Implemented rough draft of `check` write mode. (#2539)

* Add rough draft of `check` mode. Not unit tested.

* Added assert-cli; simple test case for `--write-mode=check`

* Lightly documented `check` WriteMode

* wrote clearer phrasing for config::options::WriteMode::Check

* Implemented default for WriteMode where default is Overwrite

* Simplified exit code handling

* updated README.md as per @nrc' comment

* collapsed exit-code handling

* Removed write_mode from Summary, introduced partial option parsing earlier

* Handle write-mode parsing in a slightly better way.

10 files changed, 252 insertions, 75 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 8f1414166e1..ee76b020476 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -7,6 +7,19 @@ dependencies = [
 ]
 
 [[package]]
+name = "assert_cli"
+version = "0.5.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "colored 1.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "difference 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "environment 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "error-chain 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde_json 1.0.15 (registry+https://github.com/rust-lang/crates.io-index)",
+ "skeptic 0.13.2 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "atty"
 version = "0.2.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -18,7 +31,7 @@ dependencies = [
 
 [[package]]
 name = "backtrace"
-version = "0.3.5"
+version = "0.3.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "backtrace-sys 0.1.16 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -33,35 +46,57 @@ name = "backtrace-sys"
 version = "0.1.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "cc 1.0.9 (registry+https://github.com/rust-lang/crates.io-index)",
+ "cc 1.0.10 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.40 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
 name = "bitflags"
+version = "0.9.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "bitflags"
 version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
+name = "bytecount"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
 name = "byteorder"
 version = "1.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "cargo_metadata"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "error-chain 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "semver 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde_derive 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde_json 1.0.15 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "cargo_metadata"
 version = "0.5.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "error-chain 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "semver 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)",
- "serde 1.0.37 (registry+https://github.com/rust-lang/crates.io-index)",
- "serde_derive 1.0.37 (registry+https://github.com/rust-lang/crates.io-index)",
- "serde_json 1.0.13 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde_derive 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde_json 1.0.15 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
 name = "cc"
-version = "1.0.9"
+version = "1.0.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
@@ -70,13 +105,21 @@ version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
+name = "colored"
+version = "1.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "lazy_static 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "derive-new"
-version = "0.5.2"
+version = "0.5.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "proc-macro2 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
- "quote 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "syn 0.12.15 (registry+https://github.com/rust-lang/crates.io-index)",
+ "proc-macro2 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
+ "quote 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "syn 0.13.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -85,6 +128,11 @@ version = "0.1.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
+name = "difference"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
 name = "dtoa"
 version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -104,7 +152,7 @@ dependencies = [
 
 [[package]]
 name = "env_logger"
-version = "0.5.7"
+version = "0.5.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "atty 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -115,11 +163,16 @@ dependencies = [
 ]
 
 [[package]]
+name = "environment"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
 name = "error-chain"
 version = "0.11.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "backtrace 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)",
+ "backtrace 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -142,6 +195,11 @@ version = "0.2.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
+name = "glob"
+version = "0.2.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
 name = "humantime"
 version = "1.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -173,6 +231,11 @@ dependencies = [
 
 [[package]]
 name = "lazy_static"
+version = "0.2.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "lazy_static"
 version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
@@ -232,18 +295,18 @@ dependencies = [
 
 [[package]]
 name = "proc-macro2"
-version = "0.2.3"
+version = "0.3.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "unicode-xid 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
-name = "proc-macro2"
-version = "0.3.6"
+name = "pulldown-cmark"
+version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "unicode-xid 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "bitflags 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -253,14 +316,6 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "quote"
-version = "0.4.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-dependencies = [
- "proc-macro2 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
-]
-
-[[package]]
-name = "quote"
 version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
@@ -297,20 +352,28 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "aho-corasick 0.6.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "memchr 2.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "regex-syntax 0.5.3 (registry+https://github.com/rust-lang/crates.io-index)",
+ "regex-syntax 0.5.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "thread_local 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "utf8-ranges 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
 name = "regex-syntax"
-version = "0.5.3"
+version = "0.5.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "ucd-util 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
+name = "remove_dir_all"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "winapi 0.3.4 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "rustc-ap-rustc_cratesio_shim"
 version = "103.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -390,10 +453,11 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 name = "rustfmt-nightly"
 version = "0.4.2"
 dependencies = [
+ "assert_cli 0.5.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "cargo_metadata 0.5.4 (registry+https://github.com/rust-lang/crates.io-index)",
- "derive-new 0.5.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "derive-new 0.5.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "diff 0.1.11 (registry+https://github.com/rust-lang/crates.io-index)",
- "env_logger 0.5.7 (registry+https://github.com/rust-lang/crates.io-index)",
+ "env_logger 0.5.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "getopts 0.2.17 (registry+https://github.com/rust-lang/crates.io-index)",
  "itertools 0.7.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazy_static 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -401,9 +465,9 @@ dependencies = [
  "log 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "regex 0.2.10 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc-ap-syntax 103.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
- "serde 1.0.37 (registry+https://github.com/rust-lang/crates.io-index)",
- "serde_derive 1.0.37 (registry+https://github.com/rust-lang/crates.io-index)",
- "serde_json 1.0.13 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde_derive 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde_json 1.0.15 (registry+https://github.com/rust-lang/crates.io-index)",
  "term 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "toml 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "unicode-segmentation 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -411,17 +475,35 @@ dependencies = [
 ]
 
