diff --git a/src/bootstrap/util.rs b/src/bootstrap/util.rs
index 98ae7b692bb3c..6f8a630874570 100644
--- a/src/bootstrap/util.rs
+++ b/src/bootstrap/util.rs
@@ -262,6 +262,8 @@ pub enum CiEnv {
     None,
     /// The Azure Pipelines environment, for Linux (including Docker), Windows, and macOS builds.
     AzurePipelines,
+    /// The GitHub Actions environment, for Linux (including Docker), Windows and macOS builds.
+    GitHubActions,
 }
 
 impl CiEnv {
@@ -269,6 +271,8 @@ impl CiEnv {
     pub fn current() -> CiEnv {
         if env::var("TF_BUILD").ok().map_or(false, |e| &*e == "True") {
             CiEnv::AzurePipelines
+        } else if env::var("GITHUB_ACTIONS").ok().map_or(false, |e| &*e == "true") {
+            CiEnv::GitHubActions
         } else {
             CiEnv::None
         }
diff --git a/src/ci/azure-pipelines/auto.yml b/src/ci/azure-pipelines/auto.yml
index 946eb483c2946..bfe5174bf1e57 100644
--- a/src/ci/azure-pipelines/auto.yml
+++ b/src/ci/azure-pipelines/auto.yml
@@ -19,136 +19,46 @@ jobs:
   strategy:
     matrix:
       x86_64-gnu-llvm-6.0:
-        IMAGE: x86_64-gnu-llvm-6.0
         RUST_BACKTRACE: 1
-
-      dist-x86_64-linux:
-        IMAGE: dist-x86_64-linux
-        DEPLOY: 1
-
-      # "alternate" deployments, these are "nightlies" but have LLVM assertions
-      # turned on, they're deployed to a different location primarily for
-      # additional testing.
+      dist-x86_64-linux: {}
       dist-x86_64-linux-alt:
         IMAGE: dist-x86_64-linux
-        DEPLOY_ALT: 1
-
-      # Linux builders, remaining docker images
-      arm-android:
-        IMAGE: arm-android
-
-      armhf-gnu:
-        IMAGE: armhf-gnu
-
-      dist-various-1:
-        IMAGE: dist-various-1
-        DEPLOY: 1
-
-      dist-various-2:
-        IMAGE: dist-various-2
-        DEPLOY: 1
-
-      dist-aarch64-linux:
-        IMAGE: dist-aarch64-linux
-        DEPLOY: 1
-
-      dist-android:
-        IMAGE: dist-android
-        DEPLOY: 1
-
-      dist-arm-linux:
-        IMAGE: dist-arm-linux
-        DEPLOY: 1
-
-      dist-armhf-linux:
-        IMAGE: dist-armhf-linux
-        DEPLOY: 1
-
-      dist-armv7-linux:
-        IMAGE: dist-armv7-linux
-        DEPLOY: 1
-
-      dist-i586-gnu-i586-i686-musl:
-        IMAGE: dist-i586-gnu-i586-i686-musl
-        DEPLOY: 1
-
-      dist-i686-freebsd:
-        IMAGE: dist-i686-freebsd
-        DEPLOY: 1
-
-      dist-i686-linux:
-        IMAGE: dist-i686-linux
-        DEPLOY: 1
-
-      dist-mips-linux:
-        IMAGE: dist-mips-linux
-        DEPLOY: 1
-
-      dist-mips64-linux:
-        IMAGE: dist-mips64-linux
-        DEPLOY: 1
-
-      dist-mips64el-linux:
-        IMAGE: dist-mips64el-linux
-        DEPLOY: 1
-
-      dist-mipsel-linux:
-        IMAGE: dist-mipsel-linux
-        DEPLOY: 1
-
-      dist-powerpc-linux:
-        IMAGE: dist-powerpc-linux
-        DEPLOY: 1
-
-      dist-powerpc64-linux:
-        IMAGE: dist-powerpc64-linux
-        DEPLOY: 1
-
-      dist-powerpc64le-linux:
-        IMAGE: dist-powerpc64le-linux
-        DEPLOY: 1
-
-      dist-s390x-linux:
-        IMAGE: dist-s390x-linux
-        DEPLOY: 1
-
-      dist-x86_64-freebsd:
-        IMAGE: dist-x86_64-freebsd
-        DEPLOY: 1
-
-      dist-x86_64-musl:
-        IMAGE: dist-x86_64-musl
-        DEPLOY: 1
-
-      dist-x86_64-netbsd:
-        IMAGE: dist-x86_64-netbsd
-        DEPLOY: 1
-
-      i686-gnu:
-        IMAGE: i686-gnu
-      i686-gnu-nopt:
-        IMAGE: i686-gnu-nopt
-      test-various:
-        IMAGE: test-various
-      wasm32:
-        IMAGE: wasm32
-      x86_64-gnu:
-        IMAGE: x86_64-gnu
-      x86_64-gnu-full-bootstrap:
-        IMAGE: x86_64-gnu-full-bootstrap
-      x86_64-gnu-aux:
-        IMAGE: x86_64-gnu-aux
+      arm-android: {}
+      armhf-gnu: {}
+      dist-various-1: {}
+      dist-various-2: {}
+      dist-aarch64-linux: {}
+      dist-android: {}
+      dist-arm-linux: {}
+      dist-armhf-linux: {}
+      dist-armv7-linux: {}
+      dist-i586-gnu-i586-i686-musl: {}
+      dist-i686-freebsd: {}
+      dist-i686-linux: {}
+      dist-mips-linux: {}
+      dist-mips64-linux: {}
+      dist-mips64el-linux: {}
+      dist-mipsel-linux: {}
+      dist-powerpc-linux: {}
+      dist-powerpc64-linux: {}
+      dist-powerpc64le-linux: {}
+      dist-s390x-linux: {}
+      dist-x86_64-freebsd: {}
+      dist-x86_64-musl: {}
+      dist-x86_64-netbsd: {}
+      i686-gnu: {}
+      i686-gnu-nopt: {}
+      test-various: {}
+      wasm32: {}
+      x86_64-gnu: {}
+      x86_64-gnu-full-bootstrap: {}
+      x86_64-gnu-aux: {}
       x86_64-gnu-tools:
-        IMAGE: x86_64-gnu-tools
         DEPLOY_TOOLSTATES_JSON: toolstates-linux.json
-      x86_64-gnu-debug:
-        IMAGE: x86_64-gnu-debug
-      x86_64-gnu-nopt:
-        IMAGE: x86_64-gnu-nopt
-      x86_64-gnu-distcheck:
-        IMAGE: x86_64-gnu-distcheck
-      mingw-check:
-        IMAGE: mingw-check
+      x86_64-gnu-debug: {}
+      x86_64-gnu-nopt: {}
+      x86_64-gnu-distcheck: {}
+      mingw-check: {}
 
