleanup.rs:12:9: 12:14
10+
let mut _7: (); // in scope 0 at $DIR/generator-drop-cleanup.rs:10:18: 10:18
11+
let mut _8: (); // in scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
12+
let mut _9: isize; // in scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
13+
scope 1 {
14+
debug _s => (((*_1) as variant#3).0: std::string::String); // in scope 1 at $DIR/generator-drop-cleanup.rs:11:13: 11:15
15+
}
16+
scope 2 {
17+
let mut _6: std::vec::Vec<u8>; // in scope 2 at $DIR/generator-drop-cleanup.rs:11:18: 11:31
18+
scope 3 {
19+
}
20+
}
1221

1322
bb0: {
14-
_7 = discriminant((*_1)); // bb0[0]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
15-
switchInt(move _7) -> [0u32: bb4, 3u32: bb7, otherwise: bb8]; // bb0[1]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
23+
_9 = discriminant((*_1)); // bb0[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
24+
switchInt(move _9) -> [0u32: bb7, 3u32: bb11, otherwise: bb12]; // bb0[1]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
1625
}
1726

18-
bb1: {
19-
StorageDead(_4); // bb1[0]: scope 0 at $DIR/generator-drop-cleanup.rs:9:13: 9:14
20-
StorageDead(_3); // bb1[1]: scope 0 at $DIR/generator-drop-cleanup.rs:9:14: 9:15
21-
goto -> bb5; // bb1[2]: scope 0 at $DIR/generator-drop-cleanup.rs:10:5: 10:6
27+
bb1 (cleanup): {
28+
resume; // bb1[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
2229
}
2330

24-
bb2: {
25-
return; // bb2[0]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
31+
bb2 (cleanup): {
32+
nop; // bb2[0]: scope 0 at $DIR/generator-drop-cleanup.rs:13:5: 13:6
33+
goto -> bb8; // bb2[1]: scope 0 at $DIR/generator-drop-cleanup.rs:13:5: 13:6
2634
}
2735

2836
bb3: {
29-
return; // bb3[0]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
37+
StorageDead(_5); // bb3[0]: scope 1 at $DIR/generator-drop-cleanup.rs:12:13: 12:14
38+
StorageDead(_4); // bb3[1]: scope 1 at $DIR/generator-drop-cleanup.rs:12:14: 12:15
39+
drop((((*_1) as variant#3).0: std::string::String)) -> [return: bb4, unwind: bb2]; // bb3[2]: scope 0 at $DIR/generator-drop-cleanup.rs:13:5: 13:6
3040
}
3141

3242
bb4: {
33-
goto -> bb6; // bb4[0]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
43+
nop; // bb4[0]: scope 0 at $DIR/generator-drop-cleanup.rs:13:5: 13:6
44+
goto -> bb9; // bb4[1]: scope 0 at $DIR/generator-drop-cleanup.rs:13:5: 13:6
3445
}
3546

3647
bb5: {
37-
goto -> bb2; // bb5[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:5: 10:6
48+
return; // bb5[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
3849
}
3950

4051
bb6: {
41-
goto -> bb3; // bb6[0]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
52+
return; // bb6[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
4253
}
4354

4455
bb7: {
45-
StorageLive(_3); // bb7[0]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
46-
StorageLive(_4); // bb7[1]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
47-
goto -> bb1; // bb7[2]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
56+
goto -> bb10; // bb7[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
57+
}
58+
59+
bb8 (cleanup): {
60+
goto -> bb1; // bb8[0]: scope 0 at $DIR/generator-drop-cleanup.rs:13:5: 13:6
61+
}
62+
63+
bb9: {
64+
goto -> bb5; // bb9[0]: scope 0 at $DIR/generator-drop-cleanup.rs:13:5: 13:6
65+
}
66+
67+
bb10: {
68+
goto -> bb6; // bb10[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
69+
}
70+
71+
bb11: {
72+
StorageLive(_4); // bb11[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
73+
StorageLive(_5); // bb11[1]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
74+
goto -> bb3; // bb11[2]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
4875
}
4976

50-
bb8: {
51-
return; // bb8[0]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
77+
bb12: {
78+
return; // bb12[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
5279
}
5380
}

‎src/test/ui/generator/borrowing.stderr

+6-9
Original file line numberDiff line numberDiff line change
@@ -1,19 +1,16 @@
11
error[E0597]: `a` does not live long enough
22
--> $DIR/borrowing.rs:9:33
33
|
4+
LL | let _b = {
5+
| -- borrow later stored here
6+
LL | let a = 3;
47
LL | Pin::new(&mut || yield &a).resume(())
5-
| ----------^
6-
| | |
7-
| | borrowed value does not live long enough
8+
| -- ^ borrowed value does not live long enough
9+
| |
810
| value captured here by generator
9-
| a temporary with access to the borrow is created here ...
1011
LL |
1112
LL | };
12-
| -- ... and the borrow might be used here, when that temporary is dropped and runs the destructor for generator
13-
| |
14-
| `a` dropped here while still borrowed
15-
|
16-
= note: The temporary is part of an expression at the end of a block. Consider forcing this temporary to be dropped sooner, before the block's local variables are dropped. For example, you could save the expression's value in a new local variable `x` and then make `x` be the expression at the end of the block.
13+
| - `a` dropped here while still borrowed
1714

1815
error[E0597]: `a` does not live long enough
1916
--> $DIR/borrowing.rs:16:20

‎src/test/ui/generator/retain-resume-ref.stderr

+3-4
Original file line numberDiff line numberDiff line change
@@ -4,10 +4,9 @@ error[E0499]: cannot borrow `thing` as mutable more than once at a time
44
LL | gen.as_mut().resume(&mut thing);
55
| ---------- first mutable borrow occurs here
66
LL | gen.as_mut().resume(&mut thing);
7-
| ^^^^^^^^^^ second mutable borrow occurs here
8-
LL |
9-
LL | }
10-
| - first borrow might be used here, when `gen` is dropped and runs the destructor for generator
7+
| ------ ^^^^^^^^^^ second mutable borrow occurs here
8+
| |
9+
| first borrow later used by call
1110

1211
error: aborting due to previous error
1312

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