Rollup merge of #92164 - WaffleLapkin:rustc_must_implement_one_of_attr, r=Aaron1011

Implement `#[rustc_must_implement_one_of]` attribute

This PR adds a new attribute — `#[rustc_must_implement_one_of]` that allows changing the "minimal complete definition" of a trait. It's similar to GHC's minimal `{-# MINIMAL #-}` pragma, though `#[rustc_must_implement_one_of]` is weaker atm.

Such attribute was long wanted. It can be, for example, used in `Read` trait to make transitions to recently added `read_buf` easier:
```rust
#[rustc_must_implement_one_of(read, read_buf)]
pub trait Read {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        let mut buf = ReadBuf::new(buf);
        self.read_buf(&mut buf)?;
        Ok(buf.filled_len())
    }

    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> Result<()> {
        default_read_buf(|b| self.read(b), buf)
    }
}

impl Read for Ty0 {}
//^ This will fail to compile even though all `Read` methods have default implementations

// Both of these will compile just fine
impl Read for Ty1 {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> { /* ... */ }
}
impl Read for Ty2 {
    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> Result<()> { /* ... */ }
}
```

For now, this is implemented as an internal attribute to start experimenting on the design of this feature. In the future we may want to extend it:
- Allow arbitrary requirements like `a | (b & c)`
- Allow multiple requirements like
  - ```rust
    #[rustc_must_implement_one_of(a, b)]
    #[rustc_must_implement_one_of(c, d)]
    ```
- Make it appear in rustdoc documentation
- Change the syntax?
- Etc

Eventually, we should make an RFC and make this (or rather similar) attribute public.

---

I'm fairly new to compiler development and not at all sure if the implementation makes sense, but at least it passes tests :)

compiler/rustc_middle/src/ty/trait_def.rs

@@ -1,7 +1,7 @@
 use crate::traits::specialization_graph;
 use crate::ty::fast_reject::{self, SimplifiedType, SimplifyParams, StripReferences};
 use crate::ty::fold::TypeFoldable;
-use crate::ty::{Ty, TyCtxt};
+use crate::ty::{Ident, Ty, TyCtxt};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::definitions::DefPathHash;
@@ -44,6 +44,10 @@ pub struct TraitDef {
     /// The ICH of this trait's DefPath, cached here so it doesn't have to be
     /// recomputed all the time.
     pub def_path_hash: DefPathHash,
+
+    /// List of functions from `#[rustc_must_implement_one_of]` attribute one of which
+    /// must be implemented.
+    pub must_implement_one_of: Option<Box<[Ident]>>,
 }
 
 /// Whether this trait is treated specially by the standard library
@@ -87,6 +91,7 @@ impl<'tcx> TraitDef {
         skip_array_during_method_dispatch: bool,
         specialization_kind: TraitSpecializationKind,
         def_path_hash: DefPathHash,
+        must_implement_one_of: Option<Box<[Ident]>>,
     ) -> TraitDef {
         TraitDef {
             def_id,
@@ -97,6 +102,7 @@ impl<'tcx> TraitDef {
             skip_array_during_method_dispatch,
             specialization_kind,
             def_path_hash,
+            must_implement_one_of,
         }
     }