Rollup merge of rust-lang#118234 - tgross35:type_name_of_value, r=dtolnay

Stabilize `type_name_of_val`

Make the following API stable:

```rust
// in core::any
pub fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str
```

This is a convenience method to get the type name of a value, as opposed to `type_name` that takes a type as a generic.

Const stability is not added because this relies on `type_name` which is also not const. That has a blocking issue rust-lang#97156.

Wording was also changed to direct most of the details to `type_name` so we don't have as much duplicated documentation.

Fixes tracking issue rust-lang#66359.

There were two main concerns in the tracking issue:

1. Naming: `type_name_of` and `type_name_of_val` seem like the only mentioned options. Differences in opinion here come from `std::mem::{size_of, align_of, size_of_val, align_of_val}`. This PR leaves the name as `type_name_of_val`, but I can change if desired since it is pretty verbose.
2. What this displays for `&dyn`: I don't think that having `type_name_of_val` function resolve those is worth the headache it would be, see rust-lang#66359 (comment) for some workarounds. I also amended the docs wording to leave it open-ended, in case we have means to change that behavior in the future.

`@rustbot` label -T-libs +T-libs-api +needs-fcp
r? libs-api

Author: matthiaskrgr

library/core/src/any.rs

@@ -695,44 +695,41 @@ pub const fn type_name<T: ?Sized>() -> &'static str {
     intrinsics::type_name::<T>()
 }
 
-/// Returns the name of the type of the pointed-to value as a string slice.
+/// Returns the type name of the pointed-to value as a string slice.
+///
 /// This is the same as `type_name::<T>()`, but can be used where the type of a
 /// variable is not easily available.
 ///
 /// # Note
 ///
-/// This is intended for diagnostic use. The exact contents and format of the
-/// string are not specified, other than being a best-effort description of the
-/// type. For example, `type_name_of_val::<Option<String>>(None)` could return
-/// `"Option<String>"` or `"std::option::Option<std::string::String>"`, but not
-/// `"foobar"`. In addition, the output may change between versions of the
-/// compiler.
-///
-/// This function does not resolve trait objects,
-/// meaning that `type_name_of_val(&7u32 as &dyn Debug)`
-/// may return `"dyn Debug"`, but not `"u32"`.
+/// Like [`type_name`], this is intended for diagnostic use and the exact output is not
+/// guaranteed. It provides a best-effort description, but the output may change between
+/// versions of the compiler.
 ///
-/// The type name should not be considered a unique identifier of a type;
-/// multiple types may share the same type name.
+/// In short: use this for debugging, avoid using the output to affect program behavior. More
+/// information is available at [`type_name`].
 ///
-/// The current implementation uses the same infrastructure as compiler
-/// diagnostics and debuginfo, but this is not guaranteed.
+/// Additionally, this function does not resolve trait objects. This means that
+/// `type_name_of_val(&7u32 as &dyn Debug)` may return `"dyn Debug"`, but will not return `"u32"`
+/// at this time.
 ///
 /// # Examples
 ///
 /// Prints the default integer and float types.
 ///
 /// ```rust
-/// #![feature(type_name_of_val)]
 /// use std::any::type_name_of_val;
 ///
-/// let x = 1;
-/// println!("{}", type_name_of_val(&x));
-/// let y = 1.0;
-/// println!("{}", type_name_of_val(&y));
+/// let s = "foo";
+/// let x: i32 = 1;
+/// let y: f32 = 1.0;
+///
+/// assert!(type_name_of_val(&s).contains("str"));
+/// assert!(type_name_of_val(&x).contains("i32"));
+/// assert!(type_name_of_val(&y).contains("f32"));
 /// ```
 #[must_use]
-#[unstable(feature = "type_name_of_val", issue = "66359")]
+#[stable(feature = "type_name_of_val", since = "CURRENT_RUSTC_VERSION")]
 #[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
 pub const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str {
     type_name::<T>()

tests/ui/hygiene/cross-crate-define-and-use.rs

@@ -6,7 +6,6 @@
 // check-pass
 // aux-build:use_by_macro.rs
 
-#![feature(type_name_of_val)]
 extern crate use_by_macro;
 
 use use_by_macro::*;