diff --git a/config.toml.example b/config.toml.example
index 9dd3002506e41..68bc7dfe720fe 100644
--- a/config.toml.example
+++ b/config.toml.example
@@ -339,6 +339,9 @@
 # rustc to execute.
 #lld = false
 
+# Whether to deny warnings in crates
+#deny-warnings = true
+
 # =============================================================================
 # Options for specific targets
 #
diff --git a/src/bootstrap/bin/rustc.rs b/src/bootstrap/bin/rustc.rs
index 6701f58ba8e92..3dd9b6840591b 100644
--- a/src/bootstrap/bin/rustc.rs
+++ b/src/bootstrap/bin/rustc.rs
@@ -279,6 +279,10 @@ fn main() {
         cmd.arg("--color=always");
     }
 
+    if env::var_os("RUSTC_DENY_WARNINGS").is_some() {
+        cmd.arg("-Dwarnings");
+    }
+
     if verbose > 1 {
         eprintln!("rustc command: {:?}", cmd);
         eprintln!("sysroot: {:?}", sysroot);
diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 3f5ec4933d02b..7ff64af919671 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -698,6 +698,11 @@ impl<'a> Builder<'a> {
 
         cargo.env("RUSTC_VERBOSE", format!("{}", self.verbosity));
 
+        // in std, we want to avoid denying warnings for stage 0 as that makes cfg's painful.
+        if self.config.deny_warnings && !(mode == Mode::Libstd && stage == 0) {
+            cargo.env("RUSTC_DENY_WARNINGS", "1");
+        }
+
         // Throughout the build Cargo can execute a number of build scripts
         // compiling C/C++ code and we need to pass compilers, archivers, flags, etc
         // obtained previously to those build scripts.
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index 95baf4f8cca45..239316d45c49c 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -71,6 +71,8 @@ pub struct Config {
     pub incremental: bool,
     pub dry_run: bool,
 
+    pub deny_warnings: bool,
+
     // llvm codegen options
     pub llvm_enabled: bool,
     pub llvm_assertions: bool,
@@ -301,6 +303,7 @@ struct Rust {
     codegen_backends_dir: Option<String>,
     wasm_syscall: Option<bool>,
     lld: Option<bool>,
+    deny_warnings: Option<bool>,
 }
 
 /// TOML representation of how each build target is configured.
@@ -340,6 +343,7 @@ impl Config {
         config.test_miri = false;
         config.rust_codegen_backends = vec![INTERNER.intern_str("llvm")];
         config.rust_codegen_backends_dir = "codegen-backends".to_owned();
+        config.deny_warnings = true;
 
         // set by bootstrap.py
         config.src = env::var_os("SRC").map(PathBuf::from).expect("'SRC' to be set");
@@ -366,6 +370,9 @@ impl Config {
         config.incremental = flags.incremental;
         config.dry_run = flags.dry_run;
         config.keep_stage = flags.keep_stage;
+        if let Some(value) = flags.warnings {
+            config.deny_warnings = value;
+        }
 
         if config.dry_run {
             let dir = config.out.join("tmp-dry-run");
@@ -511,6 +518,7 @@ impl Config {
             config.rustc_default_linker = rust.default_linker.clone();
             config.musl_root = rust.musl_root.clone().map(PathBuf::from);
             config.save_toolstates = rust.save_toolstates.clone().map(PathBuf::from);
+            set(&mut config.deny_warnings, rust.deny_warnings.or(flags.warnings));
 
             if let Some(ref backends) = rust.codegen_backends {
                 config.rust_codegen_backends = backends.iter()
diff --git a/src/bootstrap/flags.rs b/src/bootstrap/flags.rs
index ef902c68d12d8..3eb9dca2aa835 100644
--- a/src/bootstrap/flags.rs
+++ b/src/bootstrap/flags.rs
@@ -42,6 +42,9 @@ pub struct Flags {
     pub exclude: Vec<PathBuf>,
     pub rustc_error_format: Option<String>,
     pub dry_run: bool,
+
+    // true => deny
+    pub warnings: Option<bool>,
 }
 
 pub enum Subcommand {
@@ -118,6 +121,8 @@ To learn more about a subcommand, run `./x.py <subcommand> -h`");
         opts.optopt("", "src", "path to the root of the rust checkout", "DIR");
         opts.optopt("j", "jobs", "number of jobs to run in parallel", "JOBS");
         opts.optflag("h", "help", "print this help message");
+        opts.optopt("", "warnings", "if value is deny, will deny warnings, otherwise use default",
+            "VALUE");
         opts.optopt("", "error-format", "rustc error format", "FORMAT");
 
