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Make needs_drop less pessimistic on generators #70015

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Apr 19, 2020
Merged

Make needs_drop less pessimistic on generators #70015

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50c1c29
Make `needs_drop` less pessimistic on generators
jonas-schievink Mar 14, 2020
7bbdd2d
Adjust mir-opt test and make it drop something
jonas-schievink Mar 15, 2020
ae53315
Ignore generator-drop-cleanup on wasm32-bare
jonas-schievink Mar 25, 2020
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8 changes: 3 additions & 5 deletions src/librustc_middle/ty/util.rs
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Expand Up @@ -1047,10 +1047,7 @@ pub fn needs_drop_components(
// Foreign types can never have destructors.
ty::Foreign(..) => Ok(SmallVec::new()),

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

ty::Slice(ty) => needs_drop_components(ty, target_layout),
ty::Array(elem_ty, size) => {
Expand Down Expand Up @@ -1083,7 +1080,8 @@ pub fn needs_drop_components(
| ty::Placeholder(..)
| ty::Opaque(..)
| ty::Infer(_)
| ty::Closure(..) => Ok(smallvec![ty]),
| ty::Closure(..)
| ty::Generator(..) => Ok(smallvec![ty]),
}
}

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17 changes: 17 additions & 0 deletions src/librustc_ty/needs_drop.rs
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Expand Up @@ -99,6 +99,23 @@ where
}
}

ty::Generator(_, substs, _) => {
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Do we have assertions somewhere that could catch bugs in needs_drop (i.e., catch cases where a type has drop glue but needs_drop says no)?

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Not that I know of, but we have several mir-opt tests for generators, as well as diagnostics tests, which were (correctly) affected by this change.

let substs = substs.as_generator();
for upvar_ty in substs.upvar_tys() {
queue_type(self, upvar_ty);
}

let witness = substs.witness();
let interior_tys = match &witness.kind {
ty::GeneratorWitness(tys) => tcx.erase_late_bound_regions(tys),
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Not entirely sure the erase_late_bound_regions is completely correct here

_ => bug!(),
};

for interior_ty in interior_tys {
queue_type(self, interior_ty);
}
}

// Check for a `Drop` impl and whether this is a union or
// `ManuallyDrop`. If it's a struct or enum without a `Drop`
// impl then check whether the field types need `Drop`.
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3 changes: 3 additions & 0 deletions src/test/mir-opt/generator-drop-cleanup.rs
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@@ -1,11 +1,14 @@
#![feature(generators, generator_trait)]

// ignore-wasm32-bare compiled with panic=abort by default

// Regression test for #58892, generator drop shims should not have blocks
// spuriously marked as cleanup

// EMIT_MIR rustc.main-{{closure}}.generator_drop.0.mir
fn main() {
let gen = || {
let _s = String::new();
yield;
};
}
79 changes: 53 additions & 26 deletions src/test/mir-opt/generator-drop-cleanup/rustc.main-{{closure}}.generator_drop.0.mir
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@@ -1,53 +1,80 @@
// MIR for `main::{{closure}}#0` 0 generator_drop
// generator_layout = GeneratorLayout { field_tys: [], variant_fields: [[], [], [], []], storage_conflicts: BitMatrix { num_rows: 0, num_columns: 0, words: [], marker: PhantomData } }
// generator_layout = GeneratorLayout { field_tys: [std::string::String], variant_fields: [[], [], [], [_0]], storage_conflicts: BitMatrix { num_rows: 1, num_columns: 1, words: [1], marker: PhantomData } }

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

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

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

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

bb3: {
return; // bb3[0]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
StorageDead(_5); // bb3[0]: scope 1 at $DIR/generator-drop-cleanup.rs:12:13: 12:14
StorageDead(_4); // bb3[1]: scope 1 at $DIR/generator-drop-cleanup.rs:12:14: 12:15
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
}

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

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

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

bb7: {
StorageLive(_3); // bb7[0]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
StorageLive(_4); // bb7[1]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
goto -> bb1; // bb7[2]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
goto -> bb10; // bb7[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
}

bb8 (cleanup): {
goto -> bb1; // bb8[0]: scope 0 at $DIR/generator-drop-cleanup.rs:13:5: 13:6
}

bb9: {
goto -> bb5; // bb9[0]: scope 0 at $DIR/generator-drop-cleanup.rs:13:5: 13:6
}

bb10: {
goto -> bb6; // bb10[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
}

bb11: {
StorageLive(_4); // bb11[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
StorageLive(_5); // bb11[1]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
goto -> bb3; // bb11[2]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
}

bb8: {
return; // bb8[0]: scope 0 at $DIR/generator-drop-cleanup.rs:8:15: 10:6
bb12: {
return; // bb12[0]: scope 0 at $DIR/generator-drop-cleanup.rs:10:15: 13:6
}
}
15 changes: 6 additions & 9 deletions src/test/ui/generator/borrowing.stderr
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@@ -1,19 +1,16 @@
error[E0597]: `a` does not live long enough
--> $DIR/borrowing.rs:9:33
|
LL | let _b = {
| -- borrow later stored here
LL | let a = 3;
LL | Pin::new(&mut || yield &a).resume(())
| ----------^
| | |
| | borrowed value does not live long enough
| -- ^ borrowed value does not live long enough
| |
| value captured here by generator
| a temporary with access to the borrow is created here ...
LL |
LL | };
| -- ... and the borrow might be used here, when that temporary is dropped and runs the destructor for generator
| |
| `a` dropped here while still borrowed
|
= 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.
| - `a` dropped here while still borrowed

error[E0597]: `a` does not live long enough
--> $DIR/borrowing.rs:16:20
Expand Down
7 changes: 3 additions & 4 deletions src/test/ui/generator/retain-resume-ref.stderr
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Expand Up @@ -4,10 +4,9 @@ error[E0499]: cannot borrow `thing` as mutable more than once at a time
LL | gen.as_mut().resume(&mut thing);
| ---------- first mutable borrow occurs here
LL | gen.as_mut().resume(&mut thing);
| ^^^^^^^^^^ second mutable borrow occurs here
LL |
LL | }
| - first borrow might be used here, when `gen` is dropped and runs the destructor for generator
| ------ ^^^^^^^^^^ second mutable borrow occurs here
| |
| first borrow later used by call

error: aborting due to previous error

Expand Down