Additionally, when inlining methods defined in macros, self is not replaced.

struct Test;

macro_rules! define_method {
    () => {
        impl Test {
            fn test_method_macro(&self) {
                match self {
                    Test => println!("Test!")
                }
            }
        }
    };
}
define_method!();
fn main() {
    Test.test_method_macro();
}

becomes

// ...
fn main() {
    {
        let ref this = Test; matchself{Test=>println!("Test!")}};
}

when inlining Test.test_method_macro()

I inserted a dbg!(&body) at the beginning of fn inline on line 305 of crates/ide-assists/src/handlers/inline_call.rs, and the main difference between inlining fn test_free and fn test_macro appears to be that fn test_free contained WHITESPACE tokens, but fn test_macro did not.

My (limited, possibly wrong) understanding of what is happening (for the whitespace problem, no idea about self not being replaced¹) is that rust-analyzer converts it's AST representation from SyntaxNodes (which contain comments and whitespace) to TokenTrees (which does not contain comments or whitespace)² when expanding macros, so it may not be possible (without changes to how macros are expanded by rust-analyzer) to include the original whitespace&comments when inlining macro-produced functions.

Maybe it would be easier to just insert whitespace between tokens which cannot be adjacent? This is probably also required for inlining functions defined in proc-macros to work, since in that case there is no "original whitespace" that could even be kept.

Does rust-analyzer have a way to determine if a function was defined in a macro? rustc appears to have this (at least at some level), since if a function defined in a macro contains an error, it will say note: this error originates in the macro `define_func` (in Nightly builds, run with -Z macro-backtrace for more info)

Yes, macro expansions have no whitespace. This is a recurring thing and currently we just check for whether things come from a macro expansion in whicch ase we do whitespace insertion via https://github.com/rust-lang/rust-analyzer/blob/master/crates/ide-db/src/syntax_helpers/insert_whitespace_into_node.rs