fixes #25074; Long integer literal truncated without warning

compiler/lexer.nim

@@ -491,6 +491,25 @@ proc getNumber(L: var Lexer, result: var Token) =
           setNumber result.fNumber, (cast[ptr float64](addr(xi)))[]
         else: internalError(L.config, getLineInfo(L), "getNumber")
 
+        # Check bit width for non-base-10 literals
+        # Warn if the digit count exceeds what can fit in the target type
+        if result.base != base10 and result.tokType in {tkIntLit..tkUInt64Lit} and numDigits > 0:
+          let bitsPerDigit = case result.base
+            of base2: 1
+            of base8: 3
+            of base16: 4
+            else: raiseAssert "unreachable"
+          let bitWidth = case result.tokType
+            of tkInt8Lit, tkUInt8Lit: 8
+            of tkInt16Lit, tkUInt16Lit: 16
+            of tkInt32Lit, tkUInt32Lit: 32
+            of tkInt64Lit, tkUIntLit, tkIntLit, tkUInt64Lit: 64
+            else: raiseAssert "unreachable"
+          # Maximum digits = ceil(bitWidth / bitsPerDigit) = (bitWidth + bitsPerDigit - 1) div bitsPerDigit
+          let maxDigits = (bitWidth + bitsPerDigit - 1) div bitsPerDigit
+          if numDigits > maxDigits:
+            lexMessageLitNum(L, "number has $1 digits but type only supports $2 digits: '$3'" % [$numDigits, $maxDigits, result.literal], startpos, warnLongLiterals)
+
         # Bounds checks. Non decimal literals are allowed to overflow the range of
         # the datatype as long as their pattern don't overflow _bitwise_, hence
         # below checks of signed sizes against uint*.high is deliberate:

compiler/lineinfos.nim

@@ -93,8 +93,9 @@ type
     warnBareExcept = "BareExcept",
     warnImplicitDefaultValue = "ImplicitDefaultValue",
     warnIgnoredSymbolInjection = "IgnoredSymbolInjection",
-    warnStdPrefix = "StdPrefix"
-    warnUnknownNotes = "UnknownNotes"
+    warnStdPrefix = "StdPrefix",
+    warnUnknownNotes = "UnknownNotes",
+    warnLongLiterals = "LongLiterals",
     warnUser = "User",
     warnGlobalVarConstructorTemporary = "GlobalVarConstructorTemporary",
     # hints
@@ -202,6 +203,7 @@ const
     warnIgnoredSymbolInjection: "$1",
     warnStdPrefix: "$1 needs the 'std' prefix",
     warnUnknownNotes: "$1",
+    warnLongLiterals: "$1",
     warnUser: "$1",
     warnGlobalVarConstructorTemporary: "global variable '$1' initialization requires a temporary variable",
     hintSuccess: "operation successful: $#",

tests/lexer/t25074.nim

@@ -0,0 +1,6 @@
+discard """
+  matrix: "--warningaserror:longliterals"
+  errormsg: "number has 64 digits but type only supports 16 digits: '0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff' [LongLiterals]"
+"""
+
+echo $sizeof 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff