Rollup merge of #111665 - est31:offset_of_tests, r=WaffleLapkin

Add more tests for the offset_of macro

Implements what I [suggested in the tracking issue](https://github.com/rust-lang/rust/issues/106655#issuecomment-1535007205), plus some further improvements:

* ensuring that offset_of!(Self, ...) works iff inside an impl block
* ensuring that the output type is usize and doesn't coerce. this can be changed in the future, but if it is done, it should be a conscious decision
* improving the privacy checking test
* ensuring that generics don't let you escape the unsized check

r? `````@WaffleLapkin`````

library/core/tests/mem.rs

@@ -386,6 +386,21 @@ fn offset_of() {
     // Layout of tuples is unstable
     assert!(offset_of!((u8, u16), 0) <= size_of::<(u8, u16)>() - 1);
     assert!(offset_of!((u8, u16), 1) <= size_of::<(u8, u16)>() - 2);
+
+    #[repr(C)]
+    struct Generic<T> {
+        x: u8,
+        y: u32,
+        z: T
+    }
+
+    // Ensure that this type of generics works
+    fn offs_of_z<T>() -> usize {
+        offset_of!(Generic<T>, z)
+    }
+
+    assert_eq!(offset_of!(Generic<u8>, z), 8);
+    assert_eq!(offs_of_z::<u8>(), 8);
 }
 
 #[test]

tests/ui/offset-of/offset-of-dst-field.rs

@@ -26,8 +26,23 @@ struct Gamma {
     z: Extern,
 }
 
+struct Delta<T: ?Sized> {
+    x: u8,
+    y: u16,
+    z: T,
+}
+
 fn main() {
     offset_of!(Alpha, z); //~ ERROR the size for values of type
     offset_of!(Beta, z); //~ ERROR the size for values of type
     offset_of!(Gamma, z); //~ ERROR the size for values of type
 }
+
+fn delta() {
+    offset_of!(Delta<Alpha>, z); //~ ERROR the size for values of type
+    offset_of!(Delta<Extern>, z); //~ ERROR the size for values of type
+}
+
+fn generic_with_maybe_sized<T: ?Sized>() -> usize {
+    offset_of!(Delta<T>, z) //~ ERROR the size for values of type
+}

tests/ui/offset-of/offset-of-dst-field.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
-  --> $DIR/offset-of-dst-field.rs:30:5
+  --> $DIR/offset-of-dst-field.rs:36:5
    |
 LL |     offset_of!(Alpha, z);
    |     ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
@@ -8,7 +8,7 @@ LL |     offset_of!(Alpha, z);
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
-  --> $DIR/offset-of-dst-field.rs:31:5
+  --> $DIR/offset-of-dst-field.rs:37:5
    |
 LL |     offset_of!(Beta, z);
    |     ^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
@@ -17,7 +17,7 @@ LL |     offset_of!(Beta, z);
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: the size for values of type `Extern` cannot be known at compilation time
-  --> $DIR/offset-of-dst-field.rs:32:5
+  --> $DIR/offset-of-dst-field.rs:38:5
    |
 LL |     offset_of!(Gamma, z);
    |     ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
@@ -25,6 +25,44 @@ LL |     offset_of!(Gamma, z);
    = help: the trait `Sized` is not implemented for `Extern`
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 3 previous errors
+error[E0277]: the size for values of type `Extern` cannot be known at compilation time
+  --> $DIR/offset-of-dst-field.rs:43:5
+   |
+LL |     offset_of!(Delta<Extern>, z);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `Extern`
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/offset-of-dst-field.rs:42:5
+   |
+LL |     offset_of!(Delta<Alpha>, z);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: within `Alpha`, the trait `Sized` is not implemented for `[u8]`
+note: required because it appears within the type `Alpha`
+  --> $DIR/offset-of-dst-field.rs:5:8
+   |
+LL | struct Alpha {
+   |        ^^^^^
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the size for values of type `T` cannot be known at compilation time
+  --> $DIR/offset-of-dst-field.rs:47:5
+   |
+LL | fn generic_with_maybe_sized<T: ?Sized>() -> usize {
+   |                             - this type parameter needs to be `std::marker::Sized`
+LL |     offset_of!(Delta<T>, z)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider removing the `?Sized` bound to make the type parameter `Sized`
+   |
+LL - fn generic_with_maybe_sized<T: ?Sized>() -> usize {
+LL + fn generic_with_maybe_sized<T>() -> usize {
+   |
+
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/offset-of/offset-of-output-type.rs

