Auto merge of #104758 - Manishearth:rollup-rh1tfum, r=Manishearth

Rollup of 6 pull requests

Successful merges:

 - #103488 (Allow opaque types in trait impl headers and rely on coherence to reject unsound cases)
 - #104359 (Refactor must_use lint into two parts)
 - #104612 (Lower return type outside async block creation)
 - #104621 (Fix --extern library finding errors)
 - #104647 (enable fuzzy_provenance_casts lint in liballoc and libstd)
 - #104750 (Bump `fd-lock` in `bootstrap` again)

Failed merges:

 - #104732 (Refactor `ty::ClosureKind` related stuff)

r? `@ghost`
`@rustbot` modify labels: rollup

47 files changed, 413 insertions, 254 deletions

diff --git a/src/bootstrap/Cargo.lock b/src/bootstrap/Cargo.lock
index 0cd778a0cbb..fe8a028dfd3 100644
--- a/src/bootstrap/Cargo.lock
+++ b/src/bootstrap/Cargo.lock
@@ -225,13 +225,13 @@ dependencies = [
 
 [[package]]
 name = "fd-lock"
-version = "3.0.7"
+version = "3.0.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0c93a581058d957dc4176875aad04f82f81613e6611d64aa1a9c755bdfb16711"
+checksum = "bb21c69b9fea5e15dbc1049e4b77145dd0ba1c84019c488102de0dc4ea4b0a27"
 dependencies = [
  "cfg-if",
  "rustix",
- "windows-sys 0.42.0",
+ "windows-sys",
 ]
 
 [[package]]
@@ -319,9 +319,13 @@ dependencies = [
 
 [[package]]
 name = "io-lifetimes"
-version = "0.7.2"
+version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "24c3f4eff5495aee4c0399d7b6a0dc2b6e81be84242ffbfcf253ebacccc1d0cb"
+checksum = "a7d367024b3f3414d8e01f437f704f41a9f64ab36f9067fa73e526ad4c763c87"
+dependencies = [
+ "libc",
+ "windows-sys",
+]
 
 [[package]]
 name = "itoa"
@@ -337,15 +341,15 @@ checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
 
 [[package]]
 name = "libc"
-version = "0.2.126"
+version = "0.2.137"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "349d5a591cd28b49e1d1037471617a32ddcda5731b99419008085f72d5a53836"
+checksum = "fc7fcc620a3bff7cdd7a365be3376c97191aeaccc2a603e600951e452615bf89"
 
 [[package]]
 name = "linux-raw-sys"
-version = "0.0.46"
+version = "0.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d4d2456c373231a208ad294c33dc5bff30051eafd954cd4caae83a712b12854d"
+checksum = "8f9f08d8963a6c613f4b1a78f4f4a4dbfadf8e6545b2d72861731e4858b8b47f"
 
 [[package]]
 name = "log"
@@ -529,16 +533,16 @@ checksum = "49b3de9ec5dc0a3417da371aab17d729997c15010e7fd24ff707773a33bddb64"
 
 [[package]]
 name = "rustix"
-version = "0.35.6"
+version = "0.36.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ef258c11e17f5c01979a10543a30a4e12faef6aab217a74266e747eefa3aed88"
+checksum = "0b1fbb4dfc4eb1d390c02df47760bb19a84bb80b301ecc947ab5406394d8223e"
 dependencies = [
  "bitflags",
  "errno",
  "io-lifetimes",
  "libc",
  "linux-raw-sys",
- "windows-sys 0.36.1",
+ "windows-sys",
 ]
 
 [[package]]
@@ -727,30 +731,17 @@ checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
 
 [[package]]
 name = "windows-sys"
-version = "0.36.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ea04155a16a59f9eab786fe12a4a450e75cdb175f9e0d80da1e17db09f55b8d2"
-dependencies = [
- "windows_aarch64_msvc 0.36.1",
- "windows_i686_gnu 0.36.1",
- "windows_i686_msvc 0.36.1",
- "windows_x86_64_gnu 0.36.1",
- "windows_x86_64_msvc 0.36.1",
-]
-
-[[package]]
-name = "windows-sys"
 version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5a3e1820f08b8513f676f7ab6c1f99ff312fb97b553d30ff4dd86f9f15728aa7"
 dependencies = [
  "windows_aarch64_gnullvm",
- "windows_aarch64_msvc 0.42.0",
- "windows_i686_gnu 0.42.0",
- "windows_i686_msvc 0.42.0",
- "windows_x86_64_gnu 0.42.0",
+ "windows_aarch64_msvc",
+ "windows_i686_gnu",
+ "windows_i686_msvc",
+ "windows_x86_64_gnu",
  "windows_x86_64_gnullvm",
- "windows_x86_64_msvc 0.42.0",
+ "windows_x86_64_msvc",
 ]
 
 [[package]]
@@ -761,48 +752,24 @@ checksum = "41d2aa71f6f0cbe00ae5167d90ef3cfe66527d6f613ca78ac8024c3ccab9a19e"
 
 [[package]]
 name = "windows_aarch64_msvc"
-version = "0.36.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9bb8c3fd39ade2d67e9874ac4f3db21f0d710bee00fe7cab16949ec184eeaa47"
-
-[[package]]
-name = "windows_aarch64_msvc"
 version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dd0f252f5a35cac83d6311b2e795981f5ee6e67eb1f9a7f64eb4500fbc4dcdb4"
 
 [[package]]
 name = "windows_i686_gnu"
-version = "0.36.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "180e6ccf01daf4c426b846dfc66db1fc518f074baa793aa7d9b9aaeffad6a3b6"
-
-[[package]]
-name = "windows_i686_gnu"
 version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fbeae19f6716841636c28d695375df17562ca208b2b7d0dc47635a50ae6c5de7"
 
 [[package]]
 name = "windows_i686_msvc"
-version = "0.36.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e2e7917148b2812d1eeafaeb22a97e4813dfa60a3f8f78ebe204bcc88f12f024"
-
-[[package]]
-name = "windows_i686_msvc"
 version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "84c12f65daa39dd2babe6e442988fc329d6243fdce47d7d2d155b8d874862246"
 
 [[package]]
 name = "windows_x86_64_gnu"
-version = "0.36.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4dcd171b8776c41b97521e5da127a2d86ad280114807d0b2ab1e462bc764d9e1"
-
-[[package]]
-name = "windows_x86_64_gnu"
 version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "bf7b1b21b5362cbc318f686150e5bcea75ecedc74dd157d874d754a2ca44b0ed"
@@ -815,12 +782,6 @@ checksum = "09d525d2ba30eeb3297665bd434a54297e4170c7f1a44cad4ef58095b4cd2028"
 
