elaborate: avoid projections with unconstrained bound regions #136303

lcnr · 2025-01-30T15:58:56Z

Should only do this for where-bounds and item bounds. Gonna rewrite later I think.

This means we'll no longer need to support unconstrained regions in implied bounds, see lcnr/random-rust-snippets#15. This fixes

trait Super {
    type SAssoc;
}
trait Trait<'a>: Super<SAssoc = <Self as Trait<'a>>::TAssoc> {
    type TAssoc;
}

fn test<'a, T: 'a>() {}

fn unconstrained_lt<'arg, T: for<'a> Trait<'a>>(x: &'arg <T as Super>::SAssoc) {
    test::<'arg, <T as Super>::SAssoc>();
}

but breaks

trait Super {
    type SAssoc;
}
trait Trait<'a>: Super<SAssoc = <Self as Trait<'a>>::TAssoc> {
    type TAssoc;
}

fn test<'a, T: 'a>() {}

fn unconstrained_lt<'arg, T: for<'a> Trait<'a, TAssoc = usize>>(x: <T as Super>::SAssoc) -> usize {
    x
}

r? @compiler-errors

lcnr · 2025-01-30T16:14:31Z

@bors try

bors · 2025-01-30T16:15:43Z

⌛ Trying commit d6f2c89 with merge a3dffdb...

… r=<try> elaborate: avoid projections with unconstrained bound regions Should only do this for where-bounds and item bounds. Gonna rewrite later I think. This means we'll no longer need to support unconstrained regions in implied bounds, see lcnr/random-rust-snippets#15. This fixes ```rust trait Super { type SAssoc; } trait Trait<'a>: Super<SAssoc = <Self as Trait<'a>>::TAssoc> { type TAssoc; } fn test<'a, T: 'a>() {} fn unconstrained_lt<'arg, T: for<'a> Trait<'a>>(x: &'arg <T as Super>::SAssoc) { test::<'arg, <T as Super>::SAssoc>(); } ``` but breaks ```rust trait Super { type SAssoc; } trait Trait<'a>: Super<SAssoc = <Self as Trait<'a>>::TAssoc> { type TAssoc; } fn test<'a, T: 'a>() {} fn unconstrained_lt<'arg, T: for<'a> Trait<'a, TAssoc = usize>>(x: <T as Super>::SAssoc) -> usize { x } ``` r? `@compiler-errors`

rust-log-analyzer · 2025-01-30T16:24:39Z

The job x86_64-gnu-llvm-18 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

#21 exporting to docker image format
#21 sending tarball 28.4s done
#21 DONE 43.5s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---
Saved the actual stderr to "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/nll/issue-54189/issue-54189.stderr"
diff of stderr:

12    |
13    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
- error: aborting due to 2 previous errors
+ error[E0308]: mismatched types
+   --> $DIR/issue-54189.rs:1:44
+    |
+    |
+ LL | fn bug() -> impl for <'r> Fn() -> &'r () { || { &() } }
+    |                                            ^^^^^^^^^^ one type is more general than the other
+    = note: expected reference `&()`
+    = note: expected reference `&()`
+               found reference `&'r ()`
- For more information about this error, try `rustc --explain E0582`.
+ error: aborting due to 3 previous errors
+ 
+ Some errors have detailed explanations: E0308, E0582.
---
-   --> /checkout/tests/ui/nll/issue-54189.rs:1:44
+ error[E0308]: mismatched types
+   --> $DIR/issue-54189.rs:1:44
+    |
+ LL | fn bug() -> impl for <'r> Fn() -> &'r () { || { &() } }
+    |                                            ^^^^^^^^^^ one type is more general than the other
+    = note: expected reference `&()`
+    = note: expected reference `&()`
+               found reference `&'r ()`
+ 
+ Some errors have detailed explanations: E0308, E0582.
+ For more information about an error, try `rustc --explain E0308`.



The actual stderr differed from the expected stderr.
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args nll/issue-54189.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/nll/issue-54189.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/nll/issue-54189" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
--- stderr -------------------------------
--- stderr -------------------------------
error[E0582]: binding for associated type `Output` references lifetime `'r`, which does not appear in the trait input types
   |
   |
LL | fn bug() -> impl for <'r> Fn() -> &'r () { || { &() } }


error[E0582]: binding for associated type `Output` references lifetime `'r`, which does not appear in the trait input types
   |
   |
LL | fn bug() -> impl for <'r> Fn() -> &'r () { || { &() } }
   |
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/nll/issue-54189.rs:1:44
   |
   |
LL | fn bug() -> impl for <'r> Fn() -> &'r () { || { &() } }
   |                                            ^^^^^^^^^^ one type is more general than the other
   = note: expected reference `&()`
   = note: expected reference `&()`
              found reference `&'r ()`
error: aborting due to 3 previous errors

Some errors have detailed explanations: E0308, E0582.
For more information about an error, try `rustc --explain E0308`.
For more information about an error, try `rustc --explain E0308`.
------------------------------------------


