Rollup merge of #47547 - varkor:infinite-iterators-warning-doc, r=frewsxcv

Document the behaviour of infinite iterators on potentially-computable methods

It’s not entirely clear from the current documentation what behaviour
calling a method such as `min` on an infinite iterator like `RangeFrom`
is. One might expect this to terminate, but in fact, for infinite
iterators, `min` is always nonterminating (at least in the standard
library). This adds a quick note about this behaviour for clarification.

Author: kennytm

src/libcore/iter/iterator.rs

@@ -1431,6 +1431,10 @@ pub trait Iterator {
     /// Folding is useful whenever you have a collection of something, and want
     /// to produce a single value from it.
     ///
+    /// Note: `fold()`, and similar methods that traverse the entire iterator,
+    /// may not terminate for infinite iterators, even on traits for which a
+    /// result is determinable in finite time.
+    ///
     /// # Examples
     ///
     /// Basic usage:

src/libcore/iter/mod.rs

@@ -298,7 +298,21 @@
 //!
 //! This will print the numbers `0` through `4`, each on their own line.
 //!
+//! Bear in mind that methods on infinite iterators, even those for which a
+//! result can be determined mathematically in finite time, may not terminate.
+//! Specifically, methods such as [`min`], which in the general case require
+//! traversing every element in the iterator, are likely not to return
+//! successfully for any infinite iterators.
+//!
+//! ```no_run
+//! let ones = std::iter::repeat(1);
+//! let least = ones.min().unwrap(); // Oh no! An infinite loop!
+//! // `ones.min()` causes an infinite loop, so we won't reach this point!
+//! println!("The smallest number one is {}.", least);
+//! ```
+//!
 //! [`take`]: trait.Iterator.html#method.take
+//! [`min`]: trait.Iterator.html#method.min
 
 #![stable(feature = "rust1", since = "1.0.0")]