 [[package]]
 name = "windows_x86_64_msvc"
-version = "0.36.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c811ca4a8c853ef420abd8592ba53ddbbac90410fab6903b3e79972a631f7680"
-
-[[package]]
-name = "windows_x86_64_msvc"
 version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f40009d85759725a34da6d89a94e63d7bdc50a862acf0dbc7c8e488f1edcb6f5"
diff --git a/src/bootstrap/Cargo.toml b/src/bootstrap/Cargo.toml
index 4c24c214d2c..8751a71caf1 100644
--- a/src/bootstrap/Cargo.toml
+++ b/src/bootstrap/Cargo.toml
@@ -36,7 +36,7 @@ test = false
 
 [dependencies]
 cmake = "0.1.38"
-fd-lock = "3.0.7"
+fd-lock = "3.0.8"
 filetime = "0.2"
 getopts = "0.2.19"
 cc = "1.0.69"
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index 7085bc8d1be..b5d4ab55b4a 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -318,6 +318,7 @@ pub(crate) fn clean_predicate<'tcx>(
         | ty::PredicateKind::ObjectSafe(..)
         | ty::PredicateKind::ClosureKind(..)
         | ty::PredicateKind::ConstEquate(..)
+        | ty::PredicateKind::Ambiguous
         | ty::PredicateKind::TypeWellFormedFromEnv(..) => panic!("not user writable"),
     }
 }
diff --git a/src/test/ui/coherence/coherence-with-closure.rs b/src/test/ui/coherence/coherence-with-closure.rs
index 6e3281d8508..5b6a62b24d4 100644
--- a/src/test/ui/coherence/coherence-with-closure.rs
+++ b/src/test/ui/coherence/coherence-with-closure.rs
@@ -8,7 +8,6 @@ fn defining_use() -> OpaqueClosure {
 struct Wrapper<T>(T);
 trait Trait {}
 impl Trait for Wrapper<OpaqueClosure> {}
-//~^ ERROR cannot implement trait on type alias impl trait
 impl<T: Sync> Trait for Wrapper<T> {}
 //~^ ERROR conflicting implementations of trait `Trait` for type `Wrapper<OpaqueClosure>`
 
diff --git a/src/test/ui/coherence/coherence-with-closure.stderr b/src/test/ui/coherence/coherence-with-closure.stderr
index d2ca63fa146..431108e14d7 100644
--- a/src/test/ui/coherence/coherence-with-closure.stderr
+++ b/src/test/ui/coherence/coherence-with-closure.stderr
@@ -1,24 +1,11 @@
 error[E0119]: conflicting implementations of trait `Trait` for type `Wrapper<OpaqueClosure>`
-  --> $DIR/coherence-with-closure.rs:12:1
+  --> $DIR/coherence-with-closure.rs:11:1
    |
 LL | impl Trait for Wrapper<OpaqueClosure> {}
    | ------------------------------------- first implementation here
-LL |
 LL | impl<T: Sync> Trait for Wrapper<T> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Wrapper<OpaqueClosure>`
 
-error: cannot implement trait on type alias impl trait
-  --> $DIR/coherence-with-closure.rs:10:24
-   |
-LL | impl Trait for Wrapper<OpaqueClosure> {}
-   |                        ^^^^^^^^^^^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/coherence-with-closure.rs:3:22
-   |
-LL | type OpaqueClosure = impl Sized;
-   |                      ^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/coherence/coherence-with-generator.rs b/src/test/ui/coherence/coherence-with-generator.rs
index d34c391db9f..70665ba06f9 100644
--- a/src/test/ui/coherence/coherence-with-generator.rs
+++ b/src/test/ui/coherence/coherence-with-generator.rs
@@ -12,7 +12,6 @@ fn defining_use() -> OpaqueGenerator {
 struct Wrapper<T>(T);
 trait Trait {}
 impl Trait for Wrapper<OpaqueGenerator> {}
-//~^ ERROR cannot implement trait on type alias impl trait
 impl<T: Sync> Trait for Wrapper<T> {}
 //~^ ERROR conflicting implementations of trait `Trait` for type `Wrapper<OpaqueGenerator>`
 
diff --git a/src/test/ui/coherence/coherence-with-generator.stderr b/src/test/ui/coherence/coherence-with-generator.stderr
index 804bc1c3a6d..6d3be2e16c6 100644
--- a/src/test/ui/coherence/coherence-with-generator.stderr
+++ b/src/test/ui/coherence/coherence-with-generator.stderr
@@ -1,24 +1,11 @@
 error[E0119]: conflicting implementations of trait `Trait` for type `Wrapper<OpaqueGenerator>`
-  --> $DIR/coherence-with-generator.rs:16:1
+  --> $DIR/coherence-with-generator.rs:15:1
    |
 LL | impl Trait for Wrapper<OpaqueGenerator> {}
    | --------------------------------------- first implementation here
-LL |
 LL | impl<T: Sync> Trait for Wrapper<T> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Wrapper<OpaqueGenerator>`
 
