diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 70e5f6ac26f..737176c48f8 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -1444,6 +1444,10 @@ pub struct Cargo {
 }
 
 impl Cargo {
+    pub fn rustdocflag(&mut self, arg: &str) -> &mut Cargo {
+        self.rustdocflags.arg(arg);
+        self
+    }
     pub fn rustflag(&mut self, arg: &str) -> &mut Cargo {
         self.rustflags.arg(arg);
         self
@@ -1466,6 +1470,9 @@ impl Cargo {
     }
 
     pub fn env(&mut self, key: impl AsRef<OsStr>, value: impl AsRef<OsStr>) -> &mut Cargo {
+        // These are managed through rustflag/rustdocflag interfaces.
+        assert_ne!(key.as_ref(), "RUSTFLAGS");
+        assert_ne!(key.as_ref(), "RUSTDOCFLAGS");
         self.command.env(key.as_ref(), value.as_ref());
         self
     }
diff --git a/src/bootstrap/dist.rs b/src/bootstrap/dist.rs
index c64934cd6c9..263c7b93fe5 100644
--- a/src/bootstrap/dist.rs
+++ b/src/bootstrap/dist.rs
@@ -1005,6 +1005,7 @@ impl Step for Src {
         // (essentially libstd and all of its path dependencies)
         let std_src_dirs = [
             "src/build_helper",
+            "src/backtrace",
             "src/liballoc",
             "src/libcore",
             "src/libpanic_abort",
diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs
index b051390fc26..f8a549afc88 100644
--- a/src/bootstrap/doc.rs
+++ b/src/bootstrap/doc.rs
@@ -527,11 +527,9 @@ impl Step for Rustc {
 
         // Build cargo command.
         let mut cargo = builder.cargo(compiler, Mode::Rustc, SourceType::InTree, target, "doc");
-        cargo.env(
-            "RUSTDOCFLAGS",
-            "--document-private-items \
-            --enable-index-page -Zunstable-options",
-        );
+        cargo.rustdocflag("--document-private-items");
+        cargo.rustdocflag("--enable-index-page");
+        cargo.rustdocflag("-Zunstable-options");
         compile::rustc_cargo(builder, &mut cargo, target);
 
         // Only include compiler crates, no dependencies of those, such as `libc`.
@@ -624,7 +622,7 @@ impl Step for Rustdoc {
         cargo.arg("--no-deps");
         cargo.arg("-p").arg("rustdoc");
 
-        cargo.env("RUSTDOCFLAGS", "--document-private-items");
+        cargo.rustdocflag("--document-private-items");
         builder.run(&mut cargo.into());
     }
 }
diff --git a/src/libcore/fmt/rt/v1.rs b/src/libcore/fmt/rt/v1.rs
index f6460470bfe..37202b2774d 100644
--- a/src/libcore/fmt/rt/v1.rs
+++ b/src/libcore/fmt/rt/v1.rs
@@ -33,9 +33,13 @@ pub enum Alignment {
     Unknown,
 }
 