@@ -0,0 +1,20 @@
+#![feature(offset_of)]
+
+use std::mem::offset_of;
+
+struct S {
+    v: u8,
+    w: u16,
+}
+
+
+fn main() {
+    let _: u8 = offset_of!(S, v); //~ ERROR mismatched types
+    let _: u16 = offset_of!(S, v); //~ ERROR mismatched types
+    let _: u32 = offset_of!(S, v); //~ ERROR mismatched types
+    let _: u64 = offset_of!(S, v); //~ ERROR mismatched types
+    let _: isize = offset_of!(S, v); //~ ERROR mismatched types
+    let _: usize = offset_of!(S, v);
+
+    offset_of!(S, v) //~ ERROR mismatched types
+}

tests/ui/offset-of/offset-of-output-type.stderr

@@ -0,0 +1,64 @@
+error[E0308]: mismatched types
+  --> $DIR/offset-of-output-type.rs:12:17
+   |
+LL |     let _: u8 = offset_of!(S, v);
+   |            --   ^^^^^^^^^^^^^^^^ expected `u8`, found `usize`
+   |            |
+   |            expected due to this
+   |
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0308]: mismatched types
+  --> $DIR/offset-of-output-type.rs:13:18
+   |
+LL |     let _: u16 = offset_of!(S, v);
+   |            ---   ^^^^^^^^^^^^^^^^ expected `u16`, found `usize`
+   |            |
+   |            expected due to this
+   |
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0308]: mismatched types
+  --> $DIR/offset-of-output-type.rs:14:18
+   |
+LL |     let _: u32 = offset_of!(S, v);
+   |            ---   ^^^^^^^^^^^^^^^^ expected `u32`, found `usize`
+   |            |
+   |            expected due to this
+   |
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0308]: mismatched types
+  --> $DIR/offset-of-output-type.rs:15:18
+   |
+LL |     let _: u64 = offset_of!(S, v);
+   |            ---   ^^^^^^^^^^^^^^^^ expected `u64`, found `usize`
+   |            |
+   |            expected due to this
+   |
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0308]: mismatched types
+  --> $DIR/offset-of-output-type.rs:16:20
+   |
+LL |     let _: isize = offset_of!(S, v);
+   |            -----   ^^^^^^^^^^^^^^^^ expected `isize`, found `usize`
+   |            |
+   |            expected due to this
+   |
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0308]: mismatched types
+  --> $DIR/offset-of-output-type.rs:19:5
+   |
+LL | fn main() {
+   |           - expected `()` because of default return type
+...
+LL |     offset_of!(S, v)
+   |     ^^^^^^^^^^^^^^^^ expected `()`, found `usize`
+   |
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/offset-of/offset-of-private.rs

@@ -8,9 +8,21 @@ mod m {
         pub public: u8,
         private: u8,
     }
+    #[repr(C)]
+    pub struct FooTuple(pub u8, u8);
+    #[repr(C)]
+    struct Bar {
+        pub public: u8,
+        private: u8,
+    }
 }
 
 fn main() {
     offset_of!(m::Foo, public);
     offset_of!(m::Foo, private); //~ ERROR field `private` of struct `Foo` is private
+    offset_of!(m::FooTuple, 0);
+    offset_of!(m::FooTuple, 1); //~ ERROR field `1` of struct `FooTuple` is private
+    offset_of!(m::Bar, public); //~ ERROR struct `Bar` is private
+    offset_of!(m::Bar, private); //~ ERROR struct `Bar` is private
+    //~| ERROR field `private` of struct `Bar` is private
 }

tests/ui/offset-of/offset-of-private.stderr

@@ -1,9 +1,46 @@
+error[E0603]: struct `Bar` is private
+  --> $DIR/offset-of-private.rs:25:19
+   |
+LL |     offset_of!(m::Bar, public);
+   |                   ^^^ private struct
+   |
+note: the struct `Bar` is defined here
+  --> $DIR/offset-of-private.rs:14:5
+   |
+LL |     struct Bar {
+   |     ^^^^^^^^^^
+
+error[E0603]: struct `Bar` is private
+  --> $DIR/offset-of-private.rs:26:19
+   |
+LL |     offset_of!(m::Bar, private);
+   |                   ^^^ private struct
+   |
+note: the struct `Bar` is defined here
+  --> $DIR/offset-of-private.rs:14:5
+   |
+LL |     struct Bar {
+   |     ^^^^^^^^^^
+
 error[E0616]: field `private` of struct `Foo` is private
-  --> $DIR/offset-of-private.rs:15:24
+  --> $DIR/offset-of-private.rs:22:24
    |
 LL |     offset_of!(m::Foo, private);
    |                        ^^^^^^^ private field
 
-error: aborting due to previous error
+error[E0616]: field `1` of struct `FooTuple` is private
+  --> $DIR/offset-of-private.rs:24:29
+   |
+LL |     offset_of!(m::FooTuple, 1);
+   |                             ^ private field
 