-error: cannot implement trait on type alias impl trait
-  --> $DIR/coherence-with-generator.rs:14:24
-   |
-LL | impl Trait for Wrapper<OpaqueGenerator> {}
-   |                        ^^^^^^^^^^^^^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/coherence-with-generator.rs:3:24
-   |
-LL | type OpaqueGenerator = impl Sized;
-   |                        ^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/errors/issue-104621-extern-bad-file.rs b/src/test/ui/errors/issue-104621-extern-bad-file.rs
new file mode 100644
index 00000000000..3f13d605232
--- /dev/null
+++ b/src/test/ui/errors/issue-104621-extern-bad-file.rs
@@ -0,0 +1,8 @@
+// compile-flags: --extern foo={{src-base}}/errors/issue-104621-extern-bad-file.rs
+// only-linux
+
+extern crate foo;
+//~^ ERROR extern location for foo is of an unknown type
+//~| ERROR file name should be lib*.rlib or lib*.so
+//~| ERROR can't find crate for `foo` [E0463]
+fn main() {}
diff --git a/src/test/ui/errors/issue-104621-extern-bad-file.stderr b/src/test/ui/errors/issue-104621-extern-bad-file.stderr
new file mode 100644
index 00000000000..b8500ad0e04
--- /dev/null
+++ b/src/test/ui/errors/issue-104621-extern-bad-file.stderr
@@ -0,0 +1,21 @@
+error: extern location for foo is of an unknown type: $DIR/issue-104621-extern-bad-file.rs
+  --> $DIR/issue-104621-extern-bad-file.rs:4:1
+   |
+LL | extern crate foo;
+   | ^^^^^^^^^^^^^^^^^
+
+error: file name should be lib*.rlib or lib*.so
+  --> $DIR/issue-104621-extern-bad-file.rs:4:1
+   |
+LL | extern crate foo;
+   | ^^^^^^^^^^^^^^^^^
+
+error[E0463]: can't find crate for `foo`
+  --> $DIR/issue-104621-extern-bad-file.rs:4:1
+   |
+LL | extern crate foo;
+   | ^^^^^^^^^^^^^^^^^ can't find crate
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0463`.
diff --git a/src/test/ui/errors/issue-104621-extern-not-file.rs b/src/test/ui/errors/issue-104621-extern-not-file.rs
new file mode 100644
index 00000000000..899e45a306b
--- /dev/null
+++ b/src/test/ui/errors/issue-104621-extern-not-file.rs
@@ -0,0 +1,4 @@
+// compile-flags: --extern foo=.
+
+extern crate foo; //~ ERROR extern location for foo is not a file: .
+fn main() {}
diff --git a/src/test/ui/errors/issue-104621-extern-not-file.stderr b/src/test/ui/errors/issue-104621-extern-not-file.stderr
new file mode 100644
index 00000000000..5aaf9741360
--- /dev/null
+++ b/src/test/ui/errors/issue-104621-extern-not-file.stderr
@@ -0,0 +1,8 @@
+error: extern location for foo is not a file: .
+  --> $DIR/issue-104621-extern-not-file.rs:3:1
+   |
+LL | extern crate foo;
+   | ^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/impl-trait/auto-trait.rs b/src/test/ui/impl-trait/auto-trait.rs
index afa95645a27..35994e4a5ba 100644
--- a/src/test/ui/impl-trait/auto-trait.rs
+++ b/src/test/ui/impl-trait/auto-trait.rs
@@ -20,7 +20,6 @@ impl<T: Send> AnotherTrait for T {}
 // in the future.)
 impl AnotherTrait for D<OpaqueType> {
     //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
-    //~| ERROR cannot implement trait on type alias impl trait
 }
 
 fn main() {}
diff --git a/src/test/ui/impl-trait/auto-trait.stderr b/src/test/ui/impl-trait/auto-trait.stderr
index 5e10272b0db..81009413c9a 100644
--- a/src/test/ui/impl-trait/auto-trait.stderr
+++ b/src/test/ui/impl-trait/auto-trait.stderr
@@ -7,18 +7,6 @@ LL | impl<T: Send> AnotherTrait for T {}
 LL | impl AnotherTrait for D<OpaqueType> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<OpaqueType>`
 
-error: cannot implement trait on type alias impl trait
-  --> $DIR/auto-trait.rs:21:25
-   |
-LL | impl AnotherTrait for D<OpaqueType> {
-   |                         ^^^^^^^^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/auto-trait.rs:7:19
-   |
-LL | type OpaqueType = impl OpaqueTrait;
-   |                   ^^^^^^^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/impl-trait/negative-reasoning.rs b/src/test/ui/impl-trait/negative-reasoning.rs
index da69bb349ae..70e24a3a9d0 100644
--- a/src/test/ui/impl-trait/negative-reasoning.rs
+++ b/src/test/ui/impl-trait/negative-reasoning.rs
@@ -18,7 +18,6 @@ impl<T: std::fmt::Debug> AnotherTrait for T {}
 // This is in error, because we cannot assume that `OpaqueType: !Debug`
 impl AnotherTrait for D<OpaqueType> {
     //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
-    //~| ERROR cannot implement trait on type alias impl trait
 }
 
 fn main() {}
diff --git a/src/test/ui/impl-trait/negative-reasoning.stderr b/src/test/ui/impl-trait/negative-reasoning.stderr
index 479b451855d..6b8cc9e7374 100644
--- a/src/test/ui/impl-trait/negative-reasoning.stderr
+++ b/src/test/ui/impl-trait/negative-reasoning.stderr
@@ -9,18 +9,6 @@ LL | impl AnotherTrait for D<OpaqueType> {
    |
    = note: upstream crates may add a new impl of trait `std::fmt::Debug` for type `OpaqueType` in future versions
 
-error: cannot implement trait on type alias impl trait
-  --> $DIR/negative-reasoning.rs:19:25
-   |
-LL | impl AnotherTrait for D<OpaqueType> {
-   |                         ^^^^^^^^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/negative-reasoning.rs:7:19
-   |
-LL | type OpaqueType = impl OpaqueTrait;
-   |                   ^^^^^^^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs
index 621c4ea6e0d..af9dfe25bb4 100644
--- a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs
+++ b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs
@@ -5,13 +5,13 @@ type Foo = impl PartialEq<(Foo, i32)>;
 struct Bar;
 
 impl PartialEq<(Foo, i32)> for Bar {
-//~^ ERROR cannot implement trait on type alias impl trait
     fn eq(&self, _other: &(Foo, i32)) -> bool {
         true
     }
 }
 
 fn foo() -> Foo {
+    //~^ ERROR can't compare `Bar` with `(Bar, i32)`
     Bar
 }
 
diff --git a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.stderr b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.stderr
index 2ef1697ba34..7b63a3d0b9f 100644
--- a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.stderr
+++ b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.stderr
@@ -1,14 +1,15 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs:7:17
+error[E0277]: can't compare `Bar` with `(Bar, i32)`
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs:13:13
    |
-LL | impl PartialEq<(Foo, i32)> for Bar {
-   |                 ^^^
+LL | fn foo() -> Foo {
+   |             ^^^ no implementation for `Bar == (Bar, i32)`
+LL |
+LL |     Bar
+   |     --- return type was inferred to be `Bar` here
    |
-note: type alias impl trait defined here
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs:3:12
-   |
-LL | type Foo = impl PartialEq<(Foo, i32)>;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: the trait `PartialEq<(Bar, i32)>` is not implemented for `Bar`
+   = help: the trait `PartialEq<(Foo, i32)>` is implemented for `Bar`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.rs b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.rs
index df7966f00e1..91f1ed48133 100644
--- a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.rs
+++ b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.rs
@@ -2,11 +2,13 @@
 
 mod a {
     type Foo = impl PartialEq<(Foo, i32)>;
+    //~^ ERROR: unconstrained opaque type
 
     struct Bar;
 
     impl PartialEq<(Bar, i32)> for Bar {
         fn eq(&self, _other: &(Foo, i32)) -> bool {
+            //~^ ERROR: `eq` has an incompatible type for trait
             true
         }
     }
@@ -14,12 +16,13 @@ mod a {
 
 mod b {
     type Foo = impl PartialEq<(Foo, i32)>;
+    //~^ ERROR: unconstrained opaque type
 
     struct Bar;
 
     impl PartialEq<(Foo, i32)> for Bar {
-        //~^ ERROR cannot implement trait on type alias impl trait
         fn eq(&self, _other: &(Bar, i32)) -> bool {
+            //~^ ERROR: `eq` has an incompatible type for trait
             true
         }
     }
diff --git a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr
index 6cd63dcf81c..3dda5761ada 100644
--- a/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr
+++ b/src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr
@@ -1,14 +1,49 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:20:21
+error: unconstrained opaque type
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:4:16
    |
-LL |     impl PartialEq<(Foo, i32)> for Bar {
-   |                     ^^^
+LL |     type Foo = impl PartialEq<(Foo, i32)>;
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
+
+error[E0053]: method `eq` has an incompatible type for trait
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:10:30
+   |
+LL |     type Foo = impl PartialEq<(Foo, i32)>;
+   |                -------------------------- the found opaque type
+...
+LL |         fn eq(&self, _other: &(Foo, i32)) -> bool {
+   |                              ^^^^^^^^^^^
+   |                              |
+   |                              expected struct `a::Bar`, found opaque type
+   |                              help: change the parameter type to match the trait: `&(a::Bar, i32)`
    |
-note: type alias impl trait defined here
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:16:16
+   = note: expected fn pointer `fn(&a::Bar, &(a::Bar, i32)) -> _`
+              found fn pointer `fn(&a::Bar, &(a::Foo, i32)) -> _`
+
+error: unconstrained opaque type
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:18:16
    |
 LL |     type Foo = impl PartialEq<(Foo, i32)>;
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
+
+error[E0053]: method `eq` has an incompatible type for trait
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:24:30
+   |
+LL |     type Foo = impl PartialEq<(Foo, i32)>;
+   |                -------------------------- the expected opaque type
+...
+LL |         fn eq(&self, _other: &(Bar, i32)) -> bool {
+   |                              ^^^^^^^^^^^
+   |                              |
+   |                              expected opaque type, found struct `b::Bar`
+   |                              help: change the parameter type to match the trait: `&(b::Foo, i32)`
+   |
+   = note: expected fn pointer `fn(&b::Bar, &(b::Foo, i32)) -> _`
+              found fn pointer `fn(&b::Bar, &(b::Bar, i32)) -> _`
 
-error: aborting due to previous error
+error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0053`.
diff --git a/src/test/ui/lint/unused/must-use-ops.rs b/src/test/ui/lint/unused/must-use-ops.rs
index 3e425727e78..60f877aa8b3 100644
--- a/src/test/ui/lint/unused/must-use-ops.rs
+++ b/src/test/ui/lint/unused/must-use-ops.rs
@@ -3,12 +3,18 @@
 // check-pass
 
 #![warn(unused_must_use)]
+#![feature(never_type)]
+
+fn deref_never(x: &!) {
+    // Don't lint for uninhabited typess
+    *x;
+}
 
 fn main() {
     let val = 1;
     let val_pointer = &val;
 
-// Comparison Operators
+    // Comparison Operators
     val == 1; //~ WARNING unused comparison
     val < 1; //~ WARNING unused comparison
     val <= 1; //~ WARNING unused comparison
@@ -16,26 +22,30 @@ fn main() {
     val >= 1; //~ WARNING unused comparison
     val > 1; //~ WARNING unused comparison
 
-// Arithmetic Operators
+    // Arithmetic Operators
     val + 2; //~ WARNING unused arithmetic operation
     val - 2; //~ WARNING unused arithmetic operation
     val / 2; //~ WARNING unused arithmetic operation
     val * 2; //~ WARNING unused arithmetic operation
     val % 2; //~ WARNING unused arithmetic operation
 
-// Logical Operators
+    // Logical Operators
     true && true; //~ WARNING unused logical operation
     false || true; //~ WARNING unused logical operation
 
-// Bitwise Operators
+    // Bitwise Operators
     5 ^ val; //~ WARNING unused bitwise operation
     5 & val; //~ WARNING unused bitwise operation
     5 | val; //~ WARNING unused bitwise operation
     5 << val; //~ WARNING unused bitwise operation
     5 >> val; //~ WARNING unused bitwise operation
 
-// Unary Operators
+    // Unary Operators
     !val; //~ WARNING unused unary operation
     -val; //~ WARNING unused unary operation
     *val_pointer; //~ WARNING unused unary operation
+
+    if false {
+        deref_never(&panic!());
+    }
 }
diff --git a/src/test/ui/lint/unused/must-use-ops.stderr b/src/test/ui/lint/unused/must-use-ops.stderr
index b248dd0fe15..79a53d39cbf 100644
--- a/src/test/ui/lint/unused/must-use-ops.stderr
+++ b/src/test/ui/lint/unused/must-use-ops.stderr
@@ -1,5 +1,5 @@
 warning: unused comparison that must be used
-  --> $DIR/must-use-ops.rs:12:5
+  --> $DIR/must-use-ops.rs:18:5
    |
 LL |     val == 1;
    |     ^^^^^^^^ the comparison produces a value
@@ -15,7 +15,7 @@ LL |     let _ = val == 1;
    |     +++++++
 
 warning: unused comparison that must be used
-  --> $DIR/must-use-ops.rs:13:5
+  --> $DIR/must-use-ops.rs:19:5
    |
 LL |     val < 1;
    |     ^^^^^^^ the comparison produces a value
@@ -26,7 +26,7 @@ LL |     let _ = val < 1;
    |     +++++++
 
 warning: unused comparison that must be used
-  --> $DIR/must-use-ops.rs:14:5
+  --> $DIR/must-use-ops.rs:20:5
    |
 LL |     val <= 1;
    |     ^^^^^^^^ the comparison produces a value
@@ -37,7 +37,7 @@ LL |     let _ = val <= 1;
    |     +++++++
 
 warning: unused comparison that must be used
-  --> $DIR/must-use-ops.rs:15:5
+  --> $DIR/must-use-ops.rs:21:5
    |
 LL |     val != 1;
    |     ^^^^^^^^ the comparison produces a value
@@ -48,7 +48,7 @@ LL |     let _ = val != 1;
    |     +++++++
 
 warning: unused comparison that must be used
-  --> $DIR/must-use-ops.rs:16:5
+  --> $DIR/must-use-ops.rs:22:5
    |
 LL |     val >= 1;
    |     ^^^^^^^^ the comparison produces a value
@@ -59,7 +59,7 @@ LL |     let _ = val >= 1;
    |     +++++++
 
