Highlight 1..2 according to the current grammar.

src/etc/vim/syntax/rust.vim

@@ -172,9 +172,10 @@ syn match     rustOctNumber   display "\<0o[0-7_]\+\%([iu]\%(8\|16\|32\|64\)\=\)
 syn match     rustBinNumber   display "\<0b[01_]\+\%([iu]\%(8\|16\|32\|64\)\=\)\="
 
 " Special case for numbers of the form "1." which are float literals, unless followed by
-" an identifier, which makes them integer literals with a method call or field access.
+" an identifier, which makes them integer literals with a method call or field access,
+" or by another ".", which makes them integer literals followed by the ".." token.
 " (This must go first so the others take precedence.)
-syn match     rustFloat       display "\<[0-9][0-9_]*\.\%([^[:cntrl:][:space:][:punct:][:digit:]]\|_\)\@!"
+syn match     rustFloat       display "\<[0-9][0-9_]*\.\%([^[:cntrl:][:space:][:punct:][:digit:]]\|_\|\.\)\@!"
 " To mark a number as a normal float, it must have at least one of the three things integral values don't have:
 " a decimal point and more numbers; an exponent; and a type suffix.
 syn match     rustFloat       display "\<[0-9][0-9_]*\%(\.[0-9][0-9_]*\)\%([eE][+-]\=[0-9_]\+\)\=\(f32\|f64\)\="