         // fn usage()
@@ -374,6 +379,7 @@ Arguments:
             incremental: matches.opt_present("incremental"),
             exclude: split(matches.opt_strs("exclude"))
                 .into_iter().map(|p| p.into()).collect::<Vec<_>>(),
+            warnings: matches.opt_str("warnings").map(|v| v == "deny"),
         }
     }
 }
diff --git a/src/build_helper/lib.rs b/src/build_helper/lib.rs
index 5a12afd03e13b..e5c85ddb3a902 100644
--- a/src/build_helper/lib.rs
+++ b/src/build_helper/lib.rs
@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![deny(warnings)]
 
 use std::fs::File;
 use std::path::{Path, PathBuf};
diff --git a/src/liballoc/benches/lib.rs b/src/liballoc/benches/lib.rs
index 4d92fc67b2a50..4f69aa6670b56 100644
--- a/src/liballoc/benches/lib.rs
+++ b/src/liballoc/benches/lib.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![deny(warnings)]
-
 #![feature(rand)]
 #![feature(repr_simd)]
 #![feature(slice_sort_by_cached_key)]
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index da26e7c852c46..b08bd66b47c59 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -72,7 +72,6 @@
        test(no_crate_inject, attr(allow(unused_variables), deny(warnings))))]
 #![no_std]
 #![needs_allocator]
-#![deny(warnings)]
 #![deny(missing_debug_implementations)]
 
 #![cfg_attr(test, allow(deprecated))] // rand
diff --git a/src/liballoc/tests/lib.rs b/src/liballoc/tests/lib.rs
index a173ef10a818e..17f1d0464a5c2 100644
--- a/src/liballoc/tests/lib.rs
+++ b/src/liballoc/tests/lib.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![deny(warnings)]
-
 #![feature(allocator_api)]
 #![feature(alloc_system)]
 #![feature(attr_literals)]
diff --git a/src/liballoc_jemalloc/lib.rs b/src/liballoc_jemalloc/lib.rs
index 7a8d01e4ef8b8..df7e3f61f5f1d 100644
--- a/src/liballoc_jemalloc/lib.rs
+++ b/src/liballoc_jemalloc/lib.rs
@@ -14,7 +14,6 @@
             reason = "this library is unlikely to be stabilized in its current \
                       form or name",
             issue = "27783")]
-#![deny(warnings)]
 #![feature(alloc_system)]
 #![feature(libc)]
 #![feature(linkage)]
diff --git a/src/liballoc_system/lib.rs b/src/liballoc_system/lib.rs
index d4404e564e063..cdcb732f63547 100644
--- a/src/liballoc_system/lib.rs
+++ b/src/liballoc_system/lib.rs
@@ -10,7 +10,6 @@
 
 #![no_std]
 #![allow(unused_attributes)]
-#![deny(warnings)]
 #![unstable(feature = "alloc_system",
             reason = "this library is unlikely to be stabilized in its current \
                       form or name",
diff --git a/src/libarena/lib.rs b/src/libarena/lib.rs
index 7eaf67e6ea66e..b319f333342c1 100644
--- a/src/libarena/lib.rs
+++ b/src/libarena/lib.rs
@@ -22,7 +22,6 @@
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/",
        test(no_crate_inject, attr(deny(warnings))))]
-#![deny(warnings)]
 
 #![feature(alloc)]
 #![feature(core_intrinsics)]
diff --git a/src/libcore/benches/lib.rs b/src/libcore/benches/lib.rs
index c947b003ccbff..ced77d779182a 100644
--- a/src/libcore/benches/lib.rs
+++ b/src/libcore/benches/lib.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![deny(warnings)]
-
 #![feature(flt2dec)]
 #![feature(test)]
 
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index cf9abb26d3e1a..e194b173aa718 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -63,7 +63,6 @@
 #![no_core]
 #![deny(missing_docs)]
 #![deny(missing_debug_implementations)]
-#![deny(warnings)]
 