 [[package]]
+name = "same-file"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "kernel32-sys 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "scoped-tls"
 version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "semver"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "semver-parser 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "semver"
 version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "semver-parser 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
- "serde 1.0.37 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -431,23 +513,23 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "serde"
-version = "1.0.37"
+version = "1.0.39"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "serde_derive"
-version = "1.0.37"
+version = "1.0.39"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "proc-macro2 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "quote 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "serde_derive_internals 0.23.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde_derive_internals 0.23.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "syn 0.13.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
 name = "serde_derive_internals"
-version = "0.23.0"
+version = "0.23.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "proc-macro2 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -456,13 +538,28 @@ dependencies = [
 
 [[package]]
 name = "serde_json"
-version = "1.0.13"
+version = "1.0.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "dtoa 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "itoa 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "num-traits 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "serde 1.0.37 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "skeptic"
+version = "0.13.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "bytecount 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "cargo_metadata 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)",
+ "error-chain 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "glob 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
+ "pulldown-cmark 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde_json 1.0.15 (registry+https://github.com/rust-lang/crates.io-index)",
+ "tempdir 0.3.7 (registry+https://github.com/rust-lang/crates.io-index)",
+ "walkdir 1.0.7 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -477,22 +574,21 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "syn"
-version = "0.12.15"
+version = "0.13.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "proc-macro2 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
- "quote 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "proc-macro2 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
+ "quote 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "unicode-xid 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
-name = "syn"
-version = "0.13.1"
+name = "tempdir"
+version = "0.3.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "proc-macro2 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
- "quote 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "unicode-xid 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rand 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "remove_dir_all 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -545,7 +641,7 @@ name = "toml"
 version = "0.4.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "serde 1.0.37 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -587,6 +683,16 @@ version = "1.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
+name = "walkdir"
+version = "1.0.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "kernel32-sys 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "same-file 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)",
+ "winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "winapi"
 version = "0.2.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -625,28 +731,37 @@ dependencies = [
 
