Try MIR inlining leaf functions by default

compiler/rustc_session/src/config.rs

@@ -1827,23 +1827,6 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
             }
             Some(SymbolManglingVersion::V0) => {}
         }
-
-        if debugging_opts.mir_opt_level > 1 {
-            // Functions inlined during MIR transform can, at best, make it impossible to
-            // effectively cover inlined functions, and, at worst, break coverage map generation
-            // during LLVM codegen. For example, function counter IDs are only unique within a
-            // function. Inlining after these counters are injected can produce duplicate counters,
-            // resulting in an invalid coverage map (and ICE); so this option combination is not
-            // allowed.
-            early_warn(
-                error_format,
-                &format!(
-                    "`-Z mir-opt-level={}` (or any level > 1) enables function inlining, which \
-                    is incompatible with `-Z instrument-coverage`. Inlining will be disabled.",
-                    debugging_opts.mir_opt_level,
-                ),
-            );
-        }
     }
 
     if let Ok(graphviz_font) = std::env::var("RUSTC_GRAPHVIZ_FONT") {