ast: remove implicit pprust dependency via Display.

Instead just use `pprust::path_to_string(..)` where needed.

This has two benefits:

a) The AST definition is now independent of printing it.
   (Therefore we get closer to extracting a data-crate.)

b) Debugging should be easier as program flow is clearer.

src/librustc/lint/levels.rs

@@ -12,6 +12,7 @@
 use syntax::ast;
 use syntax::attr;
 use syntax::feature_gate;
+use syntax::print::pprust;
 use syntax::source_map::MultiSpan;
 use syntax::symbol::{Symbol, sym};
 
@@ -285,7 +286,7 @@ pub fn push(&mut self, attrs: &[ast::Attribute]) -> BuilderPush {
                             tool_ident.span,
                             E0710,
                             "an unknown tool name found in scoped lint: `{}`",
-                            meta_item.path
+                            pprust::path_to_string(&meta_item.path),
                         );
                         continue;
                     }

src/librustc_lint/builtin.rs

@@ -45,7 +45,7 @@
 use syntax_pos::{BytePos, Span};
 use syntax::symbol::{Symbol, kw, sym};
 use syntax::errors::{Applicability, DiagnosticBuilder};
-use syntax::print::pprust::expr_to_string;
+use syntax::print::pprust::{self, expr_to_string};
 use syntax::visit::FnKind;
 
 use rustc::hir::{self, GenericParamKind, PatKind};
@@ -701,7 +701,8 @@ fn check_attribute(&mut self, cx: &EarlyContext<'_>, attr: &ast::Attribute) {
             }
         }
         if attr.check_name(sym::no_start) || attr.check_name(sym::crate_id) {
-            let msg = format!("use of deprecated attribute `{}`: no longer used.", attr.path);
+            let path_str = pprust::path_to_string(&attr.path);
+            let msg = format!("use of deprecated attribute `{}`: no longer used.", path_str);
             lint_deprecated_attr(cx, attr, &msg, None);
         }
     }

src/librustc_mir/dataflow/mod.rs

 use syntax::ast::{self, MetaItem};
+use syntax::print::pprust;
 use syntax::symbol::{Symbol, sym};
 
 use rustc_index::bit_set::{BitSet, HybridBitSet};
@@ -159,9 +160,8 @@ pub(crate) fn run<P>(
                 if let Some(s) = item.value_str() {
                     return Some(s.to_string())
                 } else {
-                    sess.span_err(
-                        item.span,
-                        &format!("{} attribute requires a path", item.path));
+                    let path = pprust::path_to_string(&item.path);
+                    sess.span_err(item.span, &format!("{} attribute requires a path", path));
                     return None;
                 }
             }

src/librustc_passes/ast_validation.rs

@@ -263,7 +263,8 @@ fn no_questions_in_bounds(&self, bounds: &GenericBounds, where_: &str, is_trait:
                 let mut err = self.err_handler().struct_span_err(poly.span,
                     &format!("`?Trait` is not permitted in {}", where_));
                 if is_trait {
-                    err.note(&format!("traits are `?{}` by default", poly.trait_ref.path));
+                    let path_str = pprust::path_to_string(&poly.trait_ref.path);
+                    err.note(&format!("traits are `?{}` by default", path_str));
                 }
                 err.emit();
             }

src/librustc_resolve/build_reduced_graph.rs

@@ -37,6 +37,7 @@
 use syntax::ext::hygiene::ExpnId;
 use syntax::feature_gate::is_builtin_attr;
 use syntax::parse::token::{self, Token};
+use syntax::print::pprust;
 use syntax::{span_err, struct_span_err};
 use syntax::source_map::{respan, Spanned};
 use syntax::symbol::{kw, sym};
@@ -228,7 +229,7 @@ fn resolve_visibility(&mut self, vis: &ast::Visibility) -> ty::Visibility {
                         .span_suggestion(
                             path.span,
                             "try",
-                            format!("crate::{}", path),
+                            format!("crate::{}", pprust::path_to_string(&path)),
                             Applicability::MaybeIncorrect,
                         )
                         .emit();

src/librustc_resolve/lib.rs

@@ -38,6 +38,7 @@
 use syntax::ext::hygiene::{ExpnId, Transparency, SyntaxContext};
 use syntax::ast::{self, Name, NodeId, Ident, FloatTy, IntTy, UintTy};
 use syntax::ext::base::{SyntaxExtension, MacroKind, SpecialDerives};
+use syntax::print::pprust;
 use syntax::symbol::{kw, sym};
 
 use syntax::visit::{self, Visitor};
@@ -2011,13 +2012,13 @@ fn resolve_path_with_ribs(
                         let mut candidates =
                             self.lookup_import_candidates(ident, TypeNS, is_mod);
                         candidates.sort_by_cached_key(|c| {
-                            (c.path.segments.len(), c.path.to_string())
+                            (c.path.segments.len(), pprust::path_to_string(&c.path))
                         });
                         if let Some(candidate) = candidates.get(0) {
                             (
                                 String::from("unresolved import"),
                                 Some((
-                                    vec![(ident.span, candidate.path.to_string())],
+                                    vec![(ident.span, pprust::path_to_string(&candidate.path))],
                                     String::from("a similar path exists"),
                                     Applicability::MaybeIncorrect,
                                 )),

src/librustc_resolve/macros.rs

@@ -21,6 +21,7 @@
 use syntax::ext::compile_declarative_macro;
 use syntax::feature_gate::{emit_feature_err, is_builtin_attr_name};
 use syntax::feature_gate::GateIssue;
+use syntax::print::pprust;
 use syntax::symbol::{Symbol, kw, sym};
 use syntax_pos::{Span, DUMMY_SP};
 
