Add some #[must_use] to manually implemented builder types

... or improve their messages a bit.

Author: sdroege

gio/src/io_extension_point.rs

@@ -9,7 +9,7 @@ use crate::io_extension::IOExtension;
 
 /// Builder for extension points.
 #[derive(Debug)]
-#[must_use = "Builder doesn't do anything unless built"]
+#[must_use = "The builder must be built to be used"]
 pub struct IOExtensionPointBuilder<'a> {
     name: &'a str,
     required_type: Option<Type>,
@@ -31,6 +31,7 @@ impl<'a> IOExtensionPointBuilder<'a> {
         }
     }
 
+    #[must_use = "Building the object from the builder is usually expensive and is not expected to have side effects"]
     pub fn build(self) -> IOExtensionPoint {
         unsafe {
             let ep = IOExtensionPoint::from_glib_none(ffi::g_io_extension_point_register(

gio/src/settings.rs

@@ -6,7 +6,7 @@ use glib::translate::{from_glib_borrow, from_glib_none, IntoGlib, ToGlibPtr};
 use glib::variant::FromVariant;
 use glib::{BoolError, IsA, ToVariant};
 
-#[must_use]
+#[must_use = "The builder must be built to be used"]
 pub struct BindingBuilder<'a> {
     settings: &'a Settings,
     key: &'a str,

glib/src/enums.rs

@@ -675,6 +675,7 @@ impl Eq for FlagsValue {}
 /// ```
 ///
 /// If setting/unsetting any value fails, `build()` returns `None`.
+#[must_use = "The builder must be built to be used"]
 pub struct FlagsBuilder<'a>(&'a FlagsClass, Option<Value>);
 impl<'a> FlagsBuilder<'a> {
     fn new(flags_class: &FlagsClass) -> FlagsBuilder {
@@ -741,6 +742,7 @@ impl<'a> FlagsBuilder<'a> {
     }
 
     /// Converts to the final `Value`, unless any previous setting/unsetting of flags failed.
+    #[must_use = "Value returned from the builder should probably be used"]
     pub fn build(self) -> Option<Value> {
         self.1
     }

glib/src/gstring_builder.rs

@@ -13,6 +13,7 @@ use std::str;
 wrapper! {
     /// A mutable text buffer that grows automatically.
     #[doc(alias = "GString")]
+    #[must_use = "The builder must be built to be used"]
     pub struct GStringBuilder(Boxed<ffi::GString>);
 
     match fn {
@@ -96,6 +97,7 @@ impl GStringBuilder {
     /// # Panics
     ///
     /// If the string builder contains invalid UTF-8 this function panics.
+    #[must_use = "String returned from the builder should probably be used"]
     pub fn into_string(self) -> crate::GString {
         unsafe {
             let s = mem::ManuallyDrop::new(self);

glib/src/object.rs

@@ -3317,7 +3317,7 @@ type TransformFn =
     Option<Box<dyn Fn(&crate::Binding, &Value) -> Option<Value> + Send + Sync + 'static>>;
 
 /// Builder for object property bindings.
-#[must_use]
+#[must_use = "The builder must be built to be used"]
 pub struct BindingBuilder<'a> {
     source: &'a ObjectRef,
     source_property: &'a str,

glib/src/subclass/signal.rs

@@ -13,6 +13,7 @@ use std::{fmt, num::NonZeroU32};
 
 /// Builder for signals.
 #[allow(clippy::type_complexity)]
+#[must_use = "The builder must be built to be used"]
 pub struct SignalBuilder<'a> {
     name: &'a str,
     flags: SignalFlags,
@@ -442,6 +443,7 @@ impl<'a> SignalBuilder<'a> {
     ///
     /// This does not register the signal yet, which only happens as part of object type
     /// registration.
+    #[must_use = "Signal returned from the builder must be used for it to be registered"]
     pub fn build(self) -> Signal {
         let flags = if self.flags
             & (SignalFlags::RUN_FIRST | SignalFlags::RUN_LAST | SignalFlags::RUN_CLEANUP)