 #![feature(allow_internal_unstable)]
 #![feature(asm)]
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 971759dcdd08d..c3162899bbd01 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![deny(warnings)]
-
 #![feature(ascii_ctype)]
 #![feature(box_syntax)]
 #![feature(core_float)]
diff --git a/src/libfmt_macros/lib.rs b/src/libfmt_macros/lib.rs
index 0f45f965104ca..a551b1b770a68 100644
--- a/src/libfmt_macros/lib.rs
+++ b/src/libfmt_macros/lib.rs
@@ -19,7 +19,6 @@
        html_root_url = "https://doc.rust-lang.org/nightly/",
        html_playground_url = "https://play.rust-lang.org/",
        test(attr(deny(warnings))))]
-#![deny(warnings)]
 
 pub use self::Piece::*;
 pub use self::Position::*;
diff --git a/src/libgraphviz/lib.rs b/src/libgraphviz/lib.rs
index d8c366d2413d9..158d010151586 100644
--- a/src/libgraphviz/lib.rs
+++ b/src/libgraphviz/lib.rs
@@ -287,7 +287,6 @@
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/",
        test(attr(allow(unused_variables), deny(warnings))))]
-#![deny(warnings)]
 
 #![feature(str_escape)]
 
diff --git a/src/libpanic_abort/lib.rs b/src/libpanic_abort/lib.rs
index 5f768ef4399e8..43c5bbbc618c2 100644
--- a/src/libpanic_abort/lib.rs
+++ b/src/libpanic_abort/lib.rs
@@ -19,7 +19,6 @@
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/",
        issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/")]
-#![deny(warnings)]
 #![panic_runtime]
 #![allow(unused_features)]
 
diff --git a/src/libpanic_unwind/lib.rs b/src/libpanic_unwind/lib.rs
index a5c227cb4015c..9321d6917d156 100644
--- a/src/libpanic_unwind/lib.rs
+++ b/src/libpanic_unwind/lib.rs
@@ -28,7 +28,6 @@
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/",
        issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/")]
-#![deny(warnings)]
 
 #![feature(alloc)]
 #![feature(core_intrinsics)]
diff --git a/src/libproc_macro/lib.rs b/src/libproc_macro/lib.rs
index 6aa5572721dbd..dafdacfdd7247 100644
--- a/src/libproc_macro/lib.rs
+++ b/src/libproc_macro/lib.rs
@@ -24,7 +24,6 @@
 //! See [the book](../book/first-edition/procedural-macros.html) for more.
 
 #![stable(feature = "proc_macro_lib", since = "1.15.0")]
-#![deny(warnings)]
 #![deny(missing_docs)]
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
diff --git a/src/librustc/benches/lib.rs b/src/librustc/benches/lib.rs
index 278e0f9a26e43..5496df1342ff4 100644
--- a/src/librustc/benches/lib.rs
+++ b/src/librustc/benches/lib.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![deny(warnings)]
-
 #![feature(test)]
 
 extern crate test;
diff --git a/src/librustc/lib.rs b/src/librustc/lib.rs
index 7da664e6d0255..b54699901faf3 100644
--- a/src/librustc/lib.rs
+++ b/src/librustc/lib.rs
@@ -39,7 +39,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(box_patterns)]
 #![feature(box_syntax)]
diff --git a/src/librustc_allocator/lib.rs b/src/librustc_allocator/lib.rs
index e17fce5a2ec0a..0c7a9a91711b7 100644
--- a/src/librustc_allocator/lib.rs
+++ b/src/librustc_allocator/lib.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![deny(warnings)]
-
 #![feature(rustc_private)]
 
 extern crate rustc;
diff --git a/src/librustc_apfloat/lib.rs b/src/librustc_apfloat/lib.rs
index 276f6cd09bf7a..0f051ea5981ca 100644
--- a/src/librustc_apfloat/lib.rs
+++ b/src/librustc_apfloat/lib.rs
@@ -43,7 +43,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 #![forbid(unsafe_code)]
 
 // See librustc_cratesio_shim/Cargo.toml for a comment explaining this.
diff --git a/src/librustc_back/lib.rs b/src/librustc_back/lib.rs
index 9baee26770998..027a9c4555530 100644
--- a/src/librustc_back/lib.rs
+++ b/src/librustc_back/lib.rs
@@ -24,7 +24,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(box_syntax)]
 #![feature(const_fn)]
diff --git a/src/librustc_borrowck/lib.rs b/src/librustc_borrowck/lib.rs
index 6fe2ac2b0ca25..52a357e1a1d31 100644
--- a/src/librustc_borrowck/lib.rs
+++ b/src/librustc_borrowck/lib.rs
@@ -11,7 +11,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![allow(non_camel_case_types)]
 