---- [ui] tests/ui/traits/object/elaborated-predicates-unconstrained-late-bound.rs stdout ----
Saved the actual stderr to "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/traits/object/elaborated-predicates-unconstrained-late-bound/elaborated-predicates-unconstrained-late-bound.stderr"
diff of stderr:

- error[E0582]: binding for associated type `T` references lifetime `'a`, which does not appear in the trait input types
+ error[E0191]: the value of the associated type `T` in `B` must be specified
3    |
3    |
- LL | trait A<T>: B<T = T> {}
-    |               ----- due to this supertrait
+ LL |     type T;
+    |     ------ `T` defined here
6 ...
7 LL |         Erase::<dyn for<'a> A<&'a _>>(x.as_str())
-    |                     ^^^^^^^^^^^^^^^^
+    |                     ^^^^^^^^^^^^^^^^ help: specify the associated type: `for<'a> A<&'a _, T = Type>`
10 error: aborting due to 1 previous error
11 

- For more information about this error, try `rustc --explain E0582`.
---
To only update this specific test, also pass `--test-args traits/object/elaborated-predicates-unconstrained-late-bound.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/traits/object/elaborated-predicates-unconstrained-late-bound.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/traits/object/elaborated-predicates-unconstrained-late-bound" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
--- stderr -------------------------------
error[E0191]: the value of the associated type `T` in `B` must be specified
##[error]  --> /checkout/tests/ui/traits/object/elaborated-predicates-unconstrained-late-bound.rs:19:21
   |
   |
LL |     type T;
   |     ------ `T` defined here
...
LL |         Erase::<dyn for<'a> A<&'a _>>(x.as_str())
   |                     ^^^^^^^^^^^^^^^^ help: specify the associated type: `for<'a> A<&'a _, T = Type>`
error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0191`.
------------------------------------------
------------------------------------------


---- [ui] tests/ui/wf/ice-wf-missing-span-in-error-130012.rs stdout ----
Saved the actual stderr to "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/wf/ice-wf-missing-span-in-error-130012/ice-wf-missing-span-in-error-130012.stderr"


26 LL | impl<F: for<'b> Fun<Assoc = &'b ()>> MyTrait for F {}
28 
- error[E0308]: mismatched types
- error[E0308]: mismatched types
+ error[E0271]: type mismatch resolving `<Self as Fun>::Assoc == &()`
+    = note:    expected reference `&()`
+            found associated type `<Self as Fun>::Assoc`
+            found associated type `<Self as Fun>::Assoc`
+    = help: consider constraining the associated type `<Self as Fun>::Assoc` to `&()`
+ note: required by a bound in `MyTrait`
+   --> $DIR/ice-wf-missing-span-in-error-130012.rs:9:28
+    |
+    |
+ LL | trait MyTrait: for<'a> Fun<Assoc = &'a ()> {}
+    |                            ^^^^^^^^^^^^^^ required by this bound in `MyTrait`
+ 
+ error[E0271]: type mismatch resolving `<F as Fun>::Assoc == &()`
31    |
31    |
32 LL | impl<F: for<'b> Fun<Assoc = &'b ()>> MyTrait for F {}
-    |                                                  ^ lifetime mismatch
+    |                                                  ^ expected `&()`, found associated type
34    |
-    = note: expected reference `&()`
-    = note: expected reference `&()`
-               found reference `&'b ()`
+    = note:    expected reference `&()`
+            found associated type `<F as Fun>::Assoc`
+ note: required for `F` to implement `MyTrait`
+    |
+    |
+ LL | impl<F: for<'b> Fun<Assoc = &'b ()>> MyTrait for F {}
+    |                     |
+    |                     unsatisfied trait bound introduced here
+    |                     unsatisfied trait bound introduced here
+ help: consider constraining the associated type `<F as Fun>::Assoc` to `&()`
+    |
+ LL | impl<F: for<'b> Fun<Assoc = &'b (), Assoc = &()>> MyTrait for F {}
37 
- error: aborting due to 5 previous errors
+ error: aborting due to 6 previous errors
39 
---

Note: some mismatched output was normalized before being compared
-   --> /checkout/tests/ui/wf/ice-wf-missing-span-in-error-130012.rs:9:28
-   --> /checkout/tests/ui/wf/ice-wf-missing-span-in-error-130012.rs:14:38
+ error[E0271]: type mismatch resolving `<Self as Fun>::Assoc == &()`
+    = note:    expected reference `&()`
+            found associated type `<Self as Fun>::Assoc`
+            found associated type `<Self as Fun>::Assoc`
+    = help: consider constraining the associated type `<Self as Fun>::Assoc` to `&()`
+ note: required by a bound in `MyTrait`
+   --> $DIR/ice-wf-missing-span-in-error-130012.rs:9:28
+    |
+    |
+ LL | trait MyTrait: for<'a> Fun<Assoc = &'a ()> {}
+    |                            ^^^^^^^^^^^^^^ required by this bound in `MyTrait`
+ 
+ error[E0271]: type mismatch resolving `<F as Fun>::Assoc == &()`
+    |                                                  ^ expected `&()`, found associated type
+    = note:    expected reference `&()`
+            found associated type `<F as Fun>::Assoc`
+ note: required for `F` to implement `MyTrait`
+    |
+    |
+ LL | impl<F: for<'b> Fun<Assoc = &'b ()>> MyTrait for F {}
+    |                     |
+    |                     unsatisfied trait bound introduced here
+    |                     unsatisfied trait bound introduced here
+ help: consider constraining the associated type `<F as Fun>::Assoc` to `&()`
+    |
+ LL | impl<F: for<'b> Fun<Assoc = &'b (), Assoc = &()>> MyTrait for F {}
+ error: aborting due to 6 previous errors
+ Some errors have detailed explanations: E0271, E0582.
+ For more information about an error, try `rustc --explain E0271`.