 - job: macOS
   timeoutInMinutes: 600
@@ -176,7 +86,6 @@ jobs:
       dist-x86_64-apple:
         SCRIPT: ./x.py dist
         RUST_CONFIGURE_ARGS: --target=aarch64-apple-ios,armv7-apple-ios,armv7s-apple-ios,i386-apple-ios,x86_64-apple-ios --enable-full-tools --enable-sanitizers --enable-profiler --set rust.jemalloc
-        DEPLOY: 1
         RUSTC_RETRY_LINKER_ON_SEGFAULT: 1
         MACOSX_DEPLOYMENT_TARGET: 10.7
         NO_LLVM_ASSERTIONS: 1
@@ -186,7 +95,6 @@ jobs:
       dist-x86_64-apple-alt:
         SCRIPT: ./x.py dist
         RUST_CONFIGURE_ARGS: --enable-extended --enable-profiler --set rust.jemalloc
-        DEPLOY_ALT: 1
         RUSTC_RETRY_LINKER_ON_SEGFAULT: 1
         MACOSX_DEPLOYMENT_TARGET: 10.7
         NO_LLVM_ASSERTIONS: 1
@@ -204,7 +112,6 @@ jobs:
       dist-i686-apple:
         SCRIPT: ./x.py dist
         RUST_CONFIGURE_ARGS: --build=i686-apple-darwin --enable-full-tools --enable-profiler --set rust.jemalloc
-        DEPLOY: 1
         RUSTC_RETRY_LINKER_ON_SEGFAULT: 1
         MACOSX_DEPLOYMENT_TARGET: 10.7
         NO_LLVM_ASSERTIONS: 1
@@ -304,7 +211,6 @@ jobs:
           --enable-profiler
         SCRIPT: python x.py dist
         DIST_REQUIRE_ALL_TOOLS: 1
-        DEPLOY: 1
       dist-i686-msvc:
         RUST_CONFIGURE_ARGS: >-
           --build=i686-pc-windows-msvc
@@ -313,22 +219,18 @@ jobs:
           --enable-profiler
         SCRIPT: python x.py dist
         DIST_REQUIRE_ALL_TOOLS: 1
-        DEPLOY: 1
       dist-i686-mingw:
         RUST_CONFIGURE_ARGS: --build=i686-pc-windows-gnu --enable-full-tools --enable-profiler
         SCRIPT: python x.py dist
         CUSTOM_MINGW: 1
         DIST_REQUIRE_ALL_TOOLS: 1
-        DEPLOY: 1
       dist-x86_64-mingw:
         SCRIPT: python x.py dist
         RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-gnu --enable-full-tools --enable-profiler
         CUSTOM_MINGW: 1
         DIST_REQUIRE_ALL_TOOLS: 1
-        DEPLOY: 1
 