diff --git a/src/librustc_const_math/lib.rs b/src/librustc_const_math/lib.rs
index c4c5886d465d8..499c330be1da8 100644
--- a/src/librustc_const_math/lib.rs
+++ b/src/librustc_const_math/lib.rs
@@ -17,7 +17,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 extern crate rustc_apfloat;
 
diff --git a/src/librustc_data_structures/lib.rs b/src/librustc_data_structures/lib.rs
index 1e1628936d5ca..ba1d73dc268df 100644
--- a/src/librustc_data_structures/lib.rs
+++ b/src/librustc_data_structures/lib.rs
@@ -19,7 +19,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://www.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(collections_range)]
 #![feature(nonzero)]
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index 6f88b0aecb6b6..f6903d26e701c 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -17,7 +17,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(box_syntax)]
 #![cfg_attr(unix, feature(libc))]
diff --git a/src/librustc_errors/lib.rs b/src/librustc_errors/lib.rs
index a723e45522204..8d5f9ac93f0b4 100644
--- a/src/librustc_errors/lib.rs
+++ b/src/librustc_errors/lib.rs
@@ -11,7 +11,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(custom_attribute)]
 #![allow(unused_attributes)]
diff --git a/src/librustc_incremental/lib.rs b/src/librustc_incremental/lib.rs
index 9e72ede309d44..a5e07bcec24bb 100644
--- a/src/librustc_incremental/lib.rs
+++ b/src/librustc_incremental/lib.rs
@@ -13,7 +13,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(fs_read_write)]
 #![feature(specialization)]
diff --git a/src/librustc_lint/lib.rs b/src/librustc_lint/lib.rs
index c915181213d38..16e8600f2d89b 100644
--- a/src/librustc_lint/lib.rs
+++ b/src/librustc_lint/lib.rs
@@ -22,7 +22,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![cfg_attr(test, feature(test))]
 #![feature(box_patterns)]
diff --git a/src/librustc_llvm/lib.rs b/src/librustc_llvm/lib.rs
index 16bee5b987e2d..bf8a087ab556b 100644
--- a/src/librustc_llvm/lib.rs
+++ b/src/librustc_llvm/lib.rs
@@ -16,7 +16,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(box_syntax)]
 #![feature(concat_idents)]
diff --git a/src/librustc_metadata/lib.rs b/src/librustc_metadata/lib.rs
index e89b5a7fc1b4e..f02a34c65a94f 100644
--- a/src/librustc_metadata/lib.rs
+++ b/src/librustc_metadata/lib.rs
@@ -11,7 +11,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(box_patterns)]
 #![feature(fs_read_write)]
diff --git a/src/librustc_mir/lib.rs b/src/librustc_mir/lib.rs
index 8762e7550cded..51256c4d96f2a 100644
--- a/src/librustc_mir/lib.rs
+++ b/src/librustc_mir/lib.rs
@@ -14,8 +14,6 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 
 */
 
-#![deny(warnings)]
-
 #![feature(slice_patterns)]
 #![feature(from_ref)]
 #![feature(box_patterns)]
diff --git a/src/librustc_passes/lib.rs b/src/librustc_passes/lib.rs
index 1f6cc1f71fc08..e65c9de8df1ea 100644
--- a/src/librustc_passes/lib.rs
+++ b/src/librustc_passes/lib.rs
@@ -17,7 +17,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(rustc_diagnostic_macros)]
 
diff --git a/src/librustc_platform_intrinsics/lib.rs b/src/librustc_platform_intrinsics/lib.rs
index 4cc65ee28e89f..b57debdd99486 100644
--- a/src/librustc_platform_intrinsics/lib.rs
+++ b/src/librustc_platform_intrinsics/lib.rs
@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![deny(warnings)]
 #![allow(bad_style)]
 
 pub struct Intrinsic {
diff --git a/src/librustc_plugin/lib.rs b/src/librustc_plugin/lib.rs
index c0f830f1fbe2e..622d8e51a6ca3 100644
--- a/src/librustc_plugin/lib.rs
+++ b/src/librustc_plugin/lib.rs
@@ -63,7 +63,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(rustc_diagnostic_macros)]
 #![feature(staged_api)]
diff --git a/src/librustc_privacy/lib.rs b/src/librustc_privacy/lib.rs
index d951a7f1cc1cd..ef710ff7a7e4b 100644
--- a/src/librustc_privacy/lib.rs
+++ b/src/librustc_privacy/lib.rs
@@ -11,7 +11,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(rustc_diagnostic_macros)]
 
