Pass noReturn pragma to C code.

With GCC and Clang this generates __attribute__((noreturn)) in the
function declaration. (both tested) With VCC __declspec(noreturn) is
used.

compiler/cgen.nim

@@ -676,6 +676,9 @@ proc genProcAux(m: BModule, prc: PSym) =
   closureSetup(p, prc)
   genStmts(p, prc.getBody) # modifies p.locals, p.init, etc.
   var generatedProc: Rope
+  if sfNoReturn in prc.flags:
+    if hasNoreturnDeclspec in extccomp.CC[extccomp.cCompiler].props:
+      header = "__declspec(noreturn) " & header
   if sfPure in prc.flags:
     if hasNakedDeclspec in extccomp.CC[extccomp.cCompiler].props:
       header = "__declspec(naked) " & header
@@ -722,6 +725,8 @@ proc genProcPrototype(m: BModule, sym: PSym) =
       header = "extern \"C\" " & header
     if sfPure in sym.flags and hasNakedAttribute in CC[cCompiler].props:
       header.add(" __attribute__((naked))")
+    if sfNoReturn in sym.flags and hasNoreturnAttribute in CC[cCompiler].props:
+      header.add(" __attribute__((noreturn))")
     add(m.s[cfsProcHeaders], rfmt(nil, "$1;$n", header))
 
 proc genProcNoForward(m: BModule, prc: PSym) =

compiler/extccomp.nim

@@ -27,7 +27,9 @@ type
     hasGcGuard,               # CC supports GC_GUARD to keep stack roots
     hasGnuAsm,                # CC's asm uses the absurd GNU assembler syntax
     hasNakedDeclspec,         # CC has __declspec(naked)
-    hasNakedAttribute         # CC has __attribute__((naked))
+    hasNakedAttribute,        # CC has __attribute__((naked))
+    hasNoreturnAttribute,     # CC has __attribute__((noreturn))
+    hasNoreturnDeclspec       # CC has __declspec(noreturn)
   TInfoCCProps* = set[TInfoCCProp]
   TInfoCC* = tuple[
     name: string,        # the short name of the compiler
@@ -85,7 +87,7 @@ compiler gcc:
     structStmtFmt: "$1 $3 $2 ", # struct|union [packed] $name
     packedPragma: "__attribute__((__packed__))",
     props: {hasSwitchRange, hasComputedGoto, hasCpp, hasGcGuard, hasGnuAsm,
-            hasNakedAttribute})
+            hasNakedAttribute, hasNoreturnAttribute})
 
 # LLVM Frontend for GCC/G++
 compiler llvmGcc:
@@ -127,7 +129,7 @@ compiler vcc:
     asmStmtFrmt: "__asm{$n$1$n}$n",
     structStmtFmt: "$3$n$1 $2",
     packedPragma: "#pragma pack(1)",
-    props: {hasCpp, hasAssume, hasNakedDeclspec})
+    props: {hasCpp, hasAssume, hasNakedDeclspec, hasNoreturnDeclspec})
 
 # Intel C/C++ Compiler
 compiler icl: