The question is how to tell which code is unsafe. In some languages it is easier, in some languages it's much harder.
For example in C and in C++, every line is potentially unsafe, so you have to verify the entire program very, very carefully.
On the other hand Java is safe, unless you mess with JNI. This is a very infrequently used feature, so for most programs you can be 100% sure there are no certain class of safety issues.
Rust has specific "unsafe" keyword - if you see it in a program, you have to be very careful. You can write quite a lot of Rust without ever using it.
For example in C and in C++, every line is potentially unsafe, so you have to verify the entire program very, very carefully.
On the other hand Java is safe, unless you mess with JNI. This is a very infrequently used feature, so for most programs you can be 100% sure there are no certain class of safety issues.
Rust has specific "unsafe" keyword - if you see it in a program, you have to be very careful. You can write quite a lot of Rust without ever using it.