+/// Used by [width](https://doc.rust-lang.org/std/fmt/#width) and [precision](https://doc.rust-lang.org/std/fmt/#precision) specifiers.
 #[derive(Copy, Clone)]
 pub enum Count {
+    /// Specified with a literal number, stores the value
     Is(usize),
+    /// Specified using `$` and `*` syntaxes, stores the index into `args`
     Param(usize),
+    /// Not specified
     Implied,
 }
diff --git a/src/librustc_middle/traits/specialization_graph.rs b/src/librustc_middle/traits/specialization_graph.rs
index f4961617b81..c9aae898007 100644
--- a/src/librustc_middle/traits/specialization_graph.rs
+++ b/src/librustc_middle/traits/specialization_graph.rs
@@ -1,5 +1,6 @@
 use crate::ich::{self, StableHashingContext};
 use crate::ty::fast_reject::SimplifiedType;
+use crate::ty::fold::TypeFoldable;
 use crate::ty::{self, TyCtxt};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
@@ -226,7 +227,8 @@ pub fn ancestors(
     start_from_impl: DefId,
 ) -> Result<Ancestors<'tcx>, ErrorReported> {
     let specialization_graph = tcx.specialization_graph_of(trait_def_id);
-    if specialization_graph.has_errored {
+
+    if specialization_graph.has_errored || tcx.type_of(start_from_impl).references_error() {
         Err(ErrorReported)
     } else {
         Ok(Ancestors {
diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index 491daa80e5c..1f576a17dd9 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -346,9 +346,11 @@ pub fn build_impl(
         // such. This helps prevent dependencies of the standard library, for
         // example, from getting documented as "traits `u32` implements" which
         // isn't really too helpful.
-        if let Some(stab) = cx.tcx.lookup_stability(did) {
-            if stab.level.is_unstable() {
-                return;
+        if let Some(trait_did) = associated_trait {
+            if let Some(stab) = cx.tcx.lookup_stability(trait_did.def_id) {
+                if stab.level.is_unstable() {
+                    return;
+                }
             }
         }
     }
diff --git a/src/libstd/io/mod.rs b/src/libstd/io/mod.rs
index 823ce30febe..797318d95b7 100644
--- a/src/libstd/io/mod.rs
+++ b/src/libstd/io/mod.rs
@@ -722,7 +722,9 @@ pub trait Read {
     /// No guarantees are provided about the contents of `buf` when this
     /// function is called, implementations cannot rely on any property of the
     /// contents of `buf` being true. It is recommended that implementations
-    /// only write data to `buf` instead of reading its contents.
+    /// only write data to `buf` instead of reading its contents. The
+    /// documentation on [`read`] has a more detailed explanation on this
+    /// subject.
     ///
     /// # Errors
     ///
@@ -745,6 +747,7 @@ pub trait Read {
     ///
     /// [`File`]s implement `Read`:
     ///
+    /// [`read`]: Read::read
     /// [`File`]: crate::fs::File
     ///
     /// ```no_run
diff --git a/src/libstd/keyword_docs.rs b/src/libstd/keyword_docs.rs
index d985f10ccb4..a62987891b9 100644
--- a/src/libstd/keyword_docs.rs
+++ b/src/libstd/keyword_docs.rs
@@ -387,10 +387,11 @@ mod extern_keyword {}
 //
 /// A value of type [`bool`] representing logical **false**.
 ///
-/// The documentation for this keyword is [not yet complete]. Pull requests welcome!
+/// `false` is the logical opposite of [`true`].
 ///
-/// [`bool`]: primitive.bool.html
-/// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
+/// See the documentation for [`true`] for more information.
+///
+/// [`true`]: keyword.true.html
 mod false_keyword {}
 
