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Add a new lint unneeded_where_clauses, suggests removing empty where clauses #125277

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Add a new lint unneeded_where_clauses, suggests removing empty where clauses #125277

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mu001999
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Fixes #125242

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rustbot commented May 19, 2024

r? @wesleywiser

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@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels May 19, 2024
@cjgillot
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Would this be better placed in rustfmt?

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The job x86_64-gnu-llvm-17 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot) 
#16 exporting to docker image format
#16 sending tarball 29.5s done
#16 DONE 34.4s
##[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-17]
---
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-17', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--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', '--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-17/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999
---
normalized stderr:
warning: this empty where clause is unneeded
##[warning]  --> $DIR/issue-93341.rs:11:23
   |
LL | pub trait HasLifetime where {
   |
   = note: `#[warn(unneeded_where_clauses)]` on by default

warning: 1 warning emitted
---
To only update this specific test, also pass `--test-args generic-associated-types/issue-93341.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
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/generic-associated-types/issue-93341.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(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/generic-associated-types/issue-93341" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/generic-associated-types/issue-93341/auxiliary"
--- stderr -------------------------------
warning: this empty where clause is unneeded
##[warning]  --> /checkout/tests/ui/generic-associated-types/issue-93341.rs:11:23
   |
   |
LL | pub trait HasLifetime where {
   |
   = note: `#[warn(unneeded_where_clauses)]` on by default

warning: 1 warning emitted
---

+ warning: this empty where clause is unneeded
+   --> $DIR/issue-55731.rs:17:86
+    |
+ LL | impl<I: for<'a> DistributedIteratorMulti<&'a ()>> DistributedIterator for Connect<I> where {}
+    |
+    = note: `#[warn(unneeded_where_clauses)]` on by default
+ 
+ 
1 error: implementation of `DistributedIteratorMulti` is not general enough
3    |


10    = note: `DistributedIteratorMulti<&'0 ()>` would have to be implemented for the type `Cloned<&()>`, for any lifetime `'0`...
11    = note: ...but `DistributedIteratorMulti<&'1 ()>` is actually implemented for the type `Cloned<&'1 ()>`, for some specific lifetime `'1`
- error: aborting due to 1 previous error
+ error: aborting due to 1 previous error; 1 warning emitted
14 
15 
---
To only update this specific test, also pass `--test-args issues/issue-55731.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/issues/issue-55731.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(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/issues/issue-55731" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/issues/issue-55731/auxiliary"
--- stderr -------------------------------
warning: this empty where clause is unneeded
##[warning]  --> /checkout/tests/ui/issues/issue-55731.rs:17:86
   |
   |
LL | impl<I: for<'a> DistributedIteratorMulti<&'a ()>> DistributedIterator for Connect<I> where {}
   |
   = note: `#[warn(unneeded_where_clauses)]` on by default


error: implementation of `DistributedIteratorMulti` is not general enough
   |
   |
LL | /     multi(Map { //~ ERROR implementation of `DistributedIteratorMulti` is not general enough
LL | |         i: Cloned(PhantomData),
LL | |         f: X,
LL | |     });
   | |______^ implementation of `DistributedIteratorMulti` is not general enough
   |
   = note: `DistributedIteratorMulti<&'0 ()>` would have to be implemented for the type `Cloned<&()>`, for any lifetime `'0`...
   = note: ...but `DistributedIteratorMulti<&'1 ()>` is actually implemented for the type `Cloned<&'1 ()>`, for some specific lifetime `'1`
error: aborting due to 1 previous error; 1 warning emitted
------------------------------------------


