crate-ify compiler-rt into compiler-builtins

libcompiler-rt.a is dead, long live libcompiler-builtins.rlib

This commit moves the logic that used to build libcompiler-rt.a into a
compiler-builtins crate on top of the core crate and below the std crate.
This new crate still compiles the compiler-rt instrinsics using gcc-rs
but produces an .rlib instead of a static library.

Also, with this commit rustc no longer passes -lcompiler-rt to the
linker. This effectively makes the "no-compiler-rt" field of target
specifications a no-op. Users of `no_std` will have to explicitly add
the compiler-builtins crate to their crate dependency graph *if* they
need the compiler-rt intrinsics. Users of the `std` have to do nothing
extra as the std crate depends on compiler-builtins.

Finally, this a step towards lazy compilation of std with Cargo as the
compiler-rt intrinsics can now be built by Cargo instead of having to
be supplied by the user by some other method.

closes #34400

1 files changed, 10 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 8b8a41fc204..27b97a0ad66 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -298,6 +298,10 @@ declare_features! (
 
     // elide `'static` lifetimes in `static`s and `const`s
     (active, static_in_const, "1.13.0", Some(35897)),
+
+    // Used to identify the `compiler_builtins` crate
+    // rustc internal
+    (active, compiler_builtins, "1.13.0", None),
 );
 
 declare_features! (
@@ -537,6 +541,12 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeGat
                                                           libcore functions that are inlined \
                                                           across crates and will never be stable",
                                                           cfg_fn!(rustc_attrs))),
+    ("compiler_builtins", Whitelisted, Gated("compiler_builtins",
+                                             "the `#[compiler_builtins]` attribute is used to \
+                                              identify the `compiler_builtins` crate which \
+                                              contains compiler-rt intrinsics and will never be \
+                                              stable",
+                                          cfg_fn!(compiler_builtins))),
 
     ("allow_internal_unstable", Normal, Gated("allow_internal_unstable",
                                               EXPLAIN_ALLOW_INTERNAL_UNSTABLE,