       # "alternate" deployment, see .travis.yml for more info
       dist-x86_64-msvc-alt:
         RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-msvc --enable-extended --enable-profiler
         SCRIPT: python x.py dist
-        DEPLOY_ALT: 1
diff --git a/src/ci/azure-pipelines/pr.yml b/src/ci/azure-pipelines/pr.yml
index 566e654fdb3f0..aee4d8d5136aa 100644
--- a/src/ci/azure-pipelines/pr.yml
+++ b/src/ci/azure-pipelines/pr.yml
@@ -18,10 +18,7 @@ jobs:
     - template: steps/run.yml
   strategy:
     matrix:
-      x86_64-gnu-llvm-6.0:
-        IMAGE: x86_64-gnu-llvm-6.0
-      mingw-check:
-        IMAGE: mingw-check
+      x86_64-gnu-llvm-6.0: {}
+      mingw-check: {}
       x86_64-gnu-tools:
-        IMAGE: x86_64-gnu-tools
         CI_ONLY_WHEN_SUBMODULES_CHANGED: 1
diff --git a/src/ci/azure-pipelines/steps/run.yml b/src/ci/azure-pipelines/steps/run.yml
index 214c11fd69024..f536388b25b96 100644
--- a/src/ci/azure-pipelines/steps/run.yml
+++ b/src/ci/azure-pipelines/steps/run.yml
@@ -28,6 +28,9 @@ steps:
 - checkout: self
   fetchDepth: 2
 
+- bash: src/ci/scripts/setup-environment.sh
+  displayName: Setup environment
+
 - bash: src/ci/scripts/should-skip-this.sh
   displayName: Decide whether to run this job
 
@@ -38,38 +41,26 @@ steps:
   displayName: Show the current environment
 
 - bash: src/ci/scripts/install-sccache.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   displayName: Install sccache
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/install-clang.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   displayName: Install clang
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/switch-xcode.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   displayName: Switch to Xcode 9.3
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/install-wix.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   displayName: Install wix
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/install-innosetup.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   displayName: Install InnoSetup
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/windows-symlink-build-dir.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   displayName: Ensure the build happens on C:\ instead of D:\
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
@@ -78,35 +69,22 @@ steps:
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/install-msys2.sh
-  env:
-    AGENT_OS: $(Agent.OS)
-    SYSTEM_WORKFOLDER: $(System.Workfolder)
   displayName: Install msys2
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/install-msys2-packages.sh
-  env:
-    AGENT_OS: $(Agent.OS)
-    SYSTEM_WORKFOLDER: $(System.Workfolder)
   displayName: Install msys2 packages
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/install-mingw.sh
-  env:
-    AGENT_OS: $(Agent.OS)
-    SYSTEM_WORKFOLDER: $(System.Workfolder)
   displayName: Install MinGW
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/install-ninja.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   displayName: Install ninja
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/enable-docker-ipv6.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   displayName: Enable IPv6 on Docker
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
@@ -120,22 +98,16 @@ steps:
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/checkout-submodules.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   displayName: Checkout submodules
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 - bash: src/ci/scripts/verify-line-endings.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   displayName: Verify line endings
   condition: and(succeeded(), not(variables.SKIP_JOB))
 