 warning: unused comparison that must be used
-  --> $DIR/must-use-ops.rs:17:5
+  --> $DIR/must-use-ops.rs:23:5
    |
 LL |     val > 1;
    |     ^^^^^^^ the comparison produces a value
@@ -70,7 +70,7 @@ LL |     let _ = val > 1;
    |     +++++++
 
 warning: unused arithmetic operation that must be used
-  --> $DIR/must-use-ops.rs:20:5
+  --> $DIR/must-use-ops.rs:26:5
    |
 LL |     val + 2;
    |     ^^^^^^^ the arithmetic operation produces a value
@@ -81,7 +81,7 @@ LL |     let _ = val + 2;
    |     +++++++
 
 warning: unused arithmetic operation that must be used
-  --> $DIR/must-use-ops.rs:21:5
+  --> $DIR/must-use-ops.rs:27:5
    |
 LL |     val - 2;
    |     ^^^^^^^ the arithmetic operation produces a value
@@ -92,7 +92,7 @@ LL |     let _ = val - 2;
    |     +++++++
 
 warning: unused arithmetic operation that must be used
-  --> $DIR/must-use-ops.rs:22:5
+  --> $DIR/must-use-ops.rs:28:5
    |
 LL |     val / 2;
    |     ^^^^^^^ the arithmetic operation produces a value
@@ -103,7 +103,7 @@ LL |     let _ = val / 2;
    |     +++++++
 
 warning: unused arithmetic operation that must be used
-  --> $DIR/must-use-ops.rs:23:5
+  --> $DIR/must-use-ops.rs:29:5
    |
 LL |     val * 2;
    |     ^^^^^^^ the arithmetic operation produces a value
@@ -114,7 +114,7 @@ LL |     let _ = val * 2;
    |     +++++++
 
 warning: unused arithmetic operation that must be used
-  --> $DIR/must-use-ops.rs:24:5
+  --> $DIR/must-use-ops.rs:30:5
    |
 LL |     val % 2;
    |     ^^^^^^^ the arithmetic operation produces a value
@@ -125,7 +125,7 @@ LL |     let _ = val % 2;
    |     +++++++
 
 warning: unused logical operation that must be used
-  --> $DIR/must-use-ops.rs:27:5
+  --> $DIR/must-use-ops.rs:33:5
    |
 LL |     true && true;
    |     ^^^^^^^^^^^^ the logical operation produces a value
@@ -136,7 +136,7 @@ LL |     let _ = true && true;
    |     +++++++
 
 warning: unused logical operation that must be used
-  --> $DIR/must-use-ops.rs:28:5
+  --> $DIR/must-use-ops.rs:34:5
    |
 LL |     false || true;
    |     ^^^^^^^^^^^^^ the logical operation produces a value
@@ -147,7 +147,7 @@ LL |     let _ = false || true;
    |     +++++++
 
 warning: unused bitwise operation that must be used
-  --> $DIR/must-use-ops.rs:31:5
+  --> $DIR/must-use-ops.rs:37:5
    |
 LL |     5 ^ val;
    |     ^^^^^^^ the bitwise operation produces a value
@@ -158,7 +158,7 @@ LL |     let _ = 5 ^ val;
    |     +++++++
 
 warning: unused bitwise operation that must be used
-  --> $DIR/must-use-ops.rs:32:5
+  --> $DIR/must-use-ops.rs:38:5
    |
 LL |     5 & val;
    |     ^^^^^^^ the bitwise operation produces a value
@@ -169,7 +169,7 @@ LL |     let _ = 5 & val;
    |     +++++++
 
 warning: unused bitwise operation that must be used
-  --> $DIR/must-use-ops.rs:33:5
+  --> $DIR/must-use-ops.rs:39:5
    |
 LL |     5 | val;
    |     ^^^^^^^ the bitwise operation produces a value
@@ -180,7 +180,7 @@ LL |     let _ = 5 | val;
    |     +++++++
 
 warning: unused bitwise operation that must be used
-  --> $DIR/must-use-ops.rs:34:5
+  --> $DIR/must-use-ops.rs:40:5
    |
 LL |     5 << val;
    |     ^^^^^^^^ the bitwise operation produces a value
@@ -191,7 +191,7 @@ LL |     let _ = 5 << val;
    |     +++++++
 
 warning: unused bitwise operation that must be used
-  --> $DIR/must-use-ops.rs:35:5
+  --> $DIR/must-use-ops.rs:41:5
    |
 LL |     5 >> val;
    |     ^^^^^^^^ the bitwise operation produces a value
@@ -202,7 +202,7 @@ LL |     let _ = 5 >> val;
    |     +++++++
 
 warning: unused unary operation that must be used
-  --> $DIR/must-use-ops.rs:38:5
+  --> $DIR/must-use-ops.rs:44:5
    |
 LL |     !val;
    |     ^^^^ the unary operation produces a value
@@ -213,7 +213,7 @@ LL |     let _ = !val;
    |     +++++++
 
 warning: unused unary operation that must be used
-  --> $DIR/must-use-ops.rs:39:5
+  --> $DIR/must-use-ops.rs:45:5
    |
 LL |     -val;
    |     ^^^^ the unary operation produces a value
@@ -224,7 +224,7 @@ LL |     let _ = -val;
    |     +++++++
 
 warning: unused unary operation that must be used
-  --> $DIR/must-use-ops.rs:40:5
+  --> $DIR/must-use-ops.rs:46:5
    |
 LL |     *val_pointer;
    |     ^^^^^^^^^^^^ the unary operation produces a value
diff --git a/src/test/ui/lint/unused/must_use-array.rs b/src/test/ui/lint/unused/must_use-array.rs
index 97825dd2f6c..b7bae4b0acf 100644
--- a/src/test/ui/lint/unused/must_use-array.rs
+++ b/src/test/ui/lint/unused/must_use-array.rs
@@ -1,6 +1,7 @@
 #![deny(unused_must_use)]
 
 #[must_use]
+#[derive(Clone, Copy)]
 struct S;
 
 struct A;
@@ -34,6 +35,10 @@ fn array_of_arrays_of_arrays() -> [[[S; 1]; 2]; 1] {
     [[[S], [S]]]
 }
 
