glib/GStringPtr: Add as_str() and Deref<Target=&str>

[cgwalters]

I was updating some code to glib 0.18 and we were previously using as_str() - which is what the default Rust nomenclature for an infallible reference-to-reference conversion uses.

The to_str() is used for fallible conversions
(e.g. https://doc.rust-lang.org/std/ffi/struct.OsStr.html#method.to_str) which this is not.

[cgwalters]

Also BTW I didn't test this locally as I'm hitting a crate build error locally; I haven't dug in yet

EDIT: See #1185

walters@toolbox /v/s/w/s/g/g/gtk-rs-core (gstring-as-str)> cargo check
    Checking glib v0.19.0 (/var/srv/walters/src/github/gtk-rs/gtk-rs-core/glib)
error[E0277]: `T` doesn't implement `Debug`
    --> glib/src/gstring.rs:1318:12
     |
1318 | pub struct GStringNoTrailingNulError<T>(T);
     |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `T` cannot be formatted using `{:?}` because it doesn't implement `Debug`
     |
note: required for `GStringNoTrailingNulError<T>` to implement `Debug`
    --> glib/src/gstring.rs:1316:50
     |
1316 | #[derive(thiserror::Error, Clone, PartialEq, Eq, Debug)]
     |                                                  ^^^^^ unsatisfied trait bound introduced in this `derive` macro
note: required by a bound in `std::error::Error`
    --> /builddir/build/BUILD/rustc-1.72.0-src/library/core/src/error.rs:32:1
     = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider restricting type parameter `T`
     |
1318 | pub struct GStringNoTrailingNulError<T: std::fmt::Debug>(T);
     |                                       +++++++++++++++++

error[E0277]: `T` doesn't implement `Debug`
    --> glib/src/gstring.rs:1335:12
     |
1335 | pub struct GStringInteriorNulError<T>(T, GStrInteriorNulError);
     |            ^^^^^^^^^^^^^^^^^^^^^^^^^^ `T` cannot be formatted using `{:?}` because it doesn't implement `Debug`
     |
note: required for `GStringInteriorNulError<T>` to implement `Debug`
    --> glib/src/gstring.rs:1333:50
     |
1333 | #[derive(thiserror::Error, Clone, PartialEq, Eq, Debug)]
     |                                                  ^^^^^ unsatisfied trait bound introduced in this `derive` macro
note: required by a bound in `std::error::Error`
    --> /builddir/build/BUILD/rustc-1.72.0-src/library/core/src/error.rs:32:1
     = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider restricting type parameter `T`
     |
1335 | pub struct GStringInteriorNulError<T: std::fmt::Debug>(T, GStrInteriorNulError);
     |                                     +++++++++++++++++

error[E0277]: `T` doesn't implement `Debug`
    --> glib/src/gstring.rs:1358:12
     |
1358 | pub struct GStringUtf8Error<T>(T, std::str::Utf8Error);
     |            ^^^^^^^^^^^^^^^^^^^ `T` cannot be formatted using `{:?}` because it doesn't implement `Debug`
     |
note: required for `GStringUtf8Error<T>` to implement `Debug`
    --> glib/src/gstring.rs:1356:50
     |
1356 | #[derive(thiserror::Error, Clone, PartialEq, Eq, Debug)]
     |                                                  ^^^^^ unsatisfied trait bound introduced in this `derive` macro
note: required by a bound in `std::error::Error`
    --> /builddir/build/BUILD/rustc-1.72.0-src/library/core/src/error.rs:32:1
     = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider restricting type parameter `T`
     |
1358 | pub struct GStringUtf8Error<T: std::fmt::Debug>(T, std::str::Utf8Error);
     |                              +++++++++++++++++

error[E0277]: `T` doesn't implement `Debug`
    --> glib/src/gstring.rs:1379:10
     |
1379 | pub enum GStringFromError<T> {
     |          ^^^^^^^^^^^^^^^^^^^ `T` cannot be formatted using `{:?}` because it doesn't implement `Debug`
     |
note: required for `GStringFromError<T>` to implement `Debug`
    --> glib/src/gstring.rs:1378:28
     |
1378 | #[derive(thiserror::Error, Debug)]
     |                            ^^^^^ unsatisfied trait bound introduced in this `derive` macro
note: required by a bound in `std::error::Error`
    --> /builddir/build/BUILD/rustc-1.72.0-src/library/core/src/error.rs:32:1
     = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider restricting type parameter `T`
     |
1379 | pub enum GStringFromError<T: std::fmt::Debug> {
     |                            +++++++++++++++++

For more information about this error, try `rustc --explain E0277`.
error: could not compile `glib` (lib) due to 4 previous errors

[sdroege]

What's the status here?

[cgwalters]

Now that I can build locally, updated to fix the infinite recursion in Deref and I think this should be good to go. There's one minor outstanding question.