allow RUST_BACKTRACE=0 to act as if unset

/# This is a combination of 16 commits.
/# The first commit's message is:
allow RUST_BACKTRACE=disabled to act as if unset

When RUST_BACKTRACE is set to "disabled" then this acts as if the env.
var is unset.

/# This is the 2nd commit message:

case insensitive "DiSaBLeD" RUST_BACKTRACE value

previously it expected a lowercase "disabled" to treat the env. var as
unset

/# This is the 3rd commit message:

RUST_BACKTRACE=0 acts as if unset

previously RUST_BACKTRACE=disabled was doing the same thing

/# This is the 4th commit message:

RUST_BACKTRACE=0|n|no|off acts as if unset

previously only RUST_BACKTRACE=0 acted as if RUST_BACKTRACE was unset
Now added more options (case-insensitive): 'n','no' and 'off'
eg. RUST_BACKTRACE=oFF

/# This is the 5th commit message:

DRY on the value of 2

DRY=don't repeat yourself
Because having to remember to keep the two places of '2' in sync is not
ideal, even though this is a simple enough case.

/# This is the 6th commit message:

Revert "DRY on the value of 2"

This reverts commit 95a0479d5cf72a2b2d9d21ec0bed2823ed213fef.

Nevermind this DRY on 2, because we already have a RY on 1,
besides the code is less readable this way...

/# This is the 7th commit message:

attempt to document unsetting RUST_BACKTRACE

/# This is the 8th commit message:

curb allocations when checking for RUST_BACKTRACE

this means we don't check for case-insensitivity anymore

/# This is the 9th commit message:

as decided, RUST_BACKTRACE=0 turns off backtrace

/# This is the 10th commit message:

RUST_TEST_NOCAPTURE=0 acts as if unset

(that is, capture is on)

Any other value acts as if nocapture is enabled (that is, capture is off)

/# This is the 11th commit message:

update other RUST_TEST_NOCAPTURE occurrences

apparently only one place needs updating

/# This is the 12th commit message:

update RUST_BACKTRACE in man page

/# This is the 13th commit message:

handle an occurrence of RUST_BACKTRACE

/# This is the 14th commit message:

ensure consistency with new rules for backtrace

/# This is the 15th commit message:

a more concise comment for RUST_TEST_NOCAPTURE

/# This is the 16th commit message:

update RUST_TEST_NOCAPTURE in man page

9 files changed, 59 insertions, 17 deletions

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 8f721709b9f..10598e78ec5 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -71,7 +71,8 @@ which includes important information about what platform you're on, what
 version of Rust you're using, etc.
 
 Sometimes, a backtrace is helpful, and so including that is nice. To get
-a backtrace, set the `RUST_BACKTRACE` environment variable. The easiest way
+a backtrace, set the `RUST_BACKTRACE` environment variable to a value
+other than `0`. The easiest way
 to do this is to invoke `rustc` like this:
 
 ```bash
diff --git a/man/rustc.1 b/man/rustc.1
index 0b8b1559d90..a034e471b6e 100644
--- a/man/rustc.1
+++ b/man/rustc.1
@@ -268,7 +268,7 @@ the maximum number of threads used for this purpose.
 
 .TP
 \fBRUST_TEST_NOCAPTURE\fR
-A synonym for the --nocapture flag.
+If set to a value other than "0", a synonym for the --nocapture flag.
 
 .TP
 \fBRUST_MIN_STACK\fR
@@ -276,7 +276,7 @@ Sets the minimum stack size for new threads.
 
 .TP
 \fBRUST_BACKTRACE\fR
-If set, produces a backtrace in the output of a program which panics.
+If set to a value different than "0", produces a backtrace in the output of a program which panics.
 
 .SH "EXAMPLES"
 To build an executable from a source file with a main function:
diff --git a/src/compiletest/compiletest.rs b/src/compiletest/compiletest.rs
index 6c6a78a360b..787d77bc56c 100644
--- a/src/compiletest/compiletest.rs
+++ b/src/compiletest/compiletest.rs
@@ -263,7 +263,10 @@ pub fn test_opts(config: &Config) -> test::TestOpts {
         logfile: config.logfile.clone(),
         run_tests: true,
         bench_benchmarks: true,
-        nocapture: env::var("RUST_TEST_NOCAPTURE").is_ok(),
+        nocapture: match env::var("RUST_TEST_NOCAPTURE") {
+            Ok(val) => &val != "0",
+            Err(_) => false
+        },
         color: test::AutoColor,
     }
 }
diff --git a/src/doc/book/functions.md b/src/doc/book/functions.md
index 31c9da3fada..8a2444323f1 100644
--- a/src/doc/book/functions.md
+++ b/src/doc/book/functions.md
@@ -246,6 +246,19 @@ stack backtrace:
   13:                0x0 - <unknown>
 ```
 