+fn usize_max() -> [S; usize::MAX] {
+    [S; usize::MAX]
+}
+
 fn main() {
     empty(); // ok
     singleton(); //~ ERROR unused array of `S` that must be used
@@ -44,4 +49,6 @@ fn main() {
     //~^ ERROR unused array of boxed `T` trait objects in tuple element 1 that must be used
     array_of_arrays_of_arrays();
     //~^ ERROR unused array of arrays of arrays of `S` that must be used
+    usize_max();
+    //~^ ERROR unused array of `S` that must be used
 }
diff --git a/src/test/ui/lint/unused/must_use-array.stderr b/src/test/ui/lint/unused/must_use-array.stderr
index bba2b1ba078..61ef2088d30 100644
--- a/src/test/ui/lint/unused/must_use-array.stderr
+++ b/src/test/ui/lint/unused/must_use-array.stderr
@@ -1,5 +1,5 @@
 error: unused array of `S` that must be used
-  --> $DIR/must_use-array.rs:39:5
+  --> $DIR/must_use-array.rs:44:5
    |
 LL |     singleton();
    |     ^^^^^^^^^^^
@@ -11,34 +11,40 @@ LL | #![deny(unused_must_use)]
    |         ^^^^^^^^^^^^^^^
 
 error: unused array of `S` that must be used
-  --> $DIR/must_use-array.rs:40:5
+  --> $DIR/must_use-array.rs:45:5
    |
 LL |     many();
    |     ^^^^^^
 
 error: unused array of `S` in tuple element 0 that must be used
-  --> $DIR/must_use-array.rs:41:6
+  --> $DIR/must_use-array.rs:46:6
    |
 LL |     ([S], 0, ());
    |      ^^^
 
 error: unused array of implementers of `T` that must be used
-  --> $DIR/must_use-array.rs:42:5
+  --> $DIR/must_use-array.rs:47:5
    |
 LL |     array_of_impl_trait();
    |     ^^^^^^^^^^^^^^^^^^^^^
 
 error: unused array of boxed `T` trait objects in tuple element 1 that must be used
-  --> $DIR/must_use-array.rs:43:5
+  --> $DIR/must_use-array.rs:48:5
    |
 LL |     impl_array();
    |     ^^^^^^^^^^^^
 
 error: unused array of arrays of arrays of `S` that must be used
-  --> $DIR/must_use-array.rs:45:5
+  --> $DIR/must_use-array.rs:50:5
    |
 LL |     array_of_arrays_of_arrays();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 6 previous errors
+error: unused array of `S` that must be used
+  --> $DIR/must_use-array.rs:52:5
+   |
+LL |     usize_max();
+   |     ^^^^^^^^^^^
+
+error: aborting due to 7 previous errors
 
diff --git a/src/test/ui/traits/alias/issue-83613.rs b/src/test/ui/traits/alias/issue-83613.rs
index 04320e72076..2462e703a71 100644
--- a/src/test/ui/traits/alias/issue-83613.rs
+++ b/src/test/ui/traits/alias/issue-83613.rs
@@ -9,5 +9,4 @@ trait AnotherTrait {}
 impl<T: Send> AnotherTrait for T {}
 impl AnotherTrait for OpaqueType {}
 //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `OpaqueType`
-//~| ERROR cannot implement trait on type alias impl trait
 fn main() {}
diff --git a/src/test/ui/traits/alias/issue-83613.stderr b/src/test/ui/traits/alias/issue-83613.stderr
index b9d93160192..a78294da6c1 100644
--- a/src/test/ui/traits/alias/issue-83613.stderr
+++ b/src/test/ui/traits/alias/issue-83613.stderr
@@ -6,18 +6,6 @@ LL | impl<T: Send> AnotherTrait for T {}
 LL | impl AnotherTrait for OpaqueType {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `OpaqueType`
 
-error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-83613.rs:10:23
-   |
-LL | impl AnotherTrait for OpaqueType {}
-   |                       ^^^^^^^^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/issue-83613.rs:4:19
-   |
-LL | type OpaqueType = impl OpaqueTrait;
-   |                   ^^^^^^^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/type-alias-impl-trait/coherence.rs b/src/test/ui/type-alias-impl-trait/coherence.rs
index 98ac215ad6c..077a31494a9 100644
--- a/src/test/ui/type-alias-impl-trait/coherence.rs
+++ b/src/test/ui/type-alias-impl-trait/coherence.rs
@@ -12,6 +12,6 @@ fn use_alias<T>(val: T) -> AliasOfForeignType<T> {
 }
 
 impl<T> foreign_crate::ForeignTrait for AliasOfForeignType<T> {}
-//~^ ERROR cannot implement trait on type alias impl trait
+//~^ ERROR only traits defined in the current crate can be implemented for arbitrary types
 
 fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/coherence.stderr b/src/test/ui/type-alias-impl-trait/coherence.stderr
index 3ce25d94f6e..c923eb08ab3 100644
--- a/src/test/ui/type-alias-impl-trait/coherence.stderr
+++ b/src/test/ui/type-alias-impl-trait/coherence.stderr
@@ -1,14 +1,14 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/coherence.rs:14:41
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/coherence.rs:14:1
    |
 LL | impl<T> foreign_crate::ForeignTrait for AliasOfForeignType<T> {}
-   |                                         ^^^^^^^^^^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^---------------------
+   | |                                       |
+   | |                                       `AliasOfForeignType<T>` is not defined in the current crate
+   | impl doesn't use only types from inside the current crate
    |
-note: type alias impl trait defined here
-  --> $DIR/coherence.rs:9:30
-   |
-LL | type AliasOfForeignType<T> = impl LocalTrait;
-   |                              ^^^^^^^^^^^^^^^
+   = note: define and implement a trait or new type instead
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0117`.
diff --git a/src/test/ui/type-alias-impl-trait/coherence_generalization.rs b/src/test/ui/type-alias-impl-trait/coherence_generalization.rs
new file mode 100644
index 00000000000..5c9ad9498b6
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/coherence_generalization.rs
@@ -0,0 +1,13 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+trait Trait {}
+type Opaque<T> = impl Sized;
+fn foo<T>() -> Opaque<T> {
+    ()
+}
+
+impl<T, V> Trait for (T, V, V, u32) {}
+impl<U, V> Trait for (Opaque<U>, V, i32, V) {}
+
+fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_generic_tait.rs b/src/test/ui/type-alias-impl-trait/impl_trait_for_generic_tait.rs
new file mode 100644
index 00000000000..0efbd1c2bd5
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_generic_tait.rs
@@ -0,0 +1,23 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+trait Foo {
+    type Assoc;
+}
+
+impl Foo for i32 {
+    type Assoc = u32;
+}
+type ImplTrait = impl Sized;
+fn constrain() -> ImplTrait {
+    1u64
+}
+impl Foo for i64 {
+    type Assoc = ImplTrait;
+}
+
+trait Bar<T> {}
+
+impl<T: Foo> Bar<<T as Foo>::Assoc> for T {}
+
+fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.rs b/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.rs
new file mode 100644
index 00000000000..3f1a9d12b44
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.rs
@@ -0,0 +1,33 @@
+#![feature(type_alias_impl_trait)]
+
+trait Foo {}
+impl Foo for () {}
+impl Foo for i32 {}
+
+type Bar<T: Foo> = impl std::fmt::Debug;
+fn defining_use<T: Foo>() -> Bar<T> {
+    42
+}
+
+trait Bop {}
+
+impl Bop for Bar<()> {}
+
+// If the hidden type is the same, this is effectively a second impl for the same type.
+impl Bop for Bar<i32> {}
+//~^ ERROR conflicting implementations
+
+type Barr = impl std::fmt::Debug;
+fn defining_use2() -> Barr {
+    42
+}
+
+// Even completely different opaque types must conflict.
+impl Bop for Barr {}
+//~^ ERROR conflicting implementations
+
+// And obviously the hidden type must conflict, too.
+impl Bop for i32 {}
+//~^ ERROR conflicting implementations
+
+fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.stderr b/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.stderr
new file mode 100644
index 00000000000..aaf75cc3db9
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_same_tait.stderr
@@ -0,0 +1,30 @@
+error[E0119]: conflicting implementations of trait `Bop` for type `Bar<()>`
+  --> $DIR/impl_trait_for_same_tait.rs:17:1
+   |
+LL | impl Bop for Bar<()> {}
+   | -------------------- first implementation here
+...
+LL | impl Bop for Bar<i32> {}
+   | ^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Bar<()>`
+
+error[E0119]: conflicting implementations of trait `Bop` for type `Bar<()>`
+  --> $DIR/impl_trait_for_same_tait.rs:26:1
+   |
+LL | impl Bop for Bar<()> {}
+   | -------------------- first implementation here
+...
+LL | impl Bop for Barr {}
+   | ^^^^^^^^^^^^^^^^^ conflicting implementation for `Bar<()>`
+
+error[E0119]: conflicting implementations of trait `Bop` for type `Bar<()>`
+  --> $DIR/impl_trait_for_same_tait.rs:30:1
+   |
+LL | impl Bop for Bar<()> {}
+   | -------------------- first implementation here
+...
+LL | impl Bop for i32 {}
+   | ^^^^^^^^^^^^^^^^ conflicting implementation for `Bar<()>`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_tait.rs b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait.rs
new file mode 100644
index 00000000000..9f32c5d888b
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait.rs
@@ -0,0 +1,21 @@
+// compile-flags: --crate-type=lib
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+type Alias = impl Sized;
+
+fn constrain() -> Alias {
+    1i32
+}
+
+trait HideIt {
+    type Assoc;
+}
+
+impl HideIt for () {
+    type Assoc = Alias;
+}
+
+pub trait Yay {}
+
+impl Yay for <() as HideIt>::Assoc {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.rs b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.rs
new file mode 100644
index 00000000000..8ec20acef4d
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.rs
@@ -0,0 +1,19 @@
+#![feature(type_alias_impl_trait)]
+
+use std::fmt::Debug;
+
+type Foo = impl Debug;
+pub trait Yay { }
+impl Yay for Foo { }
+
+fn foo() {
+    is_yay::<u32>();   //~ ERROR: the trait bound `u32: Yay` is not satisfied
+    is_debug::<u32>(); // OK
+    is_yay::<Foo>();   // OK
+    is_debug::<Foo>(); // OK
+}
+
+fn is_yay<T: Yay>() { }
+fn is_debug<T: Debug>() { }
+
+fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.stderr b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.stderr
new file mode 100644
index 00000000000..1c83105a18a
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound.stderr
@@ -0,0 +1,16 @@
+error[E0277]: the trait bound `u32: Yay` is not satisfied
+  --> $DIR/impl_trait_for_tait_bound.rs:10:14
+   |
+LL |     is_yay::<u32>();
+   |              ^^^ the trait `Yay` is not implemented for `u32`
+   |
+   = help: the trait `Yay` is implemented for `Foo`
+note: required by a bound in `is_yay`
+  --> $DIR/impl_trait_for_tait_bound.rs:16:14
+   |
+LL | fn is_yay<T: Yay>() { }
+   |              ^^^ required by this bound in `is_yay`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.rs b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.rs
new file mode 100644
index 00000000000..a4b8c2d190d
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.rs
@@ -0,0 +1,16 @@
+#![feature(type_alias_impl_trait)]
+
+use std::fmt::Debug;
+
+type Foo = impl Debug;
+
+pub trait Yay { }
+impl Yay for u32 { }
+
+fn foo() {
+    is_yay::<Foo>(); //~ ERROR: the trait bound `Foo: Yay` is not satisfied
+}
+
+fn is_yay<T: Yay>() { }
+
+fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.stderr b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.stderr
new file mode 100644
index 00000000000..a6440f02c27
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/impl_trait_for_tait_bound2.stderr
@@ -0,0 +1,16 @@
+error[E0277]: the trait bound `Foo: Yay` is not satisfied
+  --> $DIR/impl_trait_for_tait_bound2.rs:11:14
+   |
+LL |     is_yay::<Foo>();
+   |              ^^^ the trait `Yay` is not implemented for `Foo`
+   |
+   = help: the trait `Yay` is implemented for `u32`
+note: required by a bound in `is_yay`
+  --> $DIR/impl_trait_for_tait_bound2.rs:14:14
+   |
+LL | fn is_yay<T: Yay>() { }
+   |              ^^^ required by this bound in `is_yay`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/type-alias-impl-trait/issue-65384.rs b/src/test/ui/type-alias-impl-trait/issue-65384.rs
index 9a119c4d2e0..9a9b2269f80 100644
--- a/src/test/ui/type-alias-impl-trait/issue-65384.rs
+++ b/src/test/ui/type-alias-impl-trait/issue-65384.rs
@@ -8,7 +8,7 @@ impl MyTrait for () {}
 type Bar = impl MyTrait;
 
 impl MyTrait for Bar {}
-//~^ ERROR: cannot implement trait on type alias impl trait
+//~^ ERROR: conflicting implementations of trait `MyTrait` for type `()`
 
 fn bazr() -> Bar { }
 
diff --git a/src/test/ui/type-alias-impl-trait/issue-65384.stderr b/src/test/ui/type-alias-impl-trait/issue-65384.stderr
index 41bcea27e1f..f6692ae3207 100644
--- a/src/test/ui/type-alias-impl-trait/issue-65384.stderr
+++ b/src/test/ui/type-alias-impl-trait/issue-65384.stderr
@@ -1,14 +1,12 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-65384.rs:10:18
+error[E0119]: conflicting implementations of trait `MyTrait` for type `()`
+  --> $DIR/issue-65384.rs:10:1
    |
+LL | impl MyTrait for () {}
+   | ------------------- first implementation here
+...
 LL | impl MyTrait for Bar {}
-   |                  ^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/issue-65384.rs:8:12
-   |
-LL | type Bar = impl MyTrait;
-   |            ^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.rs b/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.rs
index fb56cc54d63..b97e444c6d0 100644
--- a/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.rs
+++ b/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.rs
@@ -1,6 +1,8 @@
 // Regression test for issue #76202
 // Tests that we don't ICE when we have a trait impl on a TAIT.
 
