Auto merge of #90645 - terrarier2111:master, r=estebank

Implement diagnostic for String conversion

This is my first real contribution to rustc, any feedback is highly appreciated.
This should fix https://github.com/rust-lang/rust/issues/89856

Thanks to `@estebank` for guiding me.

2 files changed, 24 insertions, 0 deletions

diff --git a/src/test/ui/typeck/issue-89856.rs b/src/test/ui/typeck/issue-89856.rs
new file mode 100644
index 00000000000..b021e349e35
--- /dev/null
+++ b/src/test/ui/typeck/issue-89856.rs
@@ -0,0 +1,8 @@
+fn take_str_maybe(x: Option<&str>) -> Option<&str> { None }
+
+fn main() {
+    let string = String::from("Hello, world");
+    let option = Some(&string);
+    take_str_maybe(option);
+    //~^ ERROR: mismatched types [E0308]
+}
diff --git a/src/test/ui/typeck/issue-89856.stderr b/src/test/ui/typeck/issue-89856.stderr
new file mode 100644
index 00000000000..4cb46a34a07
--- /dev/null
+++ b/src/test/ui/typeck/issue-89856.stderr
@@ -0,0 +1,16 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-89856.rs:6:20
+   |
+LL |     take_str_maybe(option);
+   |                    ^^^^^^ expected `str`, found struct `String`
+   |
+   = note: expected enum `Option<&str>`
+              found enum `Option<&String>`
+help: try converting the passed type into a `&str`
+   |
+LL |     take_str_maybe(option.map(|x| &**x));
+   |                          ++++++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.