diff --git a/src/librustc_resolve/lib.rs b/src/librustc_resolve/lib.rs
index 2bf17cd1317d4..61ff326b2af96 100644
--- a/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@ -11,7 +11,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(rustc_diagnostic_macros)]
 
diff --git a/src/librustc_save_analysis/lib.rs b/src/librustc_save_analysis/lib.rs
index 4f46fb3545b15..fefedd4e1c819 100644
--- a/src/librustc_save_analysis/lib.rs
+++ b/src/librustc_save_analysis/lib.rs
@@ -11,7 +11,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 #![feature(custom_attribute)]
 #![feature(macro_lifetime_matcher)]
 #![allow(unused_attributes)]
diff --git a/src/librustc_traits/lib.rs b/src/librustc_traits/lib.rs
index cfa3b6912f2e4..8136f6857a5c6 100644
--- a/src/librustc_traits/lib.rs
+++ b/src/librustc_traits/lib.rs
@@ -11,8 +11,6 @@
 //! New recursive solver modeled on Chalk's recursive solver. Most of
 //! the guts are broken up into modules; see the comments in those modules.
 
-#![deny(warnings)]
-
 #![feature(crate_visibility_modifier)]
 
 #[macro_use]
diff --git a/src/librustc_trans/lib.rs b/src/librustc_trans/lib.rs
index 344f959c1414c..b9fa5b1353cee 100644
--- a/src/librustc_trans/lib.rs
+++ b/src/librustc_trans/lib.rs
@@ -17,7 +17,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(box_patterns)]
 #![feature(box_syntax)]
diff --git a/src/librustc_trans_utils/lib.rs b/src/librustc_trans_utils/lib.rs
index cf47d9b62a94e..b297fd9986501 100644
--- a/src/librustc_trans_utils/lib.rs
+++ b/src/librustc_trans_utils/lib.rs
@@ -15,7 +15,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(box_patterns)]
 #![feature(box_syntax)]
diff --git a/src/librustc_typeck/lib.rs b/src/librustc_typeck/lib.rs
index 6f71db998bd41..eb9a26855c649 100644
--- a/src/librustc_typeck/lib.rs
+++ b/src/librustc_typeck/lib.rs
@@ -68,7 +68,6 @@ This API is completely unstable and subject to change.
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![allow(non_camel_case_types)]
 
diff --git a/src/librustdoc/lib.rs b/src/librustdoc/lib.rs
index 42e87f88fd40d..730f61e0aa66d 100644
--- a/src/librustdoc/lib.rs
+++ b/src/librustdoc/lib.rs
@@ -12,7 +12,6 @@
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/",
        html_playground_url = "https://play.rust-lang.org/")]
-#![deny(warnings)]
 
 #![feature(ascii_ctype)]
 #![feature(rustc_private)]
diff --git a/src/libserialize/lib.rs b/src/libserialize/lib.rs
index f78eed3069478..22d27b6697a7c 100644
--- a/src/libserialize/lib.rs
+++ b/src/libserialize/lib.rs
@@ -19,7 +19,6 @@ Core encoding and decoding interfaces.
        html_root_url = "https://doc.rust-lang.org/nightly/",
        html_playground_url = "https://play.rust-lang.org/",
        test(attr(allow(unused_variables), deny(warnings))))]
-#![deny(warnings)]
 
 #![feature(box_syntax)]
 #![feature(core_intrinsics)]
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 3227aa9acff23..672723341eb57 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -227,10 +227,6 @@
 // Tell the compiler to link to either panic_abort or panic_unwind
 #![needs_panic_runtime]
 
-// Turn warnings into errors, but only after stage0, where it can be useful for
-// code to emit warnings during language transitions
-#![cfg_attr(not(stage0), deny(warnings))]
-
 // std may use features in a platform-specific way
 #![allow(unused_features)]
 
diff --git a/src/libstd_unicode/lib.rs b/src/libstd_unicode/lib.rs
index c22ea1671fa59..cf8c101a2f91f 100644
--- a/src/libstd_unicode/lib.rs
+++ b/src/libstd_unicode/lib.rs
@@ -27,7 +27,6 @@
        html_playground_url = "https://play.rust-lang.org/",
        issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/",
        test(no_crate_inject, attr(allow(unused_variables), deny(warnings))))]
-#![deny(warnings)]
 #![deny(missing_debug_implementations)]
 #![no_std]
 
diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index c456dc45d2182..b2976f71d49a8 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -18,7 +18,6 @@
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/",
        test(attr(deny(warnings))))]
-#![deny(warnings)]
 