 # Ensure the `aws` CLI is installed so we can deploy later on, cache docker
 # images, etc.
 - bash: src/ci/scripts/install-awscli.sh
-  env:
-    AGENT_OS: $(Agent.OS)
   condition: and(succeeded(), not(variables.SKIP_JOB))
   displayName: Install awscli
 
diff --git a/src/ci/azure-pipelines/try.yml b/src/ci/azure-pipelines/try.yml
index fe39ce3e24116..b6177b2cc9b25 100644
--- a/src/ci/azure-pipelines/try.yml
+++ b/src/ci/azure-pipelines/try.yml
@@ -14,13 +14,9 @@ jobs:
   - template: steps/run.yml
   strategy:
     matrix:
-      dist-x86_64-linux:
-        IMAGE: dist-x86_64-linux
-        DEPLOY: 1
-
+      dist-x86_64-linux: {}
       dist-x86_64-linux-alt:
         IMAGE: dist-x86_64-linux
-        DEPLOY_ALT: 1
 
 # The macOS and Windows builds here are currently disabled due to them not being
 # overly necessary on `try` builds. We also don't actually have anything that
diff --git a/src/ci/docker/run.sh b/src/ci/docker/run.sh
index cdafcbadc9ec7..f29f9f3bf1c45 100755
--- a/src/ci/docker/run.sh
+++ b/src/ci/docker/run.sh
@@ -172,6 +172,8 @@ docker \
   --env CI \
   --env TF_BUILD \
   --env BUILD_SOURCEBRANCHNAME \
+  --env GITHUB_ACTIONS \
+  --env GITHUB_REF \
   --env TOOLSTATE_REPO_ACCESS_TOKEN \
   --env TOOLSTATE_REPO \
   --env TOOLSTATE_PUBLISH \
diff --git a/src/ci/run.sh b/src/ci/run.sh
index bce35670c8d46..ae5b22493ab07 100755
--- a/src/ci/run.sh
+++ b/src/ci/run.sh
@@ -23,9 +23,7 @@ fi
 ci_dir=`cd $(dirname $0) && pwd`
 source "$ci_dir/shared.sh"
 
-branch_name=$(getCIBranch)
-
-if [ ! isCI ] || [ "$branch_name" = "auto" ] || [ "$branch_name" = "try" ]; then
+if [ ! isCI ] || isCiBranch auto || isCiBranch beta; then
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set build.print-step-timings --enable-verbose-tests"
 fi
 
diff --git a/src/ci/scripts/install-mingw.sh b/src/ci/scripts/install-mingw.sh
index 8b579587b9e1f..98373df7fce50 100755
--- a/src/ci/scripts/install-mingw.sh
+++ b/src/ci/scripts/install-mingw.sh
@@ -52,7 +52,7 @@ if isWindows; then
     if [[ "${CUSTOM_MINGW-0}" -ne 1 ]]; then
         pacman -S --noconfirm --needed mingw-w64-$arch-toolchain mingw-w64-$arch-cmake \
             mingw-w64-$arch-gcc mingw-w64-$arch-python2
-        ciCommandAddPath "${SYSTEM_WORKFOLDER}/msys2/mingw${bits}/bin"
+        ciCommandAddPath "$(ciCheckoutPath)/msys2/mingw${bits}/bin"
     else
         mingw_dir="mingw${bits}"
 
diff --git a/src/ci/scripts/install-msys2.sh b/src/ci/scripts/install-msys2.sh
index ce37c3b146977..c9fafc7fe6b41 100755
--- a/src/ci/scripts/install-msys2.sh
+++ b/src/ci/scripts/install-msys2.sh
@@ -12,8 +12,8 @@ IFS=$'\n\t'
 source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
 
 if isWindows; then
-    choco install msys2 --params="/InstallDir:${SYSTEM_WORKFOLDER}/msys2 /NoPath" -y --no-progress
-    mkdir -p "${SYSTEM_WORKFOLDER}/msys2/home/${USERNAME}"
+    choco install msys2 --params="/InstallDir:$(ciCheckoutPath)/msys2 /NoPath" -y --no-progress
+    mkdir -p "$(ciCheckoutPath)/msys2/home/${USERNAME}"
 