---
To only update this specific test, also pass `--test-args lazy-type-alias/leading-where-clause.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/lazy-type-alias/leading-where-clause.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(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/lazy-type-alias/leading-where-clause" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/lazy-type-alias/leading-where-clause/auxiliary"
--- stderr -------------------------------
error: where clauses are not allowed before the type for type aliases
##[error]  --> /checkout/tests/ui/lazy-type-alias/leading-where-clause.rs:10:1
   |
   |
LL | / where //~ ERROR where clauses are not allowed before the type for type aliases
LL | |     String: From<T>,
   |
   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
help: move it to the end of the type declaration
   |
   |
LL + 
LL ~ = T where String: From<T>;

error: where clauses are not allowed before the type for type aliases
##[error]  --> /checkout/tests/ui/lazy-type-alias/leading-where-clause.rs:15:1
   |
   |
LL | / where //~ ERROR where clauses are not allowed before the type for type aliases
LL | |     String: From<T>,
   |
   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
help: move it to the end of the type declaration
   |
   |
LL + 
LL | = (T, U)
LL | where
LL ~     U: Copy, String: From<T>;

error: where clauses are not allowed before the type for type aliases
##[error]  --> /checkout/tests/ui/lazy-type-alias/leading-where-clause.rs:21:24
   |
   |
LL | pub type EmptyLeading0 where = ();
   |                        ^^^^^
   |
   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
help: move it to the end of the type declaration
   |
LL - pub type EmptyLeading0 where = ();
LL + pub type EmptyLeading0  = () where;

error: where clauses are not allowed before the type for type aliases
##[error]  --> /checkout/tests/ui/lazy-type-alias/leading-where-clause.rs:24:27
   |
   |
LL | pub type EmptyLeading1<T> where = T where T: Copy;
   |                           ^^^^^ help: remove this `where`
   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information

warning: this empty where clause is unneeded
   |
---

+ warning: this empty where clause is unneeded
+   --> $DIR/use_of_moved_value_copy_suggestions.rs:43:51
+    |
+ LL | fn duplicate_custom_1<T>(t: S<T>) -> (S<T>, S<T>) where {
+    |
+    = note: `#[warn(unneeded_where_clauses)]` on by default
+ 
1 error[E0382]: use of moved value: `t`
1 error[E0382]: use of moved value: `t`
2   --> $DIR/use_of_moved_value_copy_suggestions.rs:7:9
3    |

198 LL |     T:, T: Copy
200 
- error: aborting due to 11 previous errors
+ error: aborting due to 11 previous errors; 1 warning emitted
202 
---
To only update this specific test, also pass `--test-args moves/use_of_moved_value_copy_suggestions.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/moves/use_of_moved_value_copy_suggestions.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(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/moves/use_of_moved_value_copy_suggestions" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/moves/use_of_moved_value_copy_suggestions/auxiliary"
--- stderr -------------------------------
warning: this empty where clause is unneeded
##[warning]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:43:51
   |
   |
LL | fn duplicate_custom_1<T>(t: S<T>) -> (S<T>, S<T>) where {
   |
   = note: `#[warn(unneeded_where_clauses)]` on by default

error[E0382]: use of moved value: `t`
error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:7:9
   |
LL | fn duplicate_t<T>(t: T) -> (T, T) {
   |                   - move occurs because `t` has type `T`, which does not implement the `Copy` trait
...
LL |     (t, t) //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
   |
help: if `T` implemented `Clone`, you could clone the value
   |
   |
LL | fn duplicate_t<T>(t: T) -> (T, T) {
   |                ^ consider constraining this type parameter with `Clone`
...
LL |     (t, t) //~ use of moved value: `t`
   |      - you could clone this value
help: consider restricting type parameter `T`
   |
LL | fn duplicate_t<T: Copy>(t: T) -> (T, T) {

error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:12:9
   |
   |
LL | fn duplicate_opt<T>(t: Option<T>) -> (Option<T>, Option<T>) {
   |                     - move occurs because `t` has type `Option<T>`, which does not implement the `Copy` trait
LL |     //~^ HELP consider restricting type parameter `T`
LL |     (t, t) //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
help: consider restricting type parameter `T`
   |
   |
LL | fn duplicate_opt<T: Copy>(t: Option<T>) -> (Option<T>, Option<T>) {

error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:17:9
   |
   |
LL | fn duplicate_tup1<T>(t: (T,)) -> ((T,), (T,)) {
   |                      - move occurs because `t` has type `(T,)`, which does not implement the `Copy` trait
LL |     //~^ HELP consider restricting type parameter `T`
LL |     (t, t) //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
help: consider restricting type parameter `T`
   |
   |
LL | fn duplicate_tup1<T: Copy>(t: (T,)) -> ((T,), (T,)) {

error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:22:9
   |
   |
LL | fn duplicate_tup2<A, B>(t: (A, B)) -> ((A, B), (A, B)) {
   |                         - move occurs because `t` has type `(A, B)`, which does not implement the `Copy` trait
LL |     //~^ HELP consider restricting type parameters
LL |     (t, t) //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
help: consider restricting type parameters
   |
   |
LL | fn duplicate_tup2<A: Copy, B: Copy>(t: (A, B)) -> ((A, B), (A, B)) {
   |                    ++++++   ++++++
error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:27:9
   |
   |
LL | fn duplicate_custom<T>(t: S<T>) -> (S<T>, S<T>) {
   |                        - move occurs because `t` has type `S<T>`, which does not implement the `Copy` trait
LL |     //~^ HELP consider restricting type parameter `T`
LL |     (t, t) //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
help: consider restricting type parameter `T`
   |
   |
LL | fn duplicate_custom<T: Copy + Trait>(t: S<T>) -> (S<T>, S<T>) {

error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:45:9
   |
   |
LL | fn duplicate_custom_1<T>(t: S<T>) -> (S<T>, S<T>) where {
   |                          - move occurs because `t` has type `S<T>`, which does not implement the `Copy` trait
LL |     //~^ HELP consider restricting type parameter `T`
LL |     (t, t) //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
help: consider restricting type parameter `T`
   |
   |
LL | fn duplicate_custom_1<T: Copy + Trait>(t: S<T>) -> (S<T>, S<T>) where {

error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:53:9
   |
   |
LL | fn duplicate_custom_2<T>(t: S<T>) -> (S<T>, S<T>)
   |                          - move occurs because `t` has type `S<T>`, which does not implement the `Copy` trait
...
LL |     (t, t) //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
help: consider further restricting this bound
   |
   |
LL |     T: A + Copy + Trait,

error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:62:9
   |
   |
LL | fn duplicate_custom_3<T>(t: S<T>) -> (S<T>, S<T>)
   |                          - move occurs because `t` has type `S<T>`, which does not implement the `Copy` trait
...
LL |     (t, t) //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
help: consider further restricting this bound
   |
   |
LL |     T: A + Copy + Trait,

error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:70:9
   |
   |
LL | fn duplicate_custom_4<T: A>(t: S<T>) -> (S<T>, S<T>)
   |                             - move occurs because `t` has type `S<T>`, which does not implement the `Copy` trait
...
LL |     (t, t) //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
help: consider further restricting this bound
   |
   |
LL | fn duplicate_custom_4<T: A + Copy + Trait>(t: S<T>) -> (S<T>, S<T>)

error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:77:9
   |
   |
LL | fn existing_colon<T:>(t: T) {
   |                       - move occurs because `t` has type `T`, which does not implement the `Copy` trait
...
LL |     [t, t]; //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
   |
help: if `T` implemented `Clone`, you could clone the value
   |
   |
LL | fn existing_colon<T:>(t: T) {
   |                   ^ consider constraining this type parameter with `Clone`
...
LL |     [t, t]; //~ use of moved value: `t`
   |      - you could clone this value
help: consider restricting type parameter `T`
   |
LL | fn existing_colon<T: Copy>(t: T) {

error[E0382]: use of moved value: `t`
##[error]  --> /checkout/tests/ui/moves/use_of_moved_value_copy_suggestions.rs:85:9
   |
   |
LL | fn existing_colon_in_where<T>(t: T) //~ HELP if `T` implemented `Clone`, you could clone the value
   |                               - move occurs because `t` has type `T`, which does not implement the `Copy` trait
...
LL |     [t, t]; //~ use of moved value: `t`
   |      -  ^ value used here after move
   |      value moved here
   |
   |
help: if `T` implemented `Clone`, you could clone the value
   |
   |
LL | fn existing_colon_in_where<T>(t: T) //~ HELP if `T` implemented `Clone`, you could clone the value
   |                            ^ consider constraining this type parameter with `Clone`
...
LL |     [t, t]; //~ use of moved value: `t`
   |      - you could clone this value
help: consider further restricting type parameter `T`
   |
LL |     T:, T: Copy

error: aborting due to 11 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0382`.
---
To only update this specific test, also pass `--test-args parser/foreign-ty-semantic-fail.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/parser/foreign-ty-semantic-fail.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(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/parser/foreign-ty-semantic-fail" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/parser/foreign-ty-semantic-fail/auxiliary"
--- stderr -------------------------------
--- stderr -------------------------------
error: bounds on `type`s in `extern` blocks have no effect
   |
   |
LL |     type A: Ord;


error: `type`s inside `extern` blocks cannot have generic parameters
   |
LL | extern "C" {
LL | extern "C" {
   | ---------- `extern` block begins here
...
LL |     type B<'a> where 'a: 'static;
   |
   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html


error: `type`s inside `extern` blocks cannot have `where` clauses
   |
LL | extern "C" {
LL | extern "C" {
   | ---------- `extern` block begins here
...
LL |     type B<'a> where 'a: 'static;
   |                ^^^^^^^^^^^^^^^^^ help: remove the `where` clause
   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html


error: `type`s inside `extern` blocks cannot have generic parameters
   |
LL | extern "C" {
LL | extern "C" {
   | ---------- `extern` block begins here
...
LL |     type C<T: Ord> where T: 'static;
   |
   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html


error: `type`s inside `extern` blocks cannot have `where` clauses
   |
LL | extern "C" {
LL | extern "C" {
   | ---------- `extern` block begins here
...
LL |     type C<T: Ord> where T: 'static;
   |                    ^^^^^^^^^^^^^^^^ help: remove the `where` clause
   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html

error: incorrect `type` inside `extern` block
##[error]  --> /checkout/tests/ui/parser/foreign-ty-semantic-fail.rs:14:10
##[error]  --> /checkout/tests/ui/parser/foreign-ty-semantic-fail.rs:14:10
   |
LL | extern "C" {
   | ---------- `extern` blocks define existing foreign types and types inside of them cannot have a body
LL |     type D = u8;
   |          ^   -- the invalid body
   |          |
   |          cannot have a body
   |          cannot have a body
   |
   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html

error: `type`s inside `extern` blocks cannot have `where` clauses
   |
LL | extern "C" {
LL | extern "C" {
   | ---------- `extern` block begins here
LL |     type E: where;
LL |     type E: where;
   |             ^^^^^ help: remove the `where` clause
   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html

warning: this empty where clause is unneeded
   |
---

+ warning: this empty where clause is unneeded
+   --> $DIR/partialeq_help.rs:5:36
+    |
+ LL | fn foo2<T: PartialEq>(a: &T, b: T) where {
+    |
+    = note: `#[warn(unneeded_where_clauses)]` on by default
+ 
+ 
1 error[E0277]: can't compare `&T` with `T`
3    |


30 LL | fn foo2<T: PartialEq>(a: &T, b: T) where &T: PartialEq<T> {
32 
- error: aborting due to 2 previous errors
+ error: aborting due to 2 previous errors; 1 warning emitted
34 
---
To only update this specific test, also pass `--test-args partialeq_help.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/partialeq_help.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(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/partialeq_help" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/partialeq_help/auxiliary"
--- stderr -------------------------------
warning: this empty where clause is unneeded
##[warning]  --> /checkout/tests/ui/partialeq_help.rs:5:36
   |
   |
LL | fn foo2<T: PartialEq>(a: &T, b: T) where {
   |
   = note: `#[warn(unneeded_where_clauses)]` on by default


error[E0277]: can't compare `&T` with `T`
   |
   |
LL |     a == b; //~ ERROR E0277
   |       ^^ no implementation for `&T == T`
   |
   = help: the trait `PartialEq<T>` is not implemented for `&T`
   |
   |
LL |     *a == b; //~ ERROR E0277
   |     +
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
   |
LL | fn foo<T: PartialEq>(a: &T, b: T) where &T: PartialEq<T> {


error[E0277]: can't compare `&T` with `T`
   |
   |
LL |     a == b; //~ ERROR E0277
   |       ^^ no implementation for `&T == T`
   |
   = help: the trait `PartialEq<T>` is not implemented for `&T`
   |
   |
LL |     *a == b; //~ ERROR E0277
   |     +
help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
   |
LL | fn foo2<T: PartialEq>(a: &T, b: T) where &T: PartialEq<T> {

error: aborting due to 2 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0277`.
---
+ 
+ warning: this empty where clause is unneeded
+   --> $DIR/empty-where-clause.rs:12:34
+    |
+ LL | struct TupleStruct(MissingType2) where;
+ 
+ warning: this empty where clause is unneeded
+   --> $DIR/empty-where-clause.rs:14:13
+    |
+    |
+ LL | enum MyEnum where {
+ 
+ error: aborting due to 3 previous errors; 3 warnings emitted
20 
21 For more information about this error, try `rustc --explain E0412`.
---
To only update this specific test, also pass `--test-args proc-macro/empty-where-clause.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/proc-macro/empty-where-clause.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(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/proc-macro/empty-where-clause" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/proc-macro/empty-where-clause/auxiliary"
--- stderr -------------------------------
error[E0412]: cannot find type `MissingType1` in this scope
##[error]  --> /checkout/tests/ui/proc-macro/empty-where-clause.rs:8:12
   |
   |
LL |     field: MissingType1 //~ ERROR cannot find

error[E0412]: cannot find type `MissingType2` in this scope
##[error]  --> /checkout/tests/ui/proc-macro/empty-where-clause.rs:12:20
   |
   |
LL | struct TupleStruct(MissingType2) where; //~ ERROR cannot find

error[E0412]: cannot find type `MissingType3` in this scope
##[error]  --> /checkout/tests/ui/proc-macro/empty-where-clause.rs:15:13
   |
   |
LL |     Variant(MissingType3) //~ ERROR cannot find

warning: this empty where clause is unneeded
##[warning]  --> /checkout/tests/ui/proc-macro/empty-where-clause.rs:7:20
   |
---

warning: this empty where clause is unneeded
##[warning]  --> /checkout/tests/ui/proc-macro/empty-where-clause.rs:12:34
   |
LL | struct TupleStruct(MissingType2) where; //~ ERROR cannot find

warning: this empty where clause is unneeded
##[warning]  --> /checkout/tests/ui/proc-macro/empty-where-clause.rs:14:13
   |
   |
LL | enum MyEnum where {

error: aborting due to 3 previous errors; 3 warnings emitted

For more information about this error, try `rustc --explain E0412`.
---
1 error[E0277]: `<impl IntoIterator as IntoIterator>::Item` doesn't implement `std::fmt::Display`
2   --> $DIR/issue-97760.rs:4:19
3    |

13 LL ~ where <I as IntoIterator>::Item: std::fmt::Display {
15 
- error: aborting due to 1 previous error
+ error: aborting due to 1 previous error; 1 warning emitted
17 
---
To only update this specific test, also pass `--test-args suggestions/issue-97760.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/suggestions/issue-97760.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(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/suggestions/issue-97760" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/suggestions/issue-97760/auxiliary"
--- stderr -------------------------------
warning: this empty where clause is unneeded
##[warning]  --> /checkout/tests/ui/suggestions/issue-97760.rs:2:1
   |
---

error[E0277]: `<impl IntoIterator as IntoIterator>::Item` doesn't implement `std::fmt::Display`
##[error]  --> /checkout/tests/ui/suggestions/issue-97760.rs:4:19
   |
LL |         println!("{x}");
   |                   ^^^ `<impl IntoIterator as IntoIterator>::Item` cannot be formatted with the default formatter
   = help: the trait `std::fmt::Display` is not implemented for `<impl IntoIterator as IntoIterator>::Item`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
help: introduce a type parameter with a trait bound instead of using `impl Trait`
help: introduce a type parameter with a trait bound instead of using `impl Trait`
   |
LL ~ pub fn print_values<I: IntoIterator>(values: &I)
LL ~ where <I as IntoIterator>::Item: std::fmt::Display {

error: aborting due to 1 previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0277`.
---

