Improve ABI docs

And defer arrays to later

Author: Jules-Bertholet

compiler/rustc_abi/src/layout.rs

@@ -196,7 +196,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             size,
             max_repr_align: None,
             unadjusted_abi_align: element.align.abi,
-            repr_c: element.repr_c,
+            repr_c: false,
             randomization_seed: element.randomization_seed.wrapping_add(Hash64::new(count)),
         })
     }

compiler/rustc_abi/src/lib.rs

@@ -2142,8 +2142,7 @@ pub struct LayoutData<FieldIdx: Idx, VariantIdx: Idx> {
     /// in some cases.
     pub unadjusted_abi_align: Align,
 
-    /// Whether this type is `repr(C)`, or a`repr(transparent)` wrapper around such,
-    /// or an array of such.
+    /// Whether this type is `repr(C)`, or a`repr(transparent)` wrapper around such.
     /// Some C ABIs pass `repr(C)` ZSTs by pointer, but `repr(Rust)` ZSTs should always
     /// be ignored.
     pub repr_c: bool,

library/core/src/primitive_docs.rs

@@ -1842,10 +1842,14 @@ mod prim_ref {}
 ///   call will be valid ABI-wise. The callee receives the result of transmuting the function pointer
 ///   from `fn()` to `fn(i32)`; that transmutation is itself a well-defined operation, it's just
 ///   almost certainly UB to later call that function pointer.)
-/// - Any two types with size 0 and alignment 1 are ABI-compatible, unless they are `repr(C)`
-///   or a `repr(transparent)` wrapper around `repr(C)`.
-/// - A `repr(transparent)` type `T` is ABI-compatible with its unique non-trivial field, i.e., the
-///   unique field that doesn't have size 0 and alignment 1 (if there is such a field).
+/// - Any two types fulfilling all the following conditions are ABI-compatible;
+///   such types are said to have "trivial ABI":
+///   - Size 0
+///   - Alignment 1
+///   - Not `repr(C)`
+///   - Not a `repr(transparent)` wrapper around a type that fails to satisfy these conditions
+/// - A `repr(transparent)` type is ABI-compatible with its unique field that does not have trivial ABI
+///   (as defined above). If there is no such field, the type has trivial ABI.
 /// - `i32` is ABI-compatible with `NonZero<i32>`, and similar for all other integer types.
 /// - If `T` is guaranteed to be subject to the [null pointer
 ///   optimization](option/index.html#representation), and `E` is an enum satisfying the following