Add test for suggestion to borrow unsized function parameters

This is a regression test for rust-lang#82820.

This test case is included in more general tests, but I think the error
regressed because there were a bunch of other diagnostic changes in the
test that obscured this regression.

Hopefully, having a test specific to the suggestion, and running rustfix
for the test, will prevent this error from regressing in the future.

Author: camelid

src/test/ui/suggestions/unsized-function-parameter.fixed

@@ -0,0 +1,23 @@
+// run-rustfix
+
+#![allow(dead_code, unused_variables)]
+
+fn foo1(bar: &str) {}
+//~^ ERROR the size for values of type `str` cannot be known at compilation time
+//~| HELP the trait `Sized` is not implemented for `str`
+//~| HELP unsized fn params are gated as an unstable feature
+//~| HELP function arguments must have a statically known size, borrowed types always have a known size
+
+fn foo2(_bar: &str) {}
+//~^ ERROR the size for values of type `str` cannot be known at compilation time
+//~| HELP the trait `Sized` is not implemented for `str`
+//~| HELP unsized fn params are gated as an unstable feature
+//~| HELP function arguments must have a statically known size, borrowed types always have a known size
+
+fn foo3(_: &str) {}
+//~^ ERROR the size for values of type `str` cannot be known at compilation time
+//~| HELP the trait `Sized` is not implemented for `str`
+//~| HELP unsized fn params are gated as an unstable feature
+//~| HELP function arguments must have a statically known size, borrowed types always have a known size
+
+fn main() {}

src/test/ui/suggestions/unsized-function-parameter.rs

@@ -0,0 +1,23 @@
+// run-rustfix
+
+#![allow(dead_code, unused_variables)]
+
+fn foo1(bar: str) {}
+//~^ ERROR the size for values of type `str` cannot be known at compilation time
+//~| HELP the trait `Sized` is not implemented for `str`
+//~| HELP unsized fn params are gated as an unstable feature
+//~| HELP function arguments must have a statically known size, borrowed types always have a known size
+
+fn foo2(_bar: str) {}
+//~^ ERROR the size for values of type `str` cannot be known at compilation time
+//~| HELP the trait `Sized` is not implemented for `str`
+//~| HELP unsized fn params are gated as an unstable feature
+//~| HELP function arguments must have a statically known size, borrowed types always have a known size
+
+fn foo3(_: str) {}
+//~^ ERROR the size for values of type `str` cannot be known at compilation time
+//~| HELP the trait `Sized` is not implemented for `str`
+//~| HELP unsized fn params are gated as an unstable feature
+//~| HELP function arguments must have a statically known size, borrowed types always have a known size
+
+fn main() {}

src/test/ui/suggestions/unsized-function-parameter.stderr

@@ -0,0 +1,42 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/unsized-function-parameter.rs:5:9
+   |
+LL | fn foo1(bar: str) {}
+   |         ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = help: unsized fn params are gated as an unstable feature
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL | fn foo1(bar: &str) {}
+   |              ^
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/unsized-function-parameter.rs:11:9
+   |
+LL | fn foo2(_bar: str) {}
+   |         ^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = help: unsized fn params are gated as an unstable feature
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL | fn foo2(_bar: &str) {}
+   |               ^
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/unsized-function-parameter.rs:17:9
+   |
+LL | fn foo3(_: str) {}
+   |         ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = help: unsized fn params are gated as an unstable feature
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL | fn foo3(_: &str) {}
+   |            ^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.