 #[doc(keyword = "fn")]
@@ -473,8 +474,8 @@ mod fn_keyword {}
 /// * `for` is also used for [higher-ranked trait bounds] as in `for<'a> &'a T: PartialEq<i32>`.
 ///
 /// for-in-loops, or to be more precise, iterator loops, are a simple syntactic sugar over a common
-/// practice within Rust, which is to loop over an iterator until that iterator returns `None` (or
-/// `break` is called).
+/// practice within Rust, which is to loop over anything that implements [`IntoIterator`] until the
+/// iterator returned by `.into_iter()` returns `None` (or the loop body uses `break`).
 ///
 /// ```rust
 /// for i in 0..5 {
@@ -680,7 +681,7 @@ mod impl_keyword {}
 //
 /// Iterate over a series of values with [`for`].
 ///
-/// The expression immediately following `in` must implement the [`Iterator`] trait.
+/// The expression immediately following `in` must implement the [`IntoIterator`] trait.
 ///
 /// ## Literal Examples:
 ///
@@ -689,7 +690,7 @@ mod impl_keyword {}
 ///
 /// (Read more about [range patterns])
 ///
-/// [`Iterator`]: ../book/ch13-04-performance.html
+/// [`IntoIterator`]: ../book/ch13-04-performance.html
 /// [range patterns]: ../reference/patterns.html?highlight=range#range-patterns
 /// [`for`]: keyword.for.html
 mod in_keyword {}
diff --git a/src/test/rustdoc/auxiliary/unstable-trait.rs b/src/test/rustdoc/auxiliary/unstable-trait.rs
new file mode 100644
index 00000000000..6f06a6e2656
--- /dev/null
+++ b/src/test/rustdoc/auxiliary/unstable-trait.rs
@@ -0,0 +1,26 @@
+#![feature(staged_api)]
+#![stable(feature = "private_general", since = "1.0.0")]
+
+#[unstable(feature = "private_trait", issue = "none")]
+pub trait Bar {}
+
+#[stable(feature = "private_general", since = "1.0.0")]
+pub struct Foo {
+    // nothing
+}
+
+impl Foo {
+    #[stable(feature = "private_general", since = "1.0.0")]
+    pub fn stable_impl() {}
+}
+
+impl Foo {
+    #[unstable(feature = "private_trait", issue = "none")]
+    pub fn bar() {}
+
+    #[stable(feature = "private_general", since = "1.0.0")]
+    pub fn bar2() {}
+}
+
+#[stable(feature = "private_general", since = "1.0.0")]
+impl Bar for Foo {}
diff --git a/src/test/rustdoc/hide-unstable-trait.rs b/src/test/rustdoc/hide-unstable-trait.rs
new file mode 100644
index 00000000000..c30d6ed7b52
--- /dev/null
+++ b/src/test/rustdoc/hide-unstable-trait.rs
@@ -0,0 +1,11 @@
+// aux-build:unstable-trait.rs
+
+#![crate_name = "foo"]
+#![feature(private_trait)]
+
+extern crate unstable_trait;
+
+// @has foo/struct.Foo.html 'bar'
+// @has foo/struct.Foo.html 'bar2'
+#[doc(inline)]
+pub use unstable_trait::Foo;
diff --git a/src/test/ui/specialization/min_specialization/impl-on-nonexisting.rs b/src/test/ui/specialization/min_specialization/impl-on-nonexisting.rs
new file mode 100644
index 00000000000..77a64320d6f
--- /dev/null
+++ b/src/test/ui/specialization/min_specialization/impl-on-nonexisting.rs
@@ -0,0 +1,7 @@
+#![feature(min_specialization)]
+
+trait Trait {}
+impl Trait for NonExistent {}
+//~^ ERROR cannot find type `NonExistent` in this scope
+
+fn main() {}
diff --git a/src/test/ui/specialization/min_specialization/impl-on-nonexisting.stderr b/src/test/ui/specialization/min_specialization/impl-on-nonexisting.stderr
new file mode 100644
index 00000000000..b032ccbe53f
--- /dev/null
+++ b/src/test/ui/specialization/min_specialization/impl-on-nonexisting.stderr
@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `NonExistent` in this scope
+  --> $DIR/impl-on-nonexisting.rs:4:16
+   |
+LL | impl Trait for NonExistent {}
+   |                ^^^^^^^^^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0412`.
diff --git a/src/tools/publish_toolstate.py b/src/tools/publish_toolstate.py
index 72437e07004..c0631fcedd3 100755
--- a/src/tools/publish_toolstate.py
+++ b/src/tools/publish_toolstate.py
@@ -26,7 +26,7 @@ except ImportError:
 MAINTAINERS = {
     'miri': {'oli-obk', 'RalfJung', 'eddyb'},
     'rls': {'Xanewok'},
-    'rustfmt': {'topecongiro'},
+    'rustfmt': {'topecongiro', 'calebcartwright'},
     'book': {'carols10cents', 'steveklabnik'},
     'nomicon': {'frewsxcv', 'Gankra'},
     'reference': {'steveklabnik', 'Havvy', 'matthewjasper', 'ehuss'},