Strip out function implementation when documenting.

This prevents compilation failure we want to document a platform-specific
module. Every function is replaced by `loop {}` using the same construct
as `--unpretty everybody_loops`.

Note also a workaround to #43636 is included: `const fn` will retain their
bodies, since the standard library has quite a number of them.

src/librustc_driver/pretty.rs

@@ -44,6 +44,7 @@
 use std::option;
 use std::path::Path;
 use std::str::FromStr;
+use std::mem;
 
 use rustc::hir::map as hir_map;
 use rustc::hir::map::blocks;
@@ -618,52 +619,53 @@ fn to_one_node_id(self, user_option: &str, sess: &Session, map: &hir_map::Map) -
     }
 }
 
-struct ReplaceBodyWithLoop {
+// Note: Also used by librustdoc, see PR #43348. Consider moving this struct elsewhere.
+pub struct ReplaceBodyWithLoop {
     within_static_or_const: bool,
 }
 
 impl ReplaceBodyWithLoop {
-    fn new() -> ReplaceBodyWithLoop {
+    pub fn new() -> ReplaceBodyWithLoop {
         ReplaceBodyWithLoop { within_static_or_const: false }
     }
+
+    fn run<R, F: FnOnce(&mut Self) -> R>(&mut self, is_const: bool, action: F) -> R {
+        let old_const = mem::replace(&mut self.within_static_or_const, is_const);
+        let ret = action(self);
+        self.within_static_or_const = old_const;
+        ret
+    }
 }
 
 impl fold::Folder for ReplaceBodyWithLoop {
     fn fold_item_kind(&mut self, i: ast::ItemKind) -> ast::ItemKind {
-        match i {
-            ast::ItemKind::Static(..) |
-            ast::ItemKind::Const(..) => {
-                self.within_static_or_const = true;
-                let ret = fold::noop_fold_item_kind(i, self);
-                self.within_static_or_const = false;
-                return ret;
-            }
-            _ => fold::noop_fold_item_kind(i, self),
-        }
+        let is_const = match i {
+            ast::ItemKind::Static(..) | ast::ItemKind::Const(..) => true,
+            ast::ItemKind::Fn(_, _, ref constness, _, _, _) =>
+                constness.node == ast::Constness::Const,
+            _ => false,
+        };
+        self.run(is_const, |s| fold::noop_fold_item_kind(i, s))
     }
 
     fn fold_trait_item(&mut self, i: ast::TraitItem) -> SmallVector<ast::TraitItem> {
-        match i.node {
-            ast::TraitItemKind::Const(..) => {
-                self.within_static_or_const = true;
-                let ret = fold::noop_fold_trait_item(i, self);
-                self.within_static_or_const = false;
-                return ret;
-            }
-            _ => fold::noop_fold_trait_item(i, self),
-        }
+        let is_const = match i.node {
+            ast::TraitItemKind::Const(..) => true,
+            ast::TraitItemKind::Method(ast::MethodSig { ref constness, .. }, _) =>
+                constness.node == ast::Constness::Const,
+            _ => false,
+        };
+        self.run(is_const, |s| fold::noop_fold_trait_item(i, s))
     }
 
     fn fold_impl_item(&mut self, i: ast::ImplItem) -> SmallVector<ast::ImplItem> {
-        match i.node {
-            ast::ImplItemKind::Const(..) => {
-                self.within_static_or_const = true;
-                let ret = fold::noop_fold_impl_item(i, self);
-                self.within_static_or_const = false;
-                return ret;
-            }
-            _ => fold::noop_fold_impl_item(i, self),
-        }
+        let is_const = match i.node {
+            ast::ImplItemKind::Const(..) => true,
+            ast::ImplItemKind::Method(ast::MethodSig { ref constness, .. }, _) =>
+                constness.node == ast::Constness::Const,
+            _ => false,
+        };
+        self.run(is_const, |s| fold::noop_fold_impl_item(i, s))
     }
 
     fn fold_block(&mut self, b: P<ast::Block>) -> P<ast::Block> {

src/librustdoc/core.rs

@@ -10,6 +10,7 @@
 
 use rustc_lint;
 use rustc_driver::{driver, target_features, abort_on_err};
+use rustc_driver::pretty::ReplaceBodyWithLoop;
 use rustc::dep_graph::DepGraph;
 use rustc::session::{self, config};
 use rustc::hir::def_id::DefId;
@@ -26,6 +27,7 @@
 
 use syntax::{ast, codemap};
 use syntax::feature_gate::UnstableFeatures;
+use syntax::fold::Folder;
 use errors;
 use errors::emitter::ColorConfig;
 
@@ -158,6 +160,7 @@ pub fn run_core(search_paths: SearchPaths,
     let krate = panictry!(driver::phase_1_parse_input(&driver::CompileController::basic(),
                                                       &sess,
                                                       &input));
+    let krate = ReplaceBodyWithLoop::new().fold_crate(krate);
 
     let name = link::find_crate_name(Some(&sess), &krate.attrs, &input);
 

src/librustdoc/test.rs

@@ -32,6 +32,7 @@
 use rustc_back::tempdir::TempDir;
 use rustc_driver::{self, driver, Compilation};
 use rustc_driver::driver::phase_2_configure_and_expand;
+use rustc_driver::pretty::ReplaceBodyWithLoop;
 use rustc_metadata::cstore::CStore;
 use rustc_resolve::MakeGlobMap;
 use rustc_trans;
@@ -39,6 +40,7 @@
 use syntax::ast;
 use syntax::codemap::CodeMap;
 use syntax::feature_gate::UnstableFeatures;
+use syntax::fold::Folder;
 use syntax_pos::{BytePos, DUMMY_SP, Pos, Span};
 use errors;
 use errors::emitter::ColorConfig;
@@ -72,6 +74,7 @@ pub fn run(input: &str,
         crate_types: vec![config::CrateTypeDylib],
         externs: externs.clone(),
         unstable_features: UnstableFeatures::from_environment(),
+        lint_cap: Some(::rustc::lint::Level::Allow),
         actually_rustdoc: true,
         ..config::basic_options().clone()
     };
@@ -94,6 +97,7 @@ pub fn run(input: &str,
     let krate = panictry!(driver::phase_1_parse_input(&driver::CompileController::basic(),
                                                       &sess,
                                                       &input));
+    let krate = ReplaceBodyWithLoop::new().fold_crate(krate);
     let driver::ExpansionResult { defs, mut hir_forest, .. } = {
         phase_2_configure_and_expand(
             &sess, &cstore, krate, None, "rustdoc-test", None, MakeGlobMap::No, |_| Ok(())