error[E0308]: mismatched types

[arcnmx]

gir generates ptr::null_mut() for &mut type_ instead of ptr::null() for a function that returns a const ptr (const gchar** argument type):

error[E0308]: mismatched types
src/auto/metadata.rs:71:123

impl<O: IsA<Metadata>> MetadataExt for O {
    fn find(&self, subject: u32, key: &str) -> (glib::GString, glib::GString) {
        unsafe {
            let mut type_ = ptr::null_mut();
            let ret = from_glib_none(ffi::wp_metadata_find(self.as_ref().to_glib_none().0, subject, key.to_glib_none().0, &mut type_));
            //                       --------------------- arguments to this function are incorrect                       ^^^^^^^^^^ types differ in muability
            (ret, from_glib_full(type_))
        }
    }
}

gir source

      <method name="find" c:identifier="wp_metadata_find">
        <doc xml:space="preserve" filename="../docs/wp-gtkdoc.h" line="1191">Finds the metadata value given its @subject and @key.</doc>
        <source-position filename="../lib/wp/metadata.h" line="50"/>
        <return-value transfer-ownership="none">
          <doc xml:space="preserve" filename="../docs/wp-gtkdoc.h" line="1203">the metadata string value, or NULL if not found.</doc>
          <type name="utf8" c:type="const gchar*"/>
        </return-value>
        <parameters>
          <instance-parameter name="self" transfer-ownership="none">
            <doc xml:space="preserve" filename="../docs/wp-gtkdoc.h" line="1193">a metadata object</doc>
            <type name="Metadata" c:type="WpMetadata*"/>
          </instance-parameter>
          <parameter name="subject" transfer-ownership="none">
            <doc xml:space="preserve" filename="../docs/wp-gtkdoc.h" line="1194">the metadata subject id</doc>
            <type name="guint32" c:type="guint32"/>
          </parameter>
          <parameter name="key" transfer-ownership="none">
            <doc xml:space="preserve" filename="../docs/wp-gtkdoc.h" line="1195">the metadata key name</doc>
            <type name="utf8" c:type="const gchar*"/>
          </parameter>
          <parameter name="type" direction="out" caller-allocates="0" transfer-ownership="full" optional="1" allow-none="1">
            <doc xml:space="preserve" filename="../docs/wp-gtkdoc.h" line="1196">the metadata type name</doc>
            <type name="utf8" c:type="const gchar**"/>
          </parameter>
        </parameters>
      </method>

• Gir.toml
• gir version

[bilelmoussaoui]

How would you write into a ptr::null()?

[arcnmx]

well it's a (const gchar*)* argument, so unless I'm mistaken, just swapping null_mut out for null in the example above should be correct?

so it becomes:

    let mut type_ = ptr::null();
    ffi::stuff(&mut type_); // this coerces to `&mut type_ as *mut (*const gchar)`

EDIT: sort of like what @sdroege said below, except it's just a simple output parameter returning a single string, there's no Vec involved afaict

EDIT2: relevant documentation for this specific function

[sdroege]

I think in Rust terms what this function returns is a *mut *const c_char or Vec<&'static glib::GStr> or so. The strings themselves are const but the returned array of const strings is not?

So this is more a transfer container than transfer full situation. And also needs an array zero-terminated=1 annotation in C

---

EDIT: This actually returns a single static string. So this is all correct and should've been a ptr::null() here. However you probably want to implement this one manually for now to avoid the useless allocation. This can just return a &'static glib::GStr.

[sdroege]

          <doc xml:space="preserve" filename="../docs/wp-gtkdoc.h" line="1203">the metadata string value, or NULL if not found.</doc>

This part, the return value, is missing a nullable annotation