-For more information about this error, try `rustc --explain E0616`.
+error[E0616]: field `private` of struct `Bar` is private
+  --> $DIR/offset-of-private.rs:26:24
+   |
+LL |     offset_of!(m::Bar, private);
+   |                        ^^^^^^^ private field
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0603, E0616.
+For more information about an error, try `rustc --explain E0603`.

tests/ui/offset-of/offset-of-self.rs

@@ -0,0 +1,58 @@
+#![feature(offset_of)]
+
+use std::mem::offset_of;
+
+struct C<T> {
+    v: T,
+    w: T,
+}
+
+struct S {
+    v: u8,
+    w: u16,
+}
+
+impl S {
+    fn v_offs() -> usize {
+        offset_of!(Self, v)
+    }
+    fn v_offs_wrong_syntax() {
+        offset_of!(Self, Self::v); //~ ERROR no rules expected the token `::`
+        offset_of!(S, Self); //~ ERROR expected identifier, found keyword `Self`
+        //~| no field `Self` on type `S`
+    }
+    fn offs_in_c() -> usize {
+        offset_of!(C<Self>, w)
+    }
+    fn offs_in_c_colon() -> usize {
+        offset_of!(C::<Self>, w)
+    }
+}
+
+mod m {
+    use std::mem::offset_of;
+    fn off() {
+        offset_of!(self::S, v); //~ ERROR cannot find type `S` in module
+        offset_of!(super::S, v);
+        offset_of!(crate::S, v);
+    }
+    impl super::n::T {
+        fn v_offs_self() -> usize {
+            offset_of!(Self, v) //~ ERROR field `v` of struct `T` is private
+        }
+    }
+}
+
+mod n {
+    pub struct T { v: u8, }
+}
+
+fn main() {
+    offset_of!(self::S, v);
+    offset_of!(Self, v); //~ ERROR cannot find type `Self` in this scope
+
+    offset_of!(S, self); //~ ERROR expected identifier, found keyword `self`
+    //~| no field `self` on type `S`
+    offset_of!(S, v.self); //~ ERROR expected identifier, found keyword `self`
+    //~| no field `self` on type `u8`
+}

tests/ui/offset-of/offset-of-self.stderr

@@ -0,0 +1,79 @@
+error: no rules expected the token `::`
+  --> $DIR/offset-of-self.rs:20:30
+   |
+LL |         offset_of!(Self, Self::v);
+   |                              ^^ no rules expected this token in macro call
+   |
+   = note: while trying to match sequence start
+
+error: expected identifier, found keyword `Self`
+  --> $DIR/offset-of-self.rs:21:23
+   |
+LL |         offset_of!(S, Self);
+   |                       ^^^^ expected identifier, found keyword
+
+error: expected identifier, found keyword `self`
+  --> $DIR/offset-of-self.rs:54:19
+   |
+LL |     offset_of!(S, self);
+   |                   ^^^^ expected identifier, found keyword
+
+error: expected identifier, found keyword `self`
+  --> $DIR/offset-of-self.rs:56:21
+   |
+LL |     offset_of!(S, v.self);
+   |                     ^^^^ expected identifier, found keyword
+
+error[E0412]: cannot find type `S` in module `self`
+  --> $DIR/offset-of-self.rs:35:26
+   |
+LL |         offset_of!(self::S, v);
+   |                          ^ not found in `self`
+   |
+help: consider importing this struct
+   |
+LL +     use S;
+   |
+help: if you import `S`, refer to it directly
+   |
+LL -         offset_of!(self::S, v);
+LL +         offset_of!(S, v);
+   |
+
+error[E0411]: cannot find type `Self` in this scope
+  --> $DIR/offset-of-self.rs:52:16
+   |
+LL | fn main() {
+   |    ---- `Self` not allowed in a function
+LL |     offset_of!(self::S, v);
+LL |     offset_of!(Self, v);
+   |                ^^^^ `Self` is only available in impls, traits, and type definitions
+
+error[E0609]: no field `Self` on type `S`
+  --> $DIR/offset-of-self.rs:21:23
+   |
+LL |         offset_of!(S, Self);
+   |                       ^^^^
+
+error[E0616]: field `v` of struct `T` is private
+  --> $DIR/offset-of-self.rs:41:30
+   |
+LL |             offset_of!(Self, v)
+   |                              ^ private field
+
+error[E0609]: no field `self` on type `S`
+  --> $DIR/offset-of-self.rs:54:19
+   |
+LL |     offset_of!(S, self);
+   |                   ^^^^
+
+error[E0609]: no field `self` on type `u8`
+  --> $DIR/offset-of-self.rs:56:21
+   |
+LL |     offset_of!(S, v.self);
+   |                     ^^^^
+
+error: aborting due to 10 previous errors
+
+Some errors have detailed explanations: E0411, E0412, E0609, E0616.
+For more information about an error, try `rustc --explain E0411`.