Rollup merge of #107633 - clubby789:option-string-coerce-fix, r=Nilstrieb

Fix suggestion for coercing Option<&String> to Option<&str>

Fixes #107604

This also makes the diagnostic `MachineApplicable`, and runs `rustfix` to check we're not producing incorrect code.

``@rustbot`` label +A-diagnostics

tests/ui/typeck/issue-89856.fixed

@@ -0,0 +1,18 @@
+// run-rustfix
+
+fn take_str_maybe(_: Option<&str>) { }
+fn main() {
+    let string = String::from("Hello, world");
+
+    let option: Option<String> = Some(string.clone());
+    take_str_maybe(option.as_deref());
+    //~^ ERROR: mismatched types [E0308]
+
+    let option_ref = Some(&string);
+    take_str_maybe(option_ref.map(|x| x.as_str()));
+    //~^ ERROR: mismatched types [E0308]
+
+    let option_ref_ref = option_ref.as_ref();
+    take_str_maybe(option_ref_ref.map(|x| x.as_str()));
+    //~^ ERROR: mismatched types [E0308]
+}

tests/ui/typeck/issue-89856.rs

@@ -1,8 +1,18 @@
-fn take_str_maybe(x: Option<&str>) -> Option<&str> { None }
+// run-rustfix
 
+fn take_str_maybe(_: Option<&str>) { }
 fn main() {
     let string = String::from("Hello, world");
-    let option = Some(&string);
+
+    let option: Option<String> = Some(string.clone());
     take_str_maybe(option);
     //~^ ERROR: mismatched types [E0308]
+
+    let option_ref = Some(&string);
+    take_str_maybe(option_ref);
+    //~^ ERROR: mismatched types [E0308]
+
+    let option_ref_ref = option_ref.as_ref();
+    take_str_maybe(option_ref_ref);
+    //~^ ERROR: mismatched types [E0308]
 }

tests/ui/typeck/issue-89856.stderr

@@ -1,23 +1,63 @@
 error[E0308]: mismatched types
-  --> $DIR/issue-89856.rs:6:20
+  --> $DIR/issue-89856.rs:8:20
    |
 LL |     take_str_maybe(option);
-   |     -------------- ^^^^^^ expected `Option<&str>`, found `Option<&String>`
+   |     -------------- ^^^^^^ expected `Option<&str>`, found `Option<String>`
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected enum `Option<&str>`
+              found enum `Option<String>`
+note: function defined here
+  --> $DIR/issue-89856.rs:3:4
+   |
+LL | fn take_str_maybe(_: Option<&str>) { }
+   |    ^^^^^^^^^^^^^^ ---------------
+help: try converting the passed type into a `&str`
+   |
+LL |     take_str_maybe(option.as_deref());
+   |                          +++++++++++
+
+error[E0308]: mismatched types
+  --> $DIR/issue-89856.rs:12:20
+   |
+LL |     take_str_maybe(option_ref);
+   |     -------------- ^^^^^^^^^^ expected `Option<&str>`, found `Option<&String>`
    |     |
    |     arguments to this function are incorrect
    |
    = note: expected enum `Option<&str>`
               found enum `Option<&String>`
 note: function defined here
-  --> $DIR/issue-89856.rs:1:4
+  --> $DIR/issue-89856.rs:3:4
    |
-LL | fn take_str_maybe(x: Option<&str>) -> Option<&str> { None }
+LL | fn take_str_maybe(_: Option<&str>) { }
    |    ^^^^^^^^^^^^^^ ---------------
 help: try converting the passed type into a `&str`
    |
-LL |     take_str_maybe(option.map(|x| &**x));
-   |                          ++++++++++++++
+LL |     take_str_maybe(option_ref.map(|x| x.as_str()));
+   |                              ++++++++++++++++++++
 
-error: aborting due to previous error
+error[E0308]: mismatched types
+  --> $DIR/issue-89856.rs:16:20
+   |
+LL |     take_str_maybe(option_ref_ref);
+   |     -------------- ^^^^^^^^^^^^^^ expected `Option<&str>`, found `Option<&&String>`
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected enum `Option<&str>`
+              found enum `Option<&&String>`
+note: function defined here
+  --> $DIR/issue-89856.rs:3:4
+   |
+LL | fn take_str_maybe(_: Option<&str>) { }
+   |    ^^^^^^^^^^^^^^ ---------------
+help: try converting the passed type into a `&str`
+   |
+LL |     take_str_maybe(option_ref_ref.map(|x| x.as_str()));
+   |                                  ++++++++++++++++++++
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0308`.