Add some regression tests related to #82494

src/test/ui/typeck/issue-75883.rs

@@ -0,0 +1,22 @@
+// Regression test for #75883.
+
+pub struct UI {}
+
+impl UI {
+    pub fn run() -> Result<_> {
+        //~^ ERROR: this enum takes 2 type arguments but only 1 type argument was supplied
+        //~| ERROR: the type placeholder `_` is not allowed within types on item signatures
+        let mut ui = UI {};
+        ui.interact();
+
+        unimplemented!();
+    }
+
+    pub fn interact(&mut self) -> Result<_> {
+        //~^ ERROR: this enum takes 2 type arguments but only 1 type argument was supplied
+        //~| ERROR: the type placeholder `_` is not allowed within types on item signatures
+        unimplemented!();
+    }
+}
+
+fn main() {}

src/test/ui/typeck/issue-75883.stderr

@@ -0,0 +1,52 @@
+error[E0107]: this enum takes 2 type arguments but only 1 type argument was supplied
+  --> $DIR/issue-75883.rs:6:21
+   |
+LL |     pub fn run() -> Result<_> {
+   |                     ^^^^^^ - supplied 1 type argument
+   |                     |
+   |                     expected 2 type arguments
+   |
+note: enum defined here, with 2 type parameters: `T`, `E`
+  --> $SRC_DIR/core/src/result.rs:LL:COL
+   |
+LL | pub enum Result<T, E> {
+   |          ^^^^^^ -  -
+help: add missing type argument
+   |
+LL |     pub fn run() -> Result<_, E> {
+   |                             ^^^
+
+error[E0107]: this enum takes 2 type arguments but only 1 type argument was supplied
+  --> $DIR/issue-75883.rs:15:35
+   |
+LL |     pub fn interact(&mut self) -> Result<_> {
+   |                                   ^^^^^^ - supplied 1 type argument
+   |                                   |
+   |                                   expected 2 type arguments
+   |
+note: enum defined here, with 2 type parameters: `T`, `E`
+  --> $SRC_DIR/core/src/result.rs:LL:COL
+   |
+LL | pub enum Result<T, E> {
+   |          ^^^^^^ -  -
+help: add missing type argument
+   |
+LL |     pub fn interact(&mut self) -> Result<_, E> {
+   |                                           ^^^
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/issue-75883.rs:15:42
+   |
+LL |     pub fn interact(&mut self) -> Result<_> {
+   |                                          ^ not allowed in type signatures
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/issue-75883.rs:6:28
+   |
+LL |     pub fn run() -> Result<_> {
+   |                            ^ not allowed in type signatures
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0107, E0121.
+For more information about an error, try `rustc --explain E0107`.

src/test/ui/typeck/issue-80779.rs

@@ -0,0 +1,13 @@
+// Regression test for #80779.
+
+pub struct T<'a>(&'a str);
+
+pub fn f<'a>(val: T<'a>) -> _ {
+    //~^ ERROR: the type placeholder `_` is not allowed within types on item signatures
+    g(val)
+}
+
+pub fn g(_: T<'static>) -> _ {}
+//~^ ERROR: the type placeholder `_` is not allowed within types on item signatures
+
+fn main() {}

src/test/ui/typeck/issue-80779.stderr

@@ -0,0 +1,21 @@
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/issue-80779.rs:10:28
+   |
+LL | pub fn g(_: T<'static>) -> _ {}
+   |                            ^
+   |                            |
+   |                            not allowed in type signatures
+   |                            help: replace with the correct return type: `()`
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/issue-80779.rs:5:29
+   |
+LL | pub fn f<'a>(val: T<'a>) -> _ {
+   |                             ^
+   |                             |
+   |                             not allowed in type signatures
+   |                             help: replace with the correct return type: `()`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0121`.