+// check-pass
+
 #![feature(type_alias_impl_trait)]
 
 trait Dummy {}
@@ -14,7 +16,12 @@ trait Test {
 }
 
 impl Test for F {
-    //~^ ERROR cannot implement trait
+    fn test(self) {}
+}
+
+// Ok because `i32` does not implement `Dummy`,
+// so it can't possibly be the hidden type of `F`.
+impl Test for i32 {
     fn test(self) {}
 }
 
diff --git a/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.stderr b/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.stderr
deleted file mode 100644
index 2d4a6854a92..00000000000
--- a/src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.stderr
+++ /dev/null
@@ -1,14 +0,0 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-76202-trait-impl-for-tait.rs:16:15
-   |
-LL | impl Test for F {
-   |               ^
-   |
-note: type alias impl trait defined here
-  --> $DIR/issue-76202-trait-impl-for-tait.rs:9:10
-   |
-LL | type F = impl Dummy;
-   |          ^^^^^^^^^^
-
-error: aborting due to previous error
-
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs b/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs
index fa25d8f762e..2ba4befea2a 100644
--- a/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs
+++ b/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs
@@ -1,6 +1,8 @@
 // Regression test for issues #84660 and #86411: both are variations on #76202.
 // Tests that we don't ICE when we have an opaque type appearing anywhere in an impl header.
 
+// check-pass
+
 #![feature(type_alias_impl_trait)]
 
 trait Foo {}
@@ -12,7 +14,7 @@ trait TraitArg<T> {
     fn f();
 }
 
-impl TraitArg<Bar> for () { //~ ERROR cannot implement trait
+impl TraitArg<Bar> for () {
     fn f() {
         println!("ho");
     }
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.stderr b/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.stderr
deleted file mode 100644
index bb70d07be59..00000000000
--- a/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.stderr
+++ /dev/null
@@ -1,14 +0,0 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-84660-trait-impl-for-tait.rs:15:15
-   |
-LL | impl TraitArg<Bar> for () {
-   |               ^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/issue-84660-trait-impl-for-tait.rs:8:12
-   |
-LL | type Bar = impl Foo;
-   |            ^^^^^^^^
-
-error: aborting due to previous error
-
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs
index f12d1b6d953..48d4b0c96ff 100644
--- a/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs
+++ b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs
@@ -13,14 +13,14 @@ trait Trait<T, In> {
     fn convert(i: In) -> Self::Out;
 }
 
-impl<In, Out> Trait<Bar, In> for Out { //~ ERROR cannot implement trait
+impl<In, Out> Trait<Bar, In> for Out {
     type Out = Out;
     fn convert(_i: In) -> Self::Out {
         unreachable!();
     }
 }
 
-impl<In, Out> Trait<(), In> for Out {
+impl<In, Out> Trait<(), In> for Out { //~ ERROR conflicting implementations of trait `Trait<Bar, _>`
     type Out = In;
     fn convert(i: In) -> Self::Out {
         i
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr
index f2d600fb46c..6a75e1bd2c0 100644
--- a/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr
+++ b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr
@@ -1,14 +1,12 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-84660-unsoundness.rs:16:21
+error[E0119]: conflicting implementations of trait `Trait<Bar, _>`
+  --> $DIR/issue-84660-unsoundness.rs:23:1
    |
 LL | impl<In, Out> Trait<Bar, In> for Out {
-   |                     ^^^
-   |
-note: type alias impl trait defined here
-  --> $DIR/issue-84660-unsoundness.rs:8:12
-   |
-LL | type Bar = impl Foo;
-   |            ^^^^^^^^
+   | ------------------------------------ first implementation here
+...
+LL | impl<In, Out> Trait<(), In> for Out {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/type-alias-impl-trait/nested-tait-inference3.rs b/src/test/ui/type-alias-impl-trait/nested-tait-inference3.rs
index ebf3a99bbf9..b0ebdd1bfab 100644
--- a/src/test/ui/type-alias-impl-trait/nested-tait-inference3.rs
+++ b/src/test/ui/type-alias-impl-trait/nested-tait-inference3.rs
@@ -4,11 +4,11 @@
 use std::fmt::Debug;
 
 type FooX = impl Debug;
+//~^ ERROR unconstrained opaque type
 
 trait Foo<A> { }
 
 impl Foo<FooX> for () { }
-//~^ cannot implement trait on type alias impl trait
 
 fn foo() -> impl Foo<FooX> {
     ()
diff --git a/src/test/ui/type-alias-impl-trait/nested-tait-inference3.stderr b/src/test/ui/type-alias-impl-trait/nested-tait-inference3.stderr
index 4a3fb16733e..b1d947a9ccf 100644
--- a/src/test/ui/type-alias-impl-trait/nested-tait-inference3.stderr
+++ b/src/test/ui/type-alias-impl-trait/nested-tait-inference3.stderr
@@ -1,14 +1,10 @@
-error: cannot implement trait on type alias impl trait
-  --> $DIR/nested-tait-inference3.rs:10:10
-   |
-LL | impl Foo<FooX> for () { }
-   |          ^^^^
-   |
-note: type alias impl trait defined here
+error: unconstrained opaque type
   --> $DIR/nested-tait-inference3.rs:6:13
    |
 LL | type FooX = impl Debug;
    |             ^^^^^^^^^^
+   |
+   = note: `FooX` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error
 
diff --git a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
index 65722f142aa..d183e28f667 100644
--- a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
+++ b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
@@ -37,6 +37,7 @@ pub fn is_min_const_fn<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, msrv: Option<
                 ty::PredicateKind::ClosureKind(..) => panic!("closure kind predicate on function: {predicate:#?}"),
                 ty::PredicateKind::Subtype(_) => panic!("subtype predicate on function: {predicate:#?}"),
                 ty::PredicateKind::Coerce(_) => panic!("coerce predicate on function: {predicate:#?}"),
+                ty::PredicateKind::Ambiguous => panic!("ambiguous predicate on function: {predicate:#?}"),
             }
         }
         match predicates.parent {