-    ciCommandAddPath "${SYSTEM_WORKFOLDER}/msys2/usr/bin"
+    ciCommandAddPath "$(ciCheckoutPath)/msys2/usr/bin"
 fi
diff --git a/src/ci/scripts/setup-environment.sh b/src/ci/scripts/setup-environment.sh
new file mode 100755
index 0000000000000..e126a06edab73
--- /dev/null
+++ b/src/ci/scripts/setup-environment.sh
@@ -0,0 +1,31 @@
+#!/bin/bash
+# This script guesses some environment variables based on the builder name and
+# the current platform, to reduce the amount of variables defined in the CI
+# configuration.
+
+set -euo pipefail
+IFS=$'\n\t'
+
+source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
+
+# Builders starting with `dist-` are dist builders, but if they also end with
+# `-alt` they are alternate dist builders.
+if [[ "${CI_JOB_NAME}" = dist-* ]]; then
+    if [[ "${CI_JOB_NAME}" = *-alt ]]; then
+        echo "alternate dist builder detected, setting DEPLOY_ALT=1"
+        ciCommandSetEnv DEPLOY_ALT 1
+    else
+        echo "normal dist builder detected, setting DEPLOY=1"
+        ciCommandSetEnv DEPLOY 1
+    fi
+fi
+
+# All the Linux builds happen inside Docker.
+if isLinux; then
+    if [[ -z "${IMAGE+x}" ]]; then
+        echo "linux builder detected, using docker to run the build"
+        ciCommandSetEnv IMAGE "${CI_JOB_NAME}"
+    else
+        echo "a custom docker image is already set"
+    fi
+fi
diff --git a/src/ci/shared.sh b/src/ci/shared.sh
index 862ded0d5dbf0..394a57158873c 100644
--- a/src/ci/shared.sh
+++ b/src/ci/shared.sh
@@ -27,27 +27,66 @@ function retry {
 }
 
 function isCI {
-  [ "$CI" = "true" ] || [ "$TF_BUILD" = "True" ]
+    [[ "${CI-false}" = "true" ]] || isAzurePipelines || isGitHubActions
+}
+
+function isAzurePipelines {
+    [[ "${TF_BUILD-False}" = "True" ]]
+}
+
+function isGitHubActions {
+    [[ "${GITHUB_ACTIONS-false}" = "true" ]]
 }
 
 function isMacOS {
-  [ "$AGENT_OS" = "Darwin" ]
+    [[ "${OSTYPE}" = "darwin"* ]]
 }
 
 function isWindows {
-  [ "$AGENT_OS" = "Windows_NT" ]
+    [[ "${OSTYPE}" = "cygwin" ]] || [[ "${OSTYPE}" = "msys" ]]
 }
 
 function isLinux {
-  [ "$AGENT_OS" = "Linux" ]
+    [[ "${OSTYPE}" = "linux-gnu" ]]
 }
 
-function getCIBranch {
-  echo "$BUILD_SOURCEBRANCHNAME"
+function isCiBranch {
+    if [[ $# -ne 1 ]]; then
+        echo "usage: $0 <branch-name>"
+        exit 1
+    fi
+    name="$1"
+
+    if isAzurePipelines; then
+        [[ "${BUILD_SOURCEBRANCHNAME}" = "${name}" ]]
+    elif isGitHubActions; then
+        [[ "${GITHUB_REF}" = "refs/heads/${name}" ]]
+    else
+        echo "isCiBranch only works inside CI!"
+        exit 1
+    fi
 }
 
 function ciCommit {
-  echo "${BUILD_SOURCEVERSION}"
+    if isAzurePipelines; then
+        echo "${BUILD_SOURCEVERSION}"
+    elif isGitHubActions; then
+        echo "${GITHUB_SHA}"
+    else
+        echo "ciCommit only works inside CI!"
+        exit 1
+    fi
+}
+
+function ciCheckoutPath {
+    if isAzurePipelines; then
+        echo "${BUILD_WORKFOLDER}"
+    elif isGitHubActions; then
+        echo "${GITHUB_WORKSPACE}"
+    else
+        echo "ciCheckoutPath only works inside CI!"
+        exit 1
+    fi
 }
 
 function ciCommandAddPath {
@@ -57,7 +96,14 @@ function ciCommandAddPath {
     fi
     path="$1"
 
