[llvm] [MC] AsmLexer assert buffer is null-terminated at CurBuf.end() (PR #154972)

[Fangrui Song via llvm-commits]

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@@ -120,6 +120,11 @@ AsmLexer::AsmLexer(const MCAsmInfo &MAI) : MAI(MAI) {
 
 void AsmLexer::setBuffer(StringRef Buf, const char *ptr,
                          bool EndStatementAtEOF) {
+  // Null terminator must be part of the actual buffer. It must reside at
+  // `Buf.end()`. It must be safe to dereference `Buf.end()`.
+  assert(*Buf.end() == '\0' &&
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MaskRay wrote:

I think it's right to require a NUL terminator, like regular `MemoryBuffer` APIs provide (`getFileOrSTDIN` `MemoryBuffer::getFile`.

We should add a comment to the header file `llvm/include/llvm/MC/MCParser/AsmLexer.h` about this requirement.

https://github.com/llvm/llvm-project/pull/154972