rollup merge of #21718: alexcrichton/stabilize-from-str

This commits adds an associated type to the `FromStr` trait representing an
error payload for parses which do not succeed. The previous return value,
`Option<Self>` did not allow for this form of payload. After the associated type
was added, the following attributes were applied:

* `FromStr` is now stable
* `FromStr::Err` is now stable
* `FromStr::from_str` is now stable
* `StrExt::parse` is now stable
* `FromStr for bool` is now stable
* `FromStr for $float` is now stable
* `FromStr for $integral` is now stable
* Errors returned from stable `FromStr` implementations are stable
* Errors implement `Display` and `Error` (both impl blocks being `#[stable]`)

Closes #15138

src/libtest/lib.rs

@@ -76,7 +76,6 @@ use std::old_io;
 use std::iter::repeat;
 use std::num::{Float, Int};
 use std::os;
-use std::str::FromStr;
 use std::sync::mpsc::{channel, Sender};
 use std::thread::{self, Thread};
 use std::thunk::{Thunk, Invoke};
@@ -820,7 +819,7 @@ fn get_concurrency() -> uint {
     use std::rt;
     match os::getenv("RUST_TEST_TASKS") {
         Some(s) => {
-            let opt_n: Option<uint> = FromStr::from_str(s.as_slice());
+            let opt_n: Option<uint> = s.parse().ok();
             match opt_n {
                 Some(n) if n > 0 => n,
                 _ => panic!("RUST_TEST_TASKS is `{}`, should be a positive integer.", s)