+If you need to override an already set `RUST_BACKTRACE`, 
+in cases when you cannot just unset the variable, 
+then set it to `0` to avoid getting a backtrace. 
+Any other value(even no value at all) turns on backtrace.
+
+```text
+$ export RUST_BACKTRACE=1
+...
+$ RUST_BACKTRACE=0 ./diverges 
+thread '<main>' panicked at 'This function never returns!', hello.rs:2
+note: Run with `RUST_BACKTRACE=1` for a backtrace.
+```
+
 `RUST_BACKTRACE` also works with Cargo’s `run` command:
 
 ```text
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index 516c55e1020..3444b770cc8 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -1059,7 +1059,10 @@ pub fn monitor<F: FnOnce() + Send + 'static>(f: F) {
             for note in &xs {
                 emitter.emit(None, &note[..], None, errors::Level::Note)
             }
-            if let None = env::var_os("RUST_BACKTRACE") {
+            if match env::var_os("RUST_BACKTRACE") {
+                Some(val) => &val != "0",
+                None => false,
+            } {
                 emitter.emit(None,
                              "run with `RUST_BACKTRACE=1` for a backtrace",
                              None,
diff --git a/src/libstd/sys/common/backtrace.rs b/src/libstd/sys/common/backtrace.rs
index b81fb3011c7..24e1a82a593 100644
--- a/src/libstd/sys/common/backtrace.rs
+++ b/src/libstd/sys/common/backtrace.rs
@@ -36,7 +36,7 @@ pub fn log_enabled() -> bool {
     }
 
     let val = match env::var_os("RUST_BACKTRACE") {
-        Some(..) => 2,
+        Some(x) => if &x == "0" { 1 } else { 2 },
         None => 1,
     };
     ENABLED.store(val, Ordering::SeqCst);
diff --git a/src/libtest/lib.rs b/src/libtest/lib.rs
index e4d7697a71b..e7fe128a7ae 100644
--- a/src/libtest/lib.rs
+++ b/src/libtest/lib.rs
@@ -349,8 +349,8 @@ By default, all tests are run in parallel. This can be altered with the
 RUST_TEST_THREADS environment variable when running tests (set it to 1).
 
 All tests have their standard output and standard error captured by default.
-This can be overridden with the --nocapture flag or the RUST_TEST_NOCAPTURE=1
-environment variable. Logging is not captured by default.
+This can be overridden with the --nocapture flag or setting RUST_TEST_NOCAPTURE
+environment variable to a value other than "0". Logging is not captured by default.
 
 Test Attributes:
 
@@ -399,7 +399,10 @@ pub fn parse_opts(args: &[String]) -> Option<OptRes> {
 
     let mut nocapture = matches.opt_present("nocapture");
     if !nocapture {
-        nocapture = env::var("RUST_TEST_NOCAPTURE").is_ok();
+        nocapture = match env::var("RUST_TEST_NOCAPTURE") {
+            Ok(val) => &val != "0",
+            Err(_) => false
+        };
     }
 
     let color = match matches.opt_str("color").as_ref().map(|s| &**s) {
diff --git a/src/test/run-pass/backtrace.rs b/src/test/run-pass/backtrace.rs
index 36cac8f50a8..a2108ff041d 100644
--- a/src/test/run-pass/backtrace.rs
+++ b/src/test/run-pass/backtrace.rs
@@ -86,6 +86,16 @@ fn runtest(me: &str) {
     assert!(!s.contains("stack backtrace") && !s.contains(&expected("foo")),
             "bad output2: {}", s);
 
+    // Make sure the stack trace is *not* printed
+    // (RUST_BACKTRACE=0 acts as if it were unset from our own environment,
+    // in case developer is running `make check` with it set.)
+    let p = template(me).arg("fail").env("RUST_BACKTRACE","0").spawn().unwrap();
+    let out = p.wait_with_output().unwrap();
+    assert!(!out.status.success());
+    let s = str::from_utf8(&out.stderr).unwrap();
+    assert!(!s.contains("stack backtrace") && !s.contains(" - foo"),
+            "bad output3: {}", s);
+
     // Make sure a stack trace is printed
     let p = template(me).arg("double-fail").spawn().unwrap();
     let out = p.wait_with_output().unwrap();
diff --git a/src/test/run-pass/multi-panic.rs b/src/test/run-pass/multi-panic.rs
index 6a0d7278b5e..8e0b14128c8 100644
--- a/src/test/run-pass/multi-panic.rs
+++ b/src/test/run-pass/multi-panic.rs
@@ -8,6 +8,17 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+fn check_for_no_backtrace(test: std::process::Output) {
+    assert!(!test.status.success());
+    let err = String::from_utf8_lossy(&test.stderr);
+    let mut it = err.lines();
+
+    assert_eq!(it.next().map(|l| l.starts_with("thread '<unnamed>' panicked at")), Some(true));
+    assert_eq!(it.next(), Some("note: Run with `RUST_BACKTRACE=1` for a backtrace."));
+    assert_eq!(it.next().map(|l| l.starts_with("thread '<main>' panicked at")), Some(true));
+    assert_eq!(it.next(), None);
+}
+
 fn main() {
     let args: Vec<String> = std::env::args().collect();
     if args.len() > 1 && args[1] == "run_test" {
@@ -21,13 +32,11 @@ fn main() {
                                                        .env_remove("RUST_BACKTRACE")
                                                        .output()
                                                        .unwrap();
-        assert!(!test.status.success());
-        let err = String::from_utf8_lossy(&test.stderr);
-        let mut it = err.lines();
-
-        assert_eq!(it.next().map(|l| l.starts_with("thread '<unnamed>' panicked at")), Some(true));
-        assert_eq!(it.next(), Some("note: Run with `RUST_BACKTRACE=1` for a backtrace."));
-        assert_eq!(it.next().map(|l| l.starts_with("thread '<main>' panicked at")), Some(true));
-        assert_eq!(it.next(), None);
+        check_for_no_backtrace(test);
+        let test = std::process::Command::new(&args[0]).arg("run_test")
+                                                       .env("RUST_BACKTRACE","0")
+                                                       .output()
+                                                       .unwrap();
+        check_for_no_backtrace(test);
     }
 }