 #![feature(unicode)]
 #![feature(rustc_diagnostic_macros)]
diff --git a/src/libsyntax_ext/lib.rs b/src/libsyntax_ext/lib.rs
index 249a64b353f59..97e34c554d116 100644
--- a/src/libsyntax_ext/lib.rs
+++ b/src/libsyntax_ext/lib.rs
@@ -13,7 +13,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(proc_macro_internals)]
 #![feature(decl_macro)]
diff --git a/src/libsyntax_pos/lib.rs b/src/libsyntax_pos/lib.rs
index 33428eb271abd..9a7d1fd8ee6cb 100644
--- a/src/libsyntax_pos/lib.rs
+++ b/src/libsyntax_pos/lib.rs
@@ -17,7 +17,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![deny(warnings)]
 
 #![feature(const_fn)]
 #![feature(custom_attribute)]
diff --git a/src/libterm/lib.rs b/src/libterm/lib.rs
index ad0e582b1c324..a012f4e776fb2 100644
--- a/src/libterm/lib.rs
+++ b/src/libterm/lib.rs
@@ -46,7 +46,6 @@
        html_playground_url = "https://play.rust-lang.org/",
        test(attr(deny(warnings))))]
 #![deny(missing_docs)]
-#![deny(warnings)]
 
 #![cfg_attr(windows, feature(libc))]
 // Handle rustfmt skips
diff --git a/src/libtest/lib.rs b/src/libtest/lib.rs
index b8be1aeff1742..9291eaa910bd7 100644
--- a/src/libtest/lib.rs
+++ b/src/libtest/lib.rs
@@ -31,7 +31,6 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/", test(attr(deny(warnings))))]
-#![deny(warnings)]
 #![feature(asm)]
 #![feature(fnbox)]
 #![cfg_attr(any(unix, target_os = "cloudabi"), feature(libc))]
diff --git a/src/libunwind/lib.rs b/src/libunwind/lib.rs
index 5347c781218ce..2b3c19c067ed4 100644
--- a/src/libunwind/lib.rs
+++ b/src/libunwind/lib.rs
@@ -10,7 +10,6 @@
 
 #![no_std]
 #![unstable(feature = "panic_unwind", issue = "32837")]
-#![deny(warnings)]
 
 #![feature(cfg_target_vendor)]
 #![feature(link_cfg)]
diff --git a/src/rustc/rustc.rs b/src/rustc/rustc.rs
index 9fa33f911a168..a888838ce432c 100644
--- a/src/rustc/rustc.rs
+++ b/src/rustc/rustc.rs
@@ -13,6 +13,7 @@
 
 // Set the stack size at link time on Windows. See rustc_driver::in_rustc_thread
 // for the rationale.
+#[allow(unused_attributes)]
 #[cfg_attr(all(windows, target_env = "msvc"), link_args = "/STACK:16777216")]
 // We only build for msvc and gnu now, but we use a exhaustive condition here
 // so we can expect either the stack size to be set or the build fails.
diff --git a/src/tools/rustdoc/main.rs b/src/tools/rustdoc/main.rs
index e726dea84f103..7aca765cead8c 100644
--- a/src/tools/rustdoc/main.rs
+++ b/src/tools/rustdoc/main.rs
@@ -9,6 +9,8 @@
 // except according to those terms.
 
 #![feature(link_args)]
+
+#[allow(unused_attributes)]
 // Set the stack size at link time on Windows. See rustc_driver::in_rustc_thread
 // for the rationale.
 #[cfg_attr(all(windows, target_env = "msvc"), link_args = "/STACK:16777216")]
diff --git a/src/tools/tidy/src/lib.rs b/src/tools/tidy/src/lib.rs
index 06eb055f68e06..fa22743664014 100644
--- a/src/tools/tidy/src/lib.rs
+++ b/src/tools/tidy/src/lib.rs
@@ -13,8 +13,6 @@
 //! This library contains the tidy lints and exposes it
 //! to be used by tools.
 
-#![deny(warnings)]
-
 extern crate serde;
 extern crate serde_json;
 #[macro_use]