Auto merge of #34491 - eddyb:return-in-peace, r=nagisa

Remove the return_address intrinsic.

This intrinsic to get the return pointer was introduced in #16248 / #16081 by @pcwalton for Servo.
However, as explained in #34227, it's impossible to ensure it's used correctly, and it broke with `-Zorbit`.

Servo's usage is being replaced in servo/servo#11872, and I expect nobody else to have abused it.
But I've also started a crater run, just in case this is a `[breaking-change]` for anyone else.

Author: bors

src/libcore/intrinsics.rs

@@ -293,12 +293,6 @@ extern "rust-intrinsic" {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn transmute<T, U>(e: T) -> U;
 
-    /// Gives the address for the return value of the enclosing function.
-    ///
-    /// Using this intrinsic in a function that does not use an out pointer
-    /// will trigger a compiler error.
-    pub fn return_address() -> *const u8;
-
     /// Returns `true` if the actual type given as `T` requires drop
     /// glue; returns `false` if the actual type provided for `T`
     /// implements `Copy`.

src/librustc_trans/diagnostics.rs

@@ -12,44 +12,6 @@
 
 register_long_diagnostics! {
 
-E0510: r##"
-`return_address` was used in an invalid context. Erroneous code example:
-
-```ignore
-#![feature(intrinsics)]
-
-extern "rust-intrinsic" {
-    fn return_address() -> *const u8;
-}
-
-unsafe fn by_value() -> i32 {
-    let _ = return_address();
-    // error: invalid use of `return_address` intrinsic: function does
-    //        not use out pointer
-    0
-}
-```
-
-Return values may be stored in a return register(s) or written into a so-called
-out pointer. In case the returned value is too big (this is
-target-ABI-dependent and generally not portable or future proof) to fit into
-the return register(s), the compiler will return the value by writing it into
-space allocated in the caller's stack frame. Example:
-
-```
-#![feature(intrinsics)]
-
-extern "rust-intrinsic" {
-    fn return_address() -> *const u8;
-}
-
-unsafe fn by_pointer() -> String {
-    let _ = return_address();
-    String::new() // ok!
-}
-```
-"##,
-
 E0511: r##"
 Invalid monomorphization of an intrinsic function was used. Erroneous code
 example:

src/librustc_trans/intrinsic.rs

@@ -617,18 +617,6 @@ pub fn trans_intrinsic_call<'a, 'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
 
         },
 
-
-        (_, "return_address") => {
-            if !fcx.fn_ty.ret.is_indirect() {
-                span_err!(tcx.sess, span, E0510,
-                          "invalid use of `return_address` intrinsic: function \
-                           does not use out pointer");
-                C_null(Type::i8p(ccx))
-            } else {
-                PointerCast(bcx, llvm::get_param(fcx.llfn, 0), Type::i8p(ccx))
-            }
-        }
-
         (_, "discriminant_value") => {
             let val_ty = substs.types.get(FnSpace, 0);
             match val_ty.sty {

src/librustc_typeck/check/intrinsic.rs

@@ -275,8 +275,6 @@ pub fn check_intrinsic_type(ccx: &CrateCtxt, it: &hir::ForeignItem) {
             "fadd_fast" | "fsub_fast" | "fmul_fast" | "fdiv_fast" | "frem_fast" =>
                 (1, vec![param(ccx, 0), param(ccx, 0)], param(ccx, 0)),
 
-            "return_address" => (0, vec![], tcx.mk_imm_ptr(tcx.types.u8)),
-
             "assume" => (0, vec![tcx.types.bool], tcx.mk_nil()),
 
             "discriminant_value" => (1, vec![