Fix strformat precision handling for strings (#7941)

* fix strformat precision handling for strings

* add some limited unicode awareness to the precision handling for strings

* improvement suggested by Varriount: use setLen and runeOffset instead of runeSubstr

Author: skilchen

lib/pure/strformat.nim

@@ -558,12 +558,16 @@ proc format*(value: string; specifier: string; res: var string) =
   ## sense to call this directly, but it is required to exist
   ## by the ``&`` macro.
   let spec = parseStandardFormatSpecifier(specifier)
+  var value = value
   case spec.typ
   of 's', '\0': discard
   else:
     raise newException(ValueError,
       "invalid type in format string for string, expected 's', but got " &
       spec.typ)
+  if spec.precision != -1:
+    if spec.precision < runelen(value):
+      setLen(value, runeOffset(value, spec.precision))
   res.add alignString(value, spec.minimumWidth, spec.align, spec.fill)
 
 when isMainModule:

tests/stdlib/tstrformat.nim

@@ -12,6 +12,32 @@ var o: Obj
 doAssert fmt"{o}" == "foobar"
 doAssert fmt"{o:10}" == "foobar    "
 
+# see issue #7933
+var str = "abc"
+doAssert fmt">7.1 :: {str:>7.1}" == ">7.1 ::       a"
+doAssert fmt">7.2 :: {str:>7.2}" == ">7.2 ::      ab"
+doAssert fmt">7.3 :: {str:>7.3}" == ">7.3 ::     abc"
+doAssert fmt">7.9 :: {str:>7.9}" == ">7.9 ::     abc"
+doAssert fmt">7.0 :: {str:>7.0}" == ">7.0 ::        "
+doAssert fmt" 7.1 :: {str:7.1}" == " 7.1 :: a      "
+doAssert fmt" 7.2 :: {str:7.2}" == " 7.2 :: ab     "
+doAssert fmt" 7.3 :: {str:7.3}" == " 7.3 :: abc    "
+doAssert fmt" 7.9 :: {str:7.9}" == " 7.9 :: abc    "
+doAssert fmt" 7.0 :: {str:7.0}" == " 7.0 ::        "
+doAssert fmt"^7.1 :: {str:^7.1}" == "^7.1 ::    a   "
+doAssert fmt"^7.2 :: {str:^7.2}" == "^7.2 ::   ab   "
+doAssert fmt"^7.3 :: {str:^7.3}" == "^7.3 ::   abc  "
+doAssert fmt"^7.9 :: {str:^7.9}" == "^7.9 ::   abc  "
+doAssert fmt"^7.0 :: {str:^7.0}" == "^7.0 ::        "
+str = "äöüe\u0309\u0319o\u0307\u0359"
+doAssert fmt"^7.1 :: {str:^7.1}" == "^7.1 ::    ä   "
+doAssert fmt"^7.2 :: {str:^7.2}" == "^7.2 ::   äö   "
+doAssert fmt"^7.3 :: {str:^7.3}" == "^7.3 ::   äöü  "
+doAssert fmt"^7.0 :: {str:^7.0}" == "^7.0 ::        "
+# this is actually wrong, but the unicode module has no support for graphemes
+doAssert fmt"^7.4 :: {str:^7.4}" == "^7.4 ::  äöüe  "
+doAssert fmt"^7.9 :: {str:^7.9}" == "^7.9 :: äöüe\u0309\u0319o\u0307\u0359"
+
 # see issue #7932
 doAssert fmt"{15:08}" == "00000015" # int, works
 doAssert fmt"{1.5:08}" == "000001.5" # float, works