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

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();

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);
     }
 }