provide diagnostic details on --mangle-props (#5621)

closes #5618

lib/propmangle.js

@@ -271,7 +271,10 @@ function mangle_properties(ast, options) {
     }
 
     function mangle(name) {
-        if (!should_mangle(name)) return name;
+        if (!should_mangle(name)) {
+            AST_Node.info("Preserving property {this}", name);
+            return name;
+        }
         var mangled = cache.get(name);
         if (!mangled) {
             if (debug) {
@@ -286,6 +289,10 @@ function mangle_properties(ast, options) {
             if (/^#/.test(name)) mangled = "#" + mangled;
             cache.set(name, mangled);
         }
+        AST_Node.info("Mapping property {name} to {mangled}", {
+            mangled: mangled,
+            name: name,
+        });
         return mangled;
     }
 

test/compress.js

@@ -459,6 +459,7 @@ function to_toplevel(input, mangle_options, expression) {
     var tokens = [];
     input.walk(new U.TreeWalker(function(node) {
         if (U.push_uniq(tokens, node.start)) node.start.line -= offset;
+        if (U.push_uniq(tokens, node.end)) node.end.line -= offset;
     }));
     var toplevel;
     if (!expression) {

test/compress/classes.js

@@ -2200,6 +2200,14 @@ mangle_properties: {
         console.log(A.s, new A().e);
     }
     expect_stdout: "PASS 42"
+    expect_warnings: [
+        "INFO: Mapping property #P to #t",
+        "INFO: Mapping property Q to s",
+        "INFO: Mapping property #p to #i",
+        "INFO: Mapping property r to e",
+        "INFO: Preserving property q",
+        "INFO: Preserving property log",
+    ]
     node_version: ">=14.6"
 }