Add --preserve-unused-functions (#291)

* Add --preserve-unused-functions

to keep otherwise pruned static / inline functions.

Closes: #287

* transpile: Reduce code duplication

* prune_unused_decls: Rename to unwanted

The criterion for whether it is desired for an item to be transpiled
have been broadened to possibly preserve unused functions as well. Thus,
the "unused" name part has been replaced with "unwanted", which
traditionally means unused but, in other configurations, can mean
"unused but not a function" now as well.

Author: chrysn

c2rust-transpile/src/c_ast/mod.rs

@@ -519,52 +519,53 @@ impl TypedAstContext {
         }
     }
 
-    pub fn prune_unused_decls(&mut self) {
+    pub fn prune_unwanted_decls(&mut self, want_unused_functions: bool) {
         // Starting from a set of root declarations, walk each one to find declarations it
         // depends on. Then walk each of those, recursively.
 
-        // Declarations we still need to walk.  Everything in here is also in `used`.
+        // Declarations we still need to walk.  Everything in here is also in `wanted`.
         let mut to_walk: Vec<CDeclId> = Vec::new();
         // Declarations accessible from a root.
-        let mut used: HashSet<CDeclId> = HashSet::new();
+        let mut wanted: HashSet<CDeclId> = HashSet::new();
 
-        // Mark all the roots as used.  Roots are all top-level functions and variables that might
+        // Mark all the roots as wanted.  Roots are all top-level functions and variables that might
         // be visible from another compilation unit.
+        //
+        // In addition, mark any other (unused) function wanted if configured.
         for &decl_id in &self.c_decls_top {
             let decl = self.index(decl_id);
-            match decl.kind {
+            let is_wanted = match decl.kind {
                 CDeclKind::Function {
                     body: Some(_),
                     is_global: true,
                     is_inline,
                     is_inline_externally_visible,
                     ..
-                } if !is_inline || is_inline_externally_visible => {
                     // Depending on the C specification and dialect, an inlined function
                     // may be externally visible. We rely on clang to determine visibility.
-                    to_walk.push(decl_id);
-                    used.insert(decl_id);
-                }
+                } if !is_inline || is_inline_externally_visible => true,
+                CDeclKind::Function {
+                    body: Some(_),
+                    ..
+                } if want_unused_functions => true,
                 CDeclKind::Variable {
                     is_defn: true,
                     is_externally_visible: true,
                     ..
-                } => {
-                    to_walk.push(decl_id);
-                    used.insert(decl_id);
-                }
+                } => true,
                 CDeclKind::Variable { ref attrs, .. } | CDeclKind::Function { ref attrs, .. }
-                    if attrs.contains(&Attribute::Used) =>
-                {
-                    to_walk.push(decl_id);
-                    used.insert(decl_id);
-                }
-                _ => {}
+                    if attrs.contains(&Attribute::Used) => true,
+                _ => false,
+            };
+
+            if is_wanted {
+                to_walk.push(decl_id);
+                wanted.insert(decl_id);
             }
         }
 
-        // Add all referenced macros to the set of used decls
-        // used.extend(self.macro_expansions.values().flatten());
+        // Add all referenced macros to the set of wanted decls
+        // wanted.extend(self.macro_expansions.values().flatten());
 
         while let Some(enclosing_decl_id) = to_walk.pop() {
             for some_id in DFNodes::new(self, SomeId::Decl(enclosing_decl_id)) {
@@ -573,13 +574,13 @@ impl TypedAstContext {
                         match self.c_types[&type_id].kind {
                             // This is a reference to a previously declared type.  If we look
                             // through it we should(?) get something that looks like a declaration,
-                            // which we can mark as used.
+                            // which we can mark as wanted.
                             CTypeKind::Elaborated(decl_type_id) => {
                                 let decl_id = self.c_types[&decl_type_id]
                                     .kind
                                     .as_decl_or_typedef()
                                     .expect("target of CTypeKind::Elaborated isn't a decl?");
-                                if used.insert(decl_id) {
+                                if wanted.insert(decl_id) {
                                     to_walk.push(decl_id);
                                 }
                             }
@@ -594,20 +595,20 @@ impl TypedAstContext {
                         let expr = self.index(expr_id);
                         if let Some(macs) = self.macro_invocations.get(&expr_id) {
                             for mac_id in macs {
-                                if used.insert(*mac_id) {
+                                if wanted.insert(*mac_id) {
                                     to_walk.push(*mac_id);
                                 }
                             }
                         }
                         if let CExprKind::DeclRef(_, decl_id, _) = &expr.kind {
-                            if used.insert(*decl_id) {
+                            if wanted.insert(*decl_id) {
                                 to_walk.push(*decl_id);
                             }
                         }
                     }
 
                     SomeId::Decl(decl_id) => {
-                        if used.insert(decl_id) {
+                        if wanted.insert(decl_id) {
                             to_walk.push(decl_id);
                         }
 
@@ -616,7 +617,7 @@ impl TypedAstContext {
                                 // Special case for enums.  The enum constant is used, so the whole
                                 // enum is also used.
                                 let parent_id = self.parents[&decl_id];
-                                if used.insert(parent_id) {
+                                if wanted.insert(parent_id) {
                                     to_walk.push(parent_id);
                                 }
                             }
@@ -633,17 +634,17 @@ impl TypedAstContext {
 
         // Unset c_main if we are not retaining its declaration
         if let Some(main_id) = self.c_main {
-            if !used.contains(&main_id) {
+            if !wanted.contains(&main_id) {
                 self.c_main = None;
             }
         }
 
         // Prune any declaration that isn't considered live
         self.c_decls
-            .retain(|&decl_id, _decl| used.contains(&decl_id));
+            .retain(|&decl_id, _decl| wanted.contains(&decl_id));
 
         // Prune top declarations that are not considered live
-        self.c_decls_top.retain(|x| used.contains(x));
+        self.c_decls_top.retain(|x| wanted.contains(x));
     }
 
     pub fn sort_top_decls(&mut self) {

c2rust-transpile/src/lib.rs

@@ -109,6 +109,7 @@ pub struct TranspilerConfig {
     pub translate_const_macros: bool,
     pub translate_fn_macros: bool,
     pub disable_refactoring: bool,
+    pub preserve_unused_functions: bool,
     pub log_level: log::LevelFilter,
 
     // Options that control build files

c2rust-transpile/src/translator/mod.rs

@@ -530,7 +530,7 @@ pub fn translate(
 
         // Headers often pull in declarations that are unused;
         // we simplify the translator output by omitting those.
-        t.ast_context.prune_unused_decls();
+        t.ast_context.prune_unwanted_decls(tcfg.preserve_unused_functions);
 
         enum Name<'a> {
             VarName(&'a str),

c2rust/src/bin/c2rust-transpile.rs

@@ -89,6 +89,7 @@ fn main() {
         translate_const_macros: matches.is_present("translate-const-macros"),
         translate_fn_macros: matches.is_present("translate-fn-macros"),
         disable_refactoring: matches.is_present("disable-refactoring"),
+        preserve_unused_functions: matches.is_present("preserve-unused-functions"),
 
         use_c_loop_info: !matches.is_present("ignore-c-loop-info"),
         use_c_multiple_info: !matches.is_present("ignore-c-multiple-info"),

c2rust/src/transpile.yaml

@@ -172,6 +172,10 @@ args:
       long: disable-refactoring
       help: Disable running refactoring tool after translation
       takes_value: false
+  - preserve-unused-functions:
+      long: preserve-unused-functions
+      help: Include static and inline functions in translation
+      takes_value: false
   - log-level:
       long: log-level
       help: Logging level