+ warning: this empty where clause is unneeded
+   --> $DIR/impl-missing-where-clause-lifetimes-from-trait.rs:6:38
+    |
+ LL |     fn foo<'a, K>(self, _: (), _: K) where {
+    |
+    = note: `#[warn(unneeded_where_clauses)]` on by default
+ 
+ 
1 error[E0195]: lifetime parameters or bounds on method `foo` do not match the trait declaration
3    |


31 LL |     fn foo<'a: 'a>(&'a self) {}
32    |           ^^^^^^^^ lifetimes do not match method in trait
- error: aborting due to 3 previous errors
+ error: aborting due to 3 previous errors; 1 warning emitted
35 
36 For more information about this error, try `rustc --explain E0195`.
---
To only update this specific test, also pass `--test-args trait-bounds/impl-missing-where-clause-lifetimes-from-trait.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/trait-bounds/impl-missing-where-clause-lifetimes-from-trait.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(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/trait-bounds/impl-missing-where-clause-lifetimes-from-trait" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/trait-bounds/impl-missing-where-clause-lifetimes-from-trait/auxiliary"
--- stderr -------------------------------
warning: this empty where clause is unneeded
##[warning]  --> /checkout/tests/ui/trait-bounds/impl-missing-where-clause-lifetimes-from-trait.rs:6:38
   |
   |
LL |     fn foo<'a, K>(self, _: (), _: K) where { //~ ERROR E0195
   |
   = note: `#[warn(unneeded_where_clauses)]` on by default


error[E0195]: lifetime parameters or bounds on method `foo` do not match the trait declaration
   |
   |
LL |     fn foo<'a, K>(self, _: T, _: K) where T: 'a, K: 'a;
   |           |                                  |
   |           |                                  this bound might be missing in the impl
   |           lifetimes in impl do not match this method in trait
...
...
LL |     fn foo<'a, K>(self, _: (), _: K) where { //~ ERROR E0195
   |           ^^^^^^^ lifetimes do not match method in trait

error[E0195]: lifetime parameters or bounds on method `foo` do not match the trait declaration
   |
LL |     fn foo<'a>(&self, state: &'a State) -> &'a T
   |           ---- lifetimes in impl do not match this method in trait
LL |     where
LL |     where
LL |         T: 'a;
   |            -- this bound might be missing in the impl
...
LL |     fn foo<'a>(&self, state: &'a State) -> &'a T { //~ ERROR E0195
   |           ^^^^ lifetimes do not match method in trait

error[E0195]: lifetime parameters or bounds on method `foo` do not match the trait declaration
   |
LL |     fn foo<'a>(&'a self) {}
   |           ---- lifetimes in impl do not match this method in trait
...
...
LL |     fn foo<'a: 'a>(&'a self) {} //~ ERROR E0195
   |           ^^^^^^^^ lifetimes do not match method in trait
error: aborting due to 3 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0195`.
------------------------------------------
------------------------------------------


---- [ui] tests/ui/where-clauses/where-clause-placement-assoc-type-in-impl.rs stdout ----
diff of stderr:

38 LL +     type Assoc3  = () where;
40 
+ warning: this empty where clause is unneeded
+    |
+    = note: `#[warn(unneeded_where_clauses)]` on by default
---
61 
- warning: 5 warnings emitted
+ warning: this empty where clause is unneeded
+    |
+    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+ warning: 7 warnings emitted
63 
64 

