Do indentation properly in Vim.

The cases that this improves are clearly documented inside the
committed file, so this commit message is comparatively brief.

src/etc/vim/indent/rust.vim

@@ -1,11 +1,137 @@
 " Vim indent file
+" Language:         Rust
+" Author:           Chris Morgan <me@chrismorgan.info>
+" Last Change:      2013 Jul 10
 
+" Only load this indent file when no other was loaded.
 if exists("b:did_indent")
-	finish
+  finish
 endif
-
 let b:did_indent = 1
 
 setlocal cindent
 setlocal cinoptions=L0,(0,Ws,JN
-setlocal cinkeys=0{,0},!^F,o,O
+setlocal cinkeys=0{,0},!^F,o,O,0[,0]
+" Don't think cinwords will actually do anything at all... never mind
+setlocal cinwords=do,for,if,else,while,loop,impl,mod,unsafe,trait,struct,enum,fn,extern
+
+" Some preliminary settings
+setlocal nolisp		" Make sure lisp indenting doesn't supersede us
+setlocal autoindent	" indentexpr isn't much help otherwise
+" Also do indentkeys, otherwise # gets shoved to column 0 :-/
+setlocal indentkeys=0{,0},!^F,o,O,0[,0]
+
+setlocal indentexpr=GetRustIndent(v:lnum)
+
+" Only define the function once.
+if exists("*GetRustIndent")
+  finish
+endif
+
+" Come here when loading the script the first time.
+
+function s:get_line_trimmed(lnum)
+	" Get the line and remove a trailing comment.
+	" Use syntax highlighting attributes when possible.
+	" NOTE: this is not accurate; /* */ or a line continuation could trick it
+	let line = getline(a:lnum)
+	let line_len = strlen(line)
+	if has('syntax_items')
+		" If the last character in the line is a comment, do a binary search for
+		" the start of the comment.  synID() is slow, a linear search would take
+		" too long on a long line.
+		if synIDattr(synID(a:lnum, line_len, 1), "name") =~ "Comment\|Todo"
+			let min = 1
+			let max = line_len
+			while min < max
+				let col = (min + max) / 2
+				if synIDattr(synID(a:lnum, col, 1), "name") =~ "Comment\|Todo"
+					let max = col
+				else
+					let min = col + 1
+				endif
+			endwhile
+			let line = strpart(line, 0, min - 1)
+		endif
+		return substitute(line, "\s*$", "", "")
+	else
+		" Sorry, this is not complete, nor fully correct (e.g. string "//").
+		" Such is life.
+		return substitute(line, "\s*//.*$", "", "")
+	endif
+endfunction
+
+function GetRustIndent(lnum)
+
+	" Starting assumption: cindent (called at the end) will do it right
+	" normally. We just want to fix up a few cases.
+
+	if has('syntax_items')
+		if synIDattr(synID(a:lnum, 1, 1), "name") == "rustString"
+			" If the start of the line is in a string, don't change the indent
+			return -1
+		elseif synIDattr(synID(a:lnum, 1, 1), "name") =~ "\\(Comment\\|Todo\\)"
+					\ && getline(a:lnum) !~ "^\\s*/\\*"
+			" If it's in a comment, let cindent take care of it now. This is
+			" for cases like "/*" where the next line should start " * ", not
+			" "* " as the code below would otherwise cause for module scope
+			" Fun fact: "  /*\n*\n*/" takes two calls to get right!
+			return cindent(a:lnum)
+		endif
+	endif
+
+	" cindent gets second and subsequent match patterns/struct members wrong,
+	" as it treats the comma as indicating an unfinished statement::
+	"
+	" match a {
+	"     b => c,
+	"         d => e,
+	"         f => g,
+	" };
+
+	" Search backwards for the previous non-empty line.
+	let prevline = s:get_line_trimmed(prevnonblank(a:lnum - 1))
+	if prevline[len(prevline) - 1] == ","
+				\ && s:get_line_trimmed(a:lnum) !~ "^\\s*[\\[\\]{}]"
+		" Oh ho! The previous line ended in a comma! I bet cindent will try to
+		" take this too far... For now, let's use the previous line's indent
+		return GetRustIndent(a:lnum - 1)
+	endif
+
+	" cindent doesn't do the module scope well at all; e.g.::
+	"
+	" static FOO : &'static [bool] = [
+	" true,
+	"     false,
+	"     false,
+	"     true,
+	"     ];
+	"
+	"     uh oh, next statement is indented further!
+
+	" Note that this does *not* apply the line continuation pattern properly;
+	" that's too hard to do correctly for my liking at present, so I'll just
+	" start with these two main cases (square brackets and not returning to
+	" column zero)
+
+	let line = getline(a:lnum)
+	call cursor(a:lnum, 1)
+	if searchpair('{\|(', '', '}\|)', 'nbW') == 0
+		if searchpair('\[', '', '\]', 'nbW') == 0
+			" Global scope, should be zero
+			return 0
+		else
+			" At the module scope, inside square brackets only
+			"if getline(a:lnum)[0] == ']' || search('\[', '', '\]', 'nW') == a:lnum
+			if line =~ "^\\s*]"
+				" It's the closing line, dedent it
+				return 0
+			else
+				return &shiftwidth
+			endif
+		endif
+	endif
+
+	" Fall back on cindent, which does it mostly right
+	return cindent(a:lnum)
+endfunction