I'm not an expert as I do not work on these tools but I don't think IDEs can rely solely on ASTs because not all code is in a compilable state. Lots of times things have to be inferred from invalid code. Jetbrains tools for example do a great job at this.
In practice though, getting the AST from the text is a computational task in and of itself and the grammar affects the runtime of that. For instance, Rust's "turbofish" syntax, `f::<T>(args)`, is used to specify the generic type for f when calling it; this is instead of the perhaps more obvious `f<T>(args)`, which is what the definition looks like. Why the extra colons? Because parsing `f<T>(args)` in an expression position would require unbounded lookahead to determine the meaning of the left angle bracket -- is it the beginning of generics or less-than? Therefore, even though Rust could be modified to accept`f<T>(args)` as a valid syntax when calling the function, the language team decided to require the colons in order to improve worst case parser performance.
It's not impossible to handle the ambiguity, it's just that you may have to look arbitrarily far ahead to resolve it. Perhaps C# simply does this. Or perhaps it limits expressions to 2^(large-ish number) bytes.