 [metadata]
 "checksum aho-corasick 0.6.4 (registry+https://github.com/rust-lang/crates.io-index)" = "d6531d44de723825aa81398a6415283229725a00fa30713812ab9323faa82fc4"
+"checksum assert_cli 0.5.4 (registry+https://github.com/rust-lang/crates.io-index)" = "72342c21057a3cb5f7c2d849bf7999a83795434dd36d74fa8c24680581bd1930"
 "checksum atty 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)" = "af80143d6f7608d746df1520709e5d141c96f240b0e62b0aa41bdfb53374d9d4"
-"checksum backtrace 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)" = "ebbbf59b1c43eefa8c3ede390fcc36820b4999f7914104015be25025e0d62af2"
+"checksum backtrace 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)" = "ebbe525f66f42d207968308ee86bc2dd60aa5fab535b22e616323a173d097d8e"
 "checksum backtrace-sys 0.1.16 (registry+https://github.com/rust-lang/crates.io-index)" = "44585761d6161b0f57afc49482ab6bd067e4edef48c12a152c237eb0203f7661"
+"checksum bitflags 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)" = "4efd02e230a02e18f92fc2735f44597385ed02ad8f831e7c1c1156ee5e1ab3a5"
 "checksum bitflags 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)" = "b3c30d3802dfb7281680d6285f2ccdaa8c2d8fee41f93805dba5c4cf50dc23cf"
+"checksum bytecount 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "af27422163679dea46a1a7239dffff64d3dcdc3ba5fe9c49c789fbfe0eb949de"
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+"checksum cargo_metadata 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)" = "1f56ec3e469bca7c276f2eea015aa05c5e381356febdbb0683c2580189604537"
 "checksum cargo_metadata 0.5.4 (registry+https://github.com/rust-lang/crates.io-index)" = "6ebd6272a2ca4fd39dbabbd6611eb03df45c2259b3b80b39a9ff8fbdcf42a4b3"
-"checksum cc 1.0.9 (registry+https://github.com/rust-lang/crates.io-index)" = "2b4911e4bdcb4100c7680e7e854ff38e23f1b34d4d9e079efae3da2801341ffc"
+"checksum cc 1.0.10 (registry+https://github.com/rust-lang/crates.io-index)" = "8b9d2900f78631a5876dc5d6c9033ede027253efcd33dd36b1309fc6cab97ee0"
 "checksum cfg-if 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)" = "d4c819a1287eb618df47cc647173c5c4c66ba19d888a6e50d605672aed3140de"
-"checksum derive-new 0.5.2 (registry+https://github.com/rust-lang/crates.io-index)" = "6fcb923bab47a948f1b01cec2f758fdebba95c9ebc255458654b2b88efe59d71"
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 "checksum diff 0.1.11 (registry+https://github.com/rust-lang/crates.io-index)" = "3c2b69f912779fbb121ceb775d74d51e915af17aaebc38d28a592843a2dd0a3a"
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 "checksum ena 0.9.2 (registry+https://github.com/rust-lang/crates.io-index)" = "f8b449f3b18c89d2dbe40548d2ee4fa58ea0a08b761992da6ecb9788e4688834"
-"checksum env_logger 0.5.7 (registry+https://github.com/rust-lang/crates.io-index)" = "0f475037312b91d34dbc3142a1ad3980ef0d070c7a855ce238afdd5e987cfecc"
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 "checksum fuchsia-zircon-sys 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)" = "3dcaa9ae7725d12cdb85b3ad99a434db70b468c09ded17e012d86b5c1010f7a7"
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+"checksum lazy_static 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)" = "76f033c7ad61445c5b347c7382dd1237847eb1bce590fe50365dcb33d546be73"
 "checksum lazy_static 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "c8f31047daa365f19be14b47c29df4f7c3b581832407daabe6ae77397619237d"
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@@ -655,16 +770,16 @@ dependencies = [
 "checksum owning_ref 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)" = "cdf84f41639e037b484f93433aa3897863b561ed65c6e59c7073d7c561710f37"
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-"checksum proc-macro2 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)" = "cd07deb3c6d1d9ff827999c7f9b04cdfd66b1b17ae508e14fe47b620f2282ae0"
 "checksum proc-macro2 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)" = "49b6a521dc81b643e9a51e0d1cf05df46d5a2f3c0280ea72bcb68276ba64a118"
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-"checksum regex-syntax 0.5.3 (registry+https://github.com/rust-lang/crates.io-index)" = "b2550876c31dc914696a6c2e01cbce8afba79a93c8ae979d2fe051c0230b3756"
+"checksum regex-syntax 0.5.5 (registry+https://github.com/rust-lang/crates.io-index)" = "bd90079345f4a4c3409214734ae220fd773c6f2e8a543d07370c6c1c369cfbfb"
+"checksum remove_dir_all 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)" = "dfc5b3ce5d5ea144bb04ebd093a9e14e9765bcfec866aecda9b6dec43b3d1e24"
 "checksum rustc-ap-rustc_cratesio_shim 103.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "463834ac5ea777cb56c073586675fac37292f8425aafb3757efca7e6a76545aa"
 "checksum rustc-ap-rustc_data_structures 103.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "5d256eeab1b8639c2a1fd341e54f3613f8150bc262e4ec9361a29bbcb162906d"
