Define the maximum width of a line and the width of indentation.

Closes #1

text/0000-line-width.md

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+- Start Date: 2016-05-04
+- RFC PR: (leave this empty)
+- Implementation Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Define the maximum width of a line and the width of indentation.
+
+
+# Details
+[details]: #details
+
+The maximum width of a line is 100 characters. The maximum width of a comment
+(including any commenting characters, e.g., `//`, but not including any
+indentation) is 80 characters. A comment should be wrapped at the minimum of the
+80 character comment limit and the 100 character line limit.
+
+Indentation should use spaces, not tabs. Each level of indentation should be
+four characters wide.
+
+Empty lines should have no whitespace, even if lines above and below are
+indented.
+
+Example:
+
+```
+// Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
+// tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
+// quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
+// consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
+// cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
+// proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
+mod foo {
+    mod bar {
+        // Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
+        // tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
+        // quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
+        // consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
+        // cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
+        // proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
+        fn baz() {
+            this_is_a_very_long_function_name_and_the_line_is_long(1 + 2 + 3 + 4 + 5 + 6 + (7 + 8));
+        }
+
+        // Note, the above line is empty, no indentation.
+        fn qux() {
+        }
+    }
+}
+
+fn main() {
+    println!("Hello, world!");
+}
+```
+
+
+# Implementation
+[implementation]: #implementation
+
+Rustfmt options:
+
+* `max_width` remains defaulted to `100`.
+* `ideal_width` is removed.
+* A new option, `comment_width`, is added with value `80` and type `usize`.
+* `tab_spaces` is renamed to `indent_width` and remains `4`.
+* `hard_tabs` remains `false`.
+
+All these options remain configurable. A `comment_width` of 0 means that Rustfmt
+will not take into account a maximum comment width and will only be constrained
+by the line width.
+
+The only implementation work required is the change from having an `ideal_width`
+(which only applies to comments) to `comment_width`. The change is that
+`ideal_width` was a line width, including indentation, whereas `comment_width`
+is not.
+
+[Note: this level of implementation is something of a grey area - the change is
+a tweak to an existing option, but is bordering on a big enough change to
+warrant implementation before the RFC is submitted. The reality is that at the
+moment comment formatting in Rustfmt is pretty bad and so implementation is a
+bit irrelevant. I hope by the time the fmt-rfcs repository is live and the
+formal process starts for this PR, comments are in a better state and this
+implementation work will be done].
+
+
+# Rationale
+[rationale]: #rationale
+
+Common choices for line width are 80 and 100 characters. Restricting to 100
+(rather than a larger number) is good for users with older hardware, reading
+code on mobile devices, using terminals, and having two files side-by-side on a
+wide screen.
+
+Indentation is commonly two or four spaces. Four seems to be more popular in the
+Rust community and is used in the Rust project itself (and pretty much every
+other project).
+
+Due to four-space indent and Rust's 'culture' of rightward drift, 100 spaces is
+more appropriate than 80. It is widely used (Rust, Servo, most other projects).
+
+Comment lines are usually prose rather than code, therefore there are different
+pressures. [Studies](http://baymard.com/blog/line-length-readability)
+have shown that wider lines of text are harder to read. Especially for long
+comments (e.g., documentation at the head of modules), 100 character-wide text
+feels extremely 'heavy'. This de facto limit is adhered to by most of the Rust
+project (e.g., [libstd](https://github.com/rust-lang/rust/blob/master/src/libstd/lib.rs)).
+
+
+# Alternatives
+[alternatives]: #alternatives
+
+The style guide currently recommends 99 character wide lines. This allows for
+diffs to be 100 characters wide. However, this rule is not followed in practice
+by the Rust project (where `make tidy` enforces a 100 character limit).
+
+We could limit the width of comment lines to 80 characters including
+indentation. This is probably easier to format manually (text editors usually
+have a line wrap function which does this). However, it means deeply indented
+comments end up very narrow.
+
+Rustfmt could be made significantly simpler by removing the `hard_tabs` option.
+However, it is a popular alternative, so should probably be left as an option.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.