---
To only update this specific test, also pass `--test-args where-clauses/where-clause-placement-assoc-type-in-impl.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
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/where-clauses/where-clause-placement-assoc-type-in-impl.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(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/where-clauses/where-clause-placement-assoc-type-in-impl" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/where-clauses/where-clause-placement-assoc-type-in-impl/auxiliary"
--- stderr -------------------------------
warning: where clause not allowed here
##[warning]  --> /checkout/tests/ui/where-clauses/where-clause-placement-assoc-type-in-impl.rs:17:16
   |
   |
LL |     type Assoc where u32: Copy = ();
   |
   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
   = note: `#[warn(deprecated_where_clause_location)]` on by default
help: move it to the end of the type declaration
help: move it to the end of the type declaration
   |
LL -     type Assoc where u32: Copy = ();
LL +     type Assoc  = () where u32: Copy;

warning: where clause not allowed here
##[warning]  --> /checkout/tests/ui/where-clauses/where-clause-placement-assoc-type-in-impl.rs:20:17
   |
   |
LL |     type Assoc2 where u32: Copy = () where i32: Copy;
   |
   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
help: move it to the end of the type declaration
   |
   |
LL -     type Assoc2 where u32: Copy = () where i32: Copy;
LL +     type Assoc2  = () where i32: Copy, u32: Copy;

warning: where clause not allowed here
##[warning]  --> /checkout/tests/ui/where-clauses/where-clause-placement-assoc-type-in-impl.rs:22:17
   |
   |
LL |     type Assoc3 where = ();
   |                 ^^^^^
   |
   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
help: move it to the end of the type declaration
   |
LL -     type Assoc3 where = ();
LL +     type Assoc3  = () where;

warning: this empty where clause is unneeded
   |
   = note: `#[warn(unneeded_where_clauses)]` on by default
   = note: `#[warn(unneeded_where_clauses)]` on by default

warning: where clause not allowed here
##[warning]  --> /checkout/tests/ui/where-clauses/where-clause-placement-assoc-type-in-impl.rs:30:17
   |
LL |     type Assoc2 where u32: Copy, i32: Copy = ();
   |
   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
help: move it to the end of the type declaration
   |
   |
LL -     type Assoc2 where u32: Copy, i32: Copy = ();
LL +     type Assoc2  = () where u32: Copy, i32: Copy;

warning: where clause not allowed here
##[warning]  --> /checkout/tests/ui/where-clauses/where-clause-placement-assoc-type-in-impl.rs:32:17
   |
   |
LL |     type Assoc3 where = () where;
   |                 ^^^^^ help: remove this `where`
   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information

warning: this empty where clause is unneeded
   |
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
warning: 7 warnings emitted
------------------------------------------


---
To only update this specific test, also pass `--test-args where-clauses/where-clause-placement-type-alias.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/where-clauses/where-clause-placement-type-alias.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(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/where-clauses/where-clause-placement-type-alias" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/where-clauses/where-clause-placement-type-alias/auxiliary"
--- stderr -------------------------------
error: where clauses are not allowed after the type for type aliases
##[error]  --> /checkout/tests/ui/where-clauses/where-clause-placement-type-alias.rs:6:15
   |
   |
LL | type Bar = () where u32: Copy;
   |
   = note: see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
   = help: add `#![feature(lazy_type_alias)]` to the crate attributes to enable


error: where clauses are not allowed after the type for type aliases
##[error]  --> /checkout/tests/ui/where-clauses/where-clause-placement-type-alias.rs:8:15
   |
LL | type Baz = () where;
   |
   = note: see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
   = help: add `#![feature(lazy_type_alias)]` to the crate attributes to enable

@mu001999
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Would this be better placed in rustfmt?

In fact, I find rustfmt has covered this, it will remove the where if it's empty.

compiler-errors
compiler-errors requested changes May 19, 2024
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I don't believe this should be made a lint in rustc, at least not first. Perhaps move this to clippy? Or yeah, as @cjgillot said, it may be better just as a formatting option.

@mu001999
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I agree, and rustfmt has covered this. We can get fn foo() {} from fn foo() where {} after formatting. So I think I can just close this.

@mu001999 mu001999 closed this May 19, 2024
@workingjubilee workingjubilee mentioned this pull request May 20, 2024
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@mu001999 mu001999 deleted the add/lint branch May 20, 2024 09:46
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Empty where clauses don't have an unused warning
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@mu001999 @rustbot @cjgillot @rust-log-analyzer @compiler-errors @wesleywiser