 "checksum rustc-ap-rustc_errors 103.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "cf6dd73033bb512845a6df347174c65ad430c92ecd35527e24d8bb186f5664ee"
@@ -672,17 +787,20 @@ dependencies = [
 "checksum rustc-ap-syntax 103.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "0f02edede4ba70963a7dac2308876f03f76f9edd48a035e5abc8fa37c57a77c8"
 "checksum rustc-ap-syntax_pos 103.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ad8e50d4c38121fa8ded3ffbf94926ec74c95f24316c3b80de84fbfb42c005cf"
 "checksum rustc-demangle 0.1.7 (registry+https://github.com/rust-lang/crates.io-index)" = "11fb43a206a04116ffd7cfcf9bcb941f8eb6cc7ff667272246b0a1c74259a3cb"
+"checksum same-file 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)" = "d931a44fdaa43b8637009e7632a02adc4f2b2e0733c08caa4cf00e8da4a117a7"
 "checksum scoped-tls 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "8674d439c964889e2476f474a3bf198cc9e199e77499960893bac5de7e9218a4"
+"checksum semver 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)" = "bee2bc909ab2d8d60dab26e8cad85b25d795b14603a0dcb627b78b9d30b6454b"
 "checksum semver 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)" = "1d7eb9ef2c18661902cc47e535f9bc51b78acd254da71d375c2f6720d9a40403"
 "checksum semver-parser 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)" = "388a1df253eca08550bef6c72392cfe7c30914bf41df5269b68cbd6ff8f570a3"
-"checksum serde 1.0.37 (registry+https://github.com/rust-lang/crates.io-index)" = "d3bcee660dcde8f52c3765dd9ca5ee36b4bf35470a738eb0bd5a8752b0389645"
-"checksum serde_derive 1.0.37 (registry+https://github.com/rust-lang/crates.io-index)" = "f1711ab8b208541fa8de00425f6a577d90f27bb60724d2bb5fd911314af9668f"
-"checksum serde_derive_internals 0.23.0 (registry+https://github.com/rust-lang/crates.io-index)" = "89b340a48245bc03ddba31d0ff1709c118df90edc6adabaca4aac77aea181cce"
-"checksum serde_json 1.0.13 (registry+https://github.com/rust-lang/crates.io-index)" = "5c508584d9913df116b91505eec55610a2f5b16e9ed793c46e4d0152872b3e74"
+"checksum serde 1.0.39 (registry+https://github.com/rust-lang/crates.io-index)" = "53e6b6859395f46cf528414659ce43e70902b2277519707c3bd91797b3320330"
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+"checksum serde_json 1.0.15 (registry+https://github.com/rust-lang/crates.io-index)" = "7bf1cbb1387028a13739cb018ee0d9b3db534f22ca3c84a5904f7eadfde14e75"
+"checksum skeptic 0.13.2 (registry+https://github.com/rust-lang/crates.io-index)" = "c8431f8fca168e2db4be547bd8329eac70d095dff1444fee4b0fa0fabc7df75a"
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 "checksum stable_deref_trait 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "15132e0e364248108c5e2c02e3ab539be8d6f5d52a01ca9bbf27ed657316f02b"
-"checksum syn 0.12.15 (registry+https://github.com/rust-lang/crates.io-index)" = "c97c05b8ebc34ddd6b967994d5c6e9852fa92f8b82b3858c39451f97346dcce5"
 "checksum syn 0.13.1 (registry+https://github.com/rust-lang/crates.io-index)" = "91b52877572087400e83d24b9178488541e3d535259e04ff17a63df1e5ceff59"
+"checksum tempdir 0.3.7 (registry+https://github.com/rust-lang/crates.io-index)" = "15f2b5fb00ccdf689e0149d1b1b3c03fead81c2b37735d812fa8bddbbf41b6d8"
 "checksum term 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)" = "fa63644f74ce96fbeb9b794f66aff2a52d601cbd5e80f4b97123e3899f4570f1"
 "checksum term 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)" = "5e6b677dd1e8214ea1ef4297f85dbcbed8e8cdddb561040cc998ca2551c37561"
 "checksum termcolor 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)" = "adc4587ead41bf016f11af03e55a624c06568b5a19db4e90fde573d805074f83"
@@ -696,6 +814,7 @@ dependencies = [
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 "checksum utf8-ranges 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "662fab6525a98beff2921d7f61a39e7d59e0b425ebc7d0d9e66d316e55124122"
 "checksum void 1.0.2 (registry+https://github.com/rust-lang/crates.io-index)" = "6a02e4885ed3bc0f2de90ea6dd45ebcbb66dacffe03547fadbb0eeae2770887d"
+"checksum walkdir 1.0.7 (registry+https://github.com/rust-lang/crates.io-index)" = "bb08f9e670fab86099470b97cd2b252d6527f0b3cc1401acdb595ffc9dd288ff"
 "checksum winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)" = "167dc9d6949a9b857f3451275e911c3f44255842c1f7a76f33c55103a909087a"
 "checksum winapi 0.3.4 (registry+https://github.com/rust-lang/crates.io-index)" = "04e3bd221fcbe8a271359c04f21a76db7d0c6028862d1bb5512d85e1e2eb5bb3"
 "checksum winapi-build 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "2d315eee3b34aca4797b2da6b13ed88266e6d612562a0c46390af8299fc699bc"
diff --git a/Cargo.toml b/Cargo.toml
index 719f56667f5..e989e123dbb 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -50,6 +50,7 @@ rustc-ap-syntax = "103.0.0"
 