The actual stderr differed from the expected stderr.
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args wf/ice-wf-missing-span-in-error-130012.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/wf/ice-wf-missing-span-in-error-130012.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/wf/ice-wf-missing-span-in-error-130012" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
--- stderr -------------------------------
--- stderr -------------------------------
error[E0582]: binding for associated type `Assoc` references lifetime `'a`, which does not appear in the trait input types
   |
   |
LL | trait MyTrait: for<'a> Fun<Assoc = &'a ()> {}


error[E0582]: binding for associated type `Assoc` references lifetime `'a`, which does not appear in the trait input types
   |
   |
LL | trait MyTrait: for<'a> Fun<Assoc = &'a ()> {}
   |
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0582]: binding for associated type `Assoc` references lifetime `'a`, which does not appear in the trait input types
   |
   |
LL | trait MyTrait: for<'a> Fun<Assoc = &'a ()> {}
   |
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0582]: binding for associated type `Assoc` references lifetime `'b`, which does not appear in the trait input types
   |
   |
LL | impl<F: for<'b> Fun<Assoc = &'b ()>> MyTrait for F {}


error[E0271]: type mismatch resolving `<Self as Fun>::Assoc == &()`
   = note:    expected reference `&()`
           found associated type `<Self as Fun>::Assoc`
           found associated type `<Self as Fun>::Assoc`
   = help: consider constraining the associated type `<Self as Fun>::Assoc` to `&()`
note: required by a bound in `MyTrait`
  --> /checkout/tests/ui/wf/ice-wf-missing-span-in-error-130012.rs:9:28
   |
   |
LL | trait MyTrait: for<'a> Fun<Assoc = &'a ()> {}
   |                            ^^^^^^^^^^^^^^ required by this bound in `MyTrait`

error[E0271]: type mismatch resolving `<F as Fun>::Assoc == &()`
   |
   |
LL | impl<F: for<'b> Fun<Assoc = &'b ()>> MyTrait for F {}
   |                                                  ^ expected `&()`, found associated type
   = note:    expected reference `&()`
   = note:    expected reference `&()`
           found associated type `<F as Fun>::Assoc`
note: required for `F` to implement `MyTrait`
   |
   |
LL | impl<F: for<'b> Fun<Assoc = &'b ()>> MyTrait for F {}
   |                     |
   |                     unsatisfied trait bound introduced here
   |                     unsatisfied trait bound introduced here
help: consider constraining the associated type `<F as Fun>::Assoc` to `&()`
   |
LL | impl<F: for<'b> Fun<Assoc = &'b (), Assoc = &()>> MyTrait for F {}

error: aborting due to 6 previous errors

Some errors have detailed explanations: E0271, E0582.

bors · 2025-01-30T18:06:09Z

☀️ Try build successful - checks-actions
Build commit: a3dffdb (a3dffdb3a32d0dedc198f9de7d43173f3eb6727b)

lcnr · 2025-01-31T14:03:12Z

@craterbot check

craterbot · 2025-01-31T14:03:30Z

👌 Experiment pr-136303 created and queued.
🤖 Automatically detected try build a3dffdb
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-01-31T14:03:34Z

🚧 Experiment pr-136303 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-02-01T10:13:40Z

🎉 Experiment pr-136303 is completed!
📊 209 regressed and 2 fixed (576018 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

lcnr · 2025-02-03T12:41:34Z

there are at least 3 actual regressions here:

gat-lending-iterator
gluon_salsa
polars-core

That makes me really unhappy :/

compiler-errors · 2025-02-06T18:04:18Z

@rustbot author -- should we close this now that we decided that we're gonna allow these projections? 🤔

lcnr added 3 commits January 30, 2025 16:37

move limit to rustc_ast_ir

31ef33b

move folders to rustc_type_ir

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elaborate: skip projection with unconstrained vars

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rustbot assigned compiler-errors Jan 30, 2025

rustbot added S-waiting-on-review T-compiler WG-trait-system-refactor labels Jan 30, 2025

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lcnr mentioned this pull request Feb 4, 2025

super trait bounds can result in unconstrained regions #136547

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elaborate: avoid projections with unconstrained bound regions #136303

elaborate: avoid projections with unconstrained bound regions #136303

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elaborate: avoid projections with unconstrained bound regions #136303

elaborate: avoid projections with unconstrained bound regions #136303

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