Optionally add type names to TypeIds.

This feature is intended to provide expensive but thorough help for
developers who have an unexpected `TypeId` value and need to determine
what type it actually is. It causes `impl Debug for TypeId` to print
the type name in addition to the opaque ID hash, and in order to do so,
adds a name field to `TypeId`. The cost of this is the increased size of
`TypeId` and the need to store type names in the binary; therefore, it
is an optional feature.

It may be enabled via `cargo -Zbuild-std -Zbuild-std-features=debug_typeid`.
(Note that `-Zbuild-std-features` disables default features which you
may wish to reenable in addition; see
<https://doc.rust-lang.org/cargo/reference/unstable.html#build-std-features>.)

Example usage and output:

```
fn main() {
    use std::any::{Any, TypeId};
    dbg!(TypeId::of::<usize>(), drop::<usize>.type_id());
}
```

```
TypeId::of::<usize>() = TypeId(0x763d199bccd319899208909ed1a860c6 = usize)
drop::<usize>.type_id() = TypeId(0xe6a34bd13f8c92dd47806da07b8cca9a = core::mem::drop<usize>)
```

Also added feature declarations for the existing `debug_refcell` feature
so it is usable from the `rust.std-features` option of `config.toml`.

library/core/Cargo.toml

@@ -36,6 +36,8 @@ optimize_for_size = []
 # Make `RefCell` store additional debugging information, which is printed out when
 # a borrow error occurs
 debug_refcell = []
+# Make `TypeId` store a reference to the name of the type, so that it can print that name.
+debug_typeid = []
 
 [lints.rust.unexpected_cfgs]
 level = "warn"

library/core/src/any.rs

@@ -616,6 +616,8 @@ pub struct TypeId {
     // We avoid using `u128` because that imposes higher alignment requirements on many platforms.
     // See issue #115620 for more information.
     t: (u64, u64),
+    #[cfg(feature = "debug_typeid")]
+    name: &'static str,
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -647,10 +649,14 @@ impl TypeId {
     #[rustc_const_unstable(feature = "const_type_id", issue = "77125")]
     pub const fn of<T: ?Sized + 'static>() -> TypeId {
         let t: u128 = intrinsics::type_id::<T>();
-
         let t1 = (t >> 64) as u64;
         let t2 = t as u64;
-        TypeId { t: (t1, t2) }
+
+        TypeId {
+            t: (t1, t2),
+            #[cfg(feature = "debug_typeid")]
+            name: type_name::<T>(),
+        }
     }
 
     fn as_u128(self) -> u128 {
@@ -681,7 +687,12 @@ impl hash::Hash for TypeId {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Debug for TypeId {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
-        write!(f, "TypeId({:#034x})", self.as_u128())
+        write!(f, "TypeId({:#034x}", self.as_u128())?;
+        #[cfg(feature = "debug_typeid")]
+        {
+            write!(f, " = {}", self.name)?;
+        }
+        write!(f, ")")
     }
 }
 

library/core/tests/any.rs

@@ -118,6 +118,14 @@ fn any_unsized() {
     is_any::<[i32]>();
 }
 
+#[cfg(feature = "debug_typeid")]
+#[test]
+fn debug_typeid_includes_name() {
+    let type_id = TypeId::of::<[usize; 2]>();
+    let debug_str = format!("{type_id:?}");
+    assert!(debug_str.ends_with("= [usize; 2])"), "{debug_str:?} did not match");
+}
+
 #[test]
 fn distinct_type_names() {
     // https://github.com/rust-lang/rust/issues/84666

library/std/Cargo.toml

@@ -109,6 +109,13 @@ panic_immediate_abort = [
 # Choose algorithms that are optimized for binary size instead of runtime performance
 optimize_for_size = ["core/optimize_for_size", "alloc/optimize_for_size"]
 
+# Make `RefCell` store additional debugging information, which is printed out when
+# a borrow error occurs
+debug_refcell = ["core/debug_refcell"]
+# Make `TypeId` store a reference to the name of the type, so that it can print that name.
+debug_typeid = ["core/debug_typeid"]
+
+
 # Enable std_detect default features for stdarch/crates/std_detect:
 # https://github.com/rust-lang/stdarch/blob/master/crates/std_detect/Cargo.toml
 std_detect_file_io = ["std_detect/std_detect_file_io"]

library/sysroot/Cargo.toml

@@ -19,11 +19,13 @@ compiler-builtins-mem = ["std/compiler-builtins-mem"]
 compiler-builtins-no-asm = ["std/compiler-builtins-no-asm"]
 compiler-builtins-no-f16-f128 = ["std/compiler-builtins-no-f16-f128"]
 compiler-builtins-mangled-names = ["std/compiler-builtins-mangled-names"]
+debug_refcell = ["std/debug_refcell"]
+debug_typeid = ["std/debug_typeid"]
 llvm-libunwind = ["std/llvm-libunwind"]
 system-llvm-libunwind = ["std/system-llvm-libunwind"]
+optimize_for_size = ["std/optimize_for_size"]
 panic-unwind = ["std/panic_unwind"]
 panic_immediate_abort = ["std/panic_immediate_abort"]
-optimize_for_size = ["std/optimize_for_size"]
 profiler = ["dep:profiler_builtins"]
 std_detect_file_io = ["std/std_detect_file_io"]
 std_detect_dlsym_getauxval = ["std/std_detect_dlsym_getauxval"]