 [dev-dependencies]
 lazy_static = "1.0.0"
+assert_cli = "0.5"
 
 [target.'cfg(unix)'.dependencies]
 libc = "0.2.11"
diff --git a/README.md b/README.md
index 49fedfdb286..020d6bf3b20 100644
--- a/README.md
+++ b/README.md
@@ -106,7 +106,7 @@ read data from stdin. Alternatively, you can use `cargo fmt` to format all
 binary and library targets of your crate.
 
 You'll probably want to specify the write mode. Currently, there are modes for
-`diff`, `replace`, `overwrite`, `display`, `coverage`, `checkstyle`, and `plain`.
+`check`, `diff`, `replace`, `overwrite`, `display`, `coverage`, `checkstyle`, and `plain`.
 
 * `overwrite` Is the default and overwrites the original files _without_ creating backups.
 * `replace` Overwrites the original files after creating backups of the files.
@@ -114,6 +114,9 @@ You'll probably want to specify the write mode. Currently, there are modes for
 * `plain` Also writes to stdout, but with no metadata.
 * `diff` Will print a diff between the original files and formatted files to stdout.
          Will also exit with an error code if there are any differences.
+* `check` Checks if the program's formatting matches what rustfmt would do. Silently exits
+          with code 0 if so, emits a diff and exits with code 1 if not. This option is
+          designed to be run in CI-like where a non-zero exit signifies incorrect formatting.
 * `checkstyle` Will output the lines that need to be corrected as a checkstyle XML file,
   that can be used by tools like Jenkins.
 
diff --git a/src/bin/main.rs b/src/bin/main.rs
index 330bec55588..ff0961fd2e2 100644
--- a/src/bin/main.rs
+++ b/src/bin/main.rs
@@ -30,7 +30,7 @@ use rustfmt::{run, FileName, Input, Summary};
 type FmtError = Box<error::Error + Send + Sync>;
 type FmtResult<T> = std::result::Result<T, FmtError>;
 
-const WRITE_MODE_LIST: &str = "[replace|overwrite|display|plain|diff|coverage|checkstyle]";
+const WRITE_MODE_LIST: &str = "[replace|overwrite|display|plain|diff|coverage|checkstyle|check]";
 
 /// Rustfmt operations.
 enum Operation {
@@ -303,6 +303,7 @@ fn execute(opts: &Options) -> FmtResult<Summary> {
             let mut out = &mut stdout();
             checkstyle::output_header(&mut out, config.write_mode())?;
             let mut error_summary = Summary::default();
+
             for file in files {
                 if !file.exists() {
                     eprintln!("Error: file `{}` does not exist", file.to_str().unwrap());
@@ -350,22 +351,28 @@ fn execute(opts: &Options) -> FmtResult<Summary> {
     }
 }
 
+fn determine_write_mode(opts: &Options) -> WriteMode {
+    let matches = opts.parse(env::args().skip(1)).unwrap();
+    let options = CliOptions::from_matches(&matches).unwrap();
+    match options.write_mode {
+        Some(m) => m,
+        None => WriteMode::default(),
+    }
+}
+
 fn main() {
     env_logger::init();
-
     let opts = make_opts();
+    // Only handles arguments passed in through the CLI.
+    let write_mode = determine_write_mode(&opts);
 