-    echo "##vso[task.prependpath]${path}"
+    if isAzurePipelines; then
+        echo "##vso[task.prependpath]${path}"
+    elif isGitHubActions; then
+        echo "::add-path::${value}"
+    else
+        echo "ciCommandAddPath only works inside CI!"
+        exit 1
+    fi
 }
 
 function ciCommandSetEnv {
@@ -68,5 +114,12 @@ function ciCommandSetEnv {
     name="$1"
     value="$2"
 
-    echo "##vso[task.setvariable variable=${name}]${value}"
+    if isAzurePipelines; then
+        echo "##vso[task.setvariable variable=${name}]${value}"
+    elif isGitHubActions; then
+        echo "::set-env name=${name}::${value}"
+    else
+        echo "ciCommandSetEnv only works inside CI!"
+        exit 1
+    fi
 }
diff --git a/src/etc/gdb_rust_pretty_printing.py b/src/etc/gdb_rust_pretty_printing.py
index a6b09722e1c94..5da01b96fa5e3 100755
--- a/src/etc/gdb_rust_pretty_printing.py
+++ b/src/etc/gdb_rust_pretty_printing.py
@@ -284,10 +284,20 @@ def to_string(self):
                 ("(len: %i, cap: %i)" % (length, cap)))
 
     def children(self):
+        saw_inaccessible = False
         (length, data_ptr, cap) = rustpp.extract_length_ptr_and_cap_from_std_vec(self.__val)
         gdb_ptr = data_ptr.get_wrapped_value()
         for index in xrange(0, length):
-            yield (str(index), (gdb_ptr + index).dereference())
+            if saw_inaccessible:
+                return
+            try:
+                # rust-lang/rust#64343: passing deref expr to `str` allows
+                # catching exception on garbage pointer
+                str((gdb_ptr + index).dereference())
+                yield (str(index), (gdb_ptr + index).dereference())
+            except RuntimeError:
+                saw_inaccessible = True
+                yield (str(index), "inaccessible")
 
 
 class RustStdVecDequePrinter(object):
diff --git a/src/libcore/cmp.rs b/src/libcore/cmp.rs
index 1e2e595f1dd0a..eb5121dd0e081 100644
--- a/src/libcore/cmp.rs
+++ b/src/libcore/cmp.rs
@@ -135,10 +135,15 @@ use self::Ordering::*;
 /// By changing `impl PartialEq for Book` to `impl PartialEq<BookFormat> for Book`,
 /// we allow `BookFormat`s to be compared with `Book`s.
 ///
-/// You can also combine these implementations to let the `==` operator work with
-/// two different types:
-///
-/// ```
+/// A comparison like the one above, which ignores some fields of the struct,
+/// can be dangerous. It can easily lead to an unintended violation of the
+/// requirements for a partial equivalence relation. For example, if we kept
+/// the above implementation of `PartialEq<Book>` for `BookFormat` and added an
+/// implementation of `PartialEq<Book>` for `Book` (either via a `#[derive]` or
+/// via the manual implementation from the first example) then the result would
+/// violate transitivity:
+///
+/// ```should_panic
 /// #[derive(PartialEq)]
 /// enum BookFormat {
 ///     Paperback,
@@ -146,6 +151,7 @@ use self::Ordering::*;
 ///     Ebook,
 /// }
 ///
+/// #[derive(PartialEq)]
 /// struct Book {
 ///     isbn: i32,
 ///     format: BookFormat,
@@ -163,18 +169,16 @@ use self::Ordering::*;
 ///     }
 /// }
 ///
-/// impl PartialEq for Book {
-///     fn eq(&self, other: &Book) -> bool {
-///         self.isbn == other.isbn
-///     }
-/// }
+/// fn main() {
+///     let b1 = Book { isbn: 1, format: BookFormat::Paperback };
+///     let b2 = Book { isbn: 2, format: BookFormat::Paperback };
 ///
-/// let b1 = Book { isbn: 3, format: BookFormat::Paperback };
-/// let b2 = Book { isbn: 3, format: BookFormat::Ebook };
+///     assert!(b1 == BookFormat::Paperback);
+///     assert!(BookFormat::Paperback == b2);
 ///
-/// assert!(b1 == BookFormat::Paperback);
-/// assert!(BookFormat::Ebook != b1);
-/// assert!(b1 == b2);
+///     // The following should hold by transitivity but doesn't.
+///     assert!(b1 == b2); // <-- PANICS
+/// }
 /// ```
 ///
 /// # Examples