     let exit_code = match execute(&opts) {
         Ok(summary) => {
-            if summary.has_operational_errors() {
+            if summary.has_operational_errors()
+                || summary.has_diff && write_mode == WriteMode::Check
+                || summary.has_parsing_errors() || summary.has_formatting_errors()
+            {
                 1
-            } else if summary.has_parsing_errors() {
-                2
-            } else if summary.has_formatting_errors() {
-                3
-            } else if summary.has_diff {
-                // should only happen in diff mode
-                4
             } else {
                 assert!(summary.has_no_errors());
                 0
diff --git a/src/config/mod.rs b/src/config/mod.rs
index 8cc55ebac4b..274a40a2ae7 100644
--- a/src/config/mod.rs
+++ b/src/config/mod.rs
@@ -120,7 +120,7 @@ create_config! {
     // Control options (changes the operation of rustfmt, rather than the formatting)
     write_mode: WriteMode, WriteMode::Overwrite, false,
         "What Write Mode to use when none is supplied: \
-         Replace, Overwrite, Display, Plain, Diff, Coverage";
+         Replace, Overwrite, Display, Plain, Diff, Coverage, Check";
     color: Color, Color::Auto, false,
         "What Color option to use when none is supplied: Always, Never, Auto";
     required_version: String, env!("CARGO_PKG_VERSION").to_owned(), false,
diff --git a/src/config/options.rs b/src/config/options.rs
index eca206a75fe..0bb53f046ac 100644
--- a/src/config/options.rs
+++ b/src/config/options.rs
@@ -183,6 +183,9 @@ configuration_option_enum! { WriteMode:
     Checkstyle,
     // Output the changed lines (for internal value only)
     Modified,
+    // Checks if a diff can be generated. If so, rustfmt outputs a diff and quits with exit code 1.
+    // This option is designed to be run in CI where a non-zero exit signifies non-standard code formatting.
+    Check,
 }
 
 configuration_option_enum! { Color:
@@ -250,6 +253,12 @@ impl ::std::str::FromStr for WidthHeuristics {
     }
 }
 
+impl Default for WriteMode {
+    fn default() -> WriteMode {
+        WriteMode::Overwrite
+    }
+}
+
 /// A set of directories, files and modules that rustfmt should ignore.
 #[derive(Default, Deserialize, Serialize, Clone, Debug)]
 pub struct IgnoreList(HashSet<PathBuf>);
diff --git a/src/config/summary.rs b/src/config/summary.rs
index 99bf752f2b9..3654ea89df4 100644
--- a/src/config/summary.rs
+++ b/src/config/summary.rs
@@ -104,10 +104,7 @@ impl Summary {
     pub fn print_exit_codes() {
         let exit_codes = r#"Exit Codes:
     0 = No errors
-    1 = Encountered operational errors e.g. an IO error
-    2 = Failed to reformat code because of parsing errors
-    3 = Code is valid, but it is impossible to format it properly
-    4 = Formatted code differs from existing code (write-mode diff only)"#;
+    1 = Encountered error in formatting code"#;
         println!("{}", exit_codes);
     }
 }
diff --git a/src/filemap.rs b/src/filemap.rs
index ee2253043a1..ce9134b131e 100644
--- a/src/filemap.rs
+++ b/src/filemap.rs
@@ -178,6 +178,19 @@ where
             let diff = create_diff(filename, text, config)?;
             output_checkstyle_file(out, filename, diff)?;
         }
+        WriteMode::Check => {
+            let filename = filename_to_path();
+            if let Ok((ori, fmt)) = source_and_formatted_text(text, filename, config) {
+                let mismatch = make_diff(&ori, &fmt, 3);
+                let has_diff = !mismatch.is_empty();
+                print_diff(
+                    mismatch,
+                    |line_num| format!("Diff in {} at line {}:", filename.display(), line_num),
+                    config.color(),
+                );
+                return Ok(has_diff);
+            }
+        }
     }
 
     // when we are not in diff mode, don't indicate differing files
diff --git a/src/lib.rs b/src/lib.rs
index c14737a8ebf..ddb3ebeb2e9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -14,6 +14,7 @@
 #![allow(stable_features)]
 #![feature(match_default_bindings)]
 #![feature(type_ascription)]
+#![feature(unicode_internals)]
 
 #[macro_use]
 extern crate derive_new;
diff --git a/tests/lib.rs b/tests/lib.rs
index 3e388af0022..b9b51853a2d 100644
--- a/tests/lib.rs
+++ b/tests/lib.rs
@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+extern crate assert_cli;
 #[macro_use]
 extern crate lazy_static;
 #[macro_use]
@@ -862,3 +863,29 @@ fn configuration_snippet_tests() {
     println!("Ran {} configurations tests.", blocks.len());
     assert_eq!(failures, 0, "{} configurations tests failed", failures);
 }
+
+#[test]
+fn verify_check_works() {
+    assert_cli::Assert::command(&[
+        "cargo",
+        "run",
+        "--bin=rustfmt",
+        "--",
+        "--write-mode=check",
+        "src/bin/main.rs",
+    ]).succeeds()
+        .unwrap();
+}
+
+#[test]
+fn verify_diff_works() {
+    assert_cli::Assert::command(&[
+        "cargo",
+        "run",
+        "--bin=rustfmt",
+        "--",
+        "--write-mode=diff",
+        "src/bin/main.rs",
+    ]).succeeds()
+        .unwrap();
+}