@@ -324,7 +325,8 @@ fn smart_resolve_macro_path(
 
         Ok(if ext.macro_kind() != kind {
             let expected = kind.descr_expected();
-            let msg = format!("expected {}, found {} `{}`", expected, res.descr(), path);
+            let path_str = pprust::path_to_string(path);
+            let msg = format!("expected {}, found {} `{}`", expected, res.descr(), path_str);
             self.session.struct_span_err(path.span, &msg)
                         .span_label(path.span, format!("not {} {}", kind.article(), expected))
                         .emit();
@@ -805,14 +807,16 @@ fn check_stability_and_deprecation(&self, ext: &SyntaxExtension, path: &ast::Pat
                 }
             }
             if let Some(depr) = &stability.rustc_depr {
-                let (message, lint) = stability::rustc_deprecation_message(depr, &path.to_string());
+                let path = pprust::path_to_string(path);
+                let (message, lint) = stability::rustc_deprecation_message(depr, &path);
                 stability::early_report_deprecation(
                     self.session, &message, depr.suggestion, lint, span
                 );
             }
         }
         if let Some(depr) = &ext.deprecation {
-            let (message, lint) = stability::deprecation_message(depr, &path.to_string());
+            let path = pprust::path_to_string(&path);
+            let (message, lint) = stability::deprecation_message(depr, &path);
             stability::early_report_deprecation(self.session, &message, None, lint, span);
         }
     }

src/librustdoc/html/render.rs

@@ -46,6 +46,7 @@
 use syntax::ast;
 use syntax::edition::Edition;
 use syntax::ext::base::MacroKind;
+use syntax::print::pprust;
 use syntax::source_map::FileName;
 use syntax::feature_gate::UnstableFeatures;
 use syntax::symbol::{Symbol, sym};
@@ -2957,7 +2958,7 @@ fn item_enum(w: &mut Buffer, cx: &Context, it: &clean::Item, e: &clean::Enum) {
 }
 
 fn render_attribute(attr: &ast::MetaItem) -> Option<String> {
-    let path = attr.path.to_string();
+    let path = pprust::path_to_string(&attr.path);
 
     if attr.is_word() {
         Some(path)

src/libsyntax/ast.rs

@@ -7,7 +7,6 @@
 
 use crate::ext::hygiene::ExpnId;
 use crate::parse::token::{self, DelimToken};
-use crate::print::pprust;
 use crate::ptr::P;
 use crate::source_map::{dummy_spanned, respan, Spanned};
 use crate::symbol::{kw, sym, Symbol};
@@ -86,12 +85,6 @@ fn eq(&self, symbol: &Symbol) -> bool {
     }
 }
 
-impl fmt::Display for Path {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", pprust::path_to_string(self))
-    }
-}
-
 impl Path {
     // Convert a span and an identifier to the corresponding
     // one-segment path.

src/libsyntax/attr/builtin.rs

@@ -5,6 +5,7 @@
 use crate::ext::base::ExtCtxt;
 use crate::feature_gate::{Features, GatedCfg};
 use crate::parse::ParseSess;
+use crate::print::pprust;
 
 use errors::{Applicability, Handler};
 use syntax_pos::hygiene::Transparency;
@@ -243,7 +244,11 @@ fn find_stability_generic<'a, I>(sess: &ParseSess,
             let meta = meta.as_ref().unwrap();
             let get = |meta: &MetaItem, item: &mut Option<Symbol>| {
                 if item.is_some() {
-                    handle_errors(sess, meta.span, AttrError::MultipleItem(meta.path.to_string()));
+                    handle_errors(
+                        sess,
+                        meta.span,
+                        AttrError::MultipleItem(pprust::path_to_string(&meta.path)),
+                    );
                     return false
                 }
                 if let Some(v) = meta.value_str() {
@@ -271,7 +276,10 @@ fn find_stability_generic<'a, I>(sess: &ParseSess,
                                     handle_errors(
                                         sess,
                                         mi.span,
-                                        AttrError::UnknownMetaItem(mi.path.to_string(), expected),
+                                        AttrError::UnknownMetaItem(
+                                            pprust::path_to_string(&mi.path),
+                                            expected,
+                                        ),
                                     );
                                     continue 'outer
                                 }
@@ -362,7 +370,7 @@ fn find_stability_generic<'a, I>(sess: &ParseSess,
                                         sess,
                                         meta.span(),
                                         AttrError::UnknownMetaItem(
-                                            mi.path.to_string(),
+                                            pprust::path_to_string(&mi.path),
                                             &["feature", "reason", "issue", "soft"]
                                         ),
                                     );
@@ -434,7 +442,8 @@ fn find_stability_generic<'a, I>(sess: &ParseSess,
                                             sess,
                                             meta.span(),
                                             AttrError::UnknownMetaItem(
-                                                mi.path.to_string(), &["since", "note"],
+                                                pprust::path_to_string(&mi.path),
+                                                &["since", "note"],
                                             ),
                                         );
                                         continue 'outer
@@ -597,8 +606,11 @@ pub fn eval_condition<F>(cfg: &ast::MetaItem, sess: &ParseSess, eval: &mut F)
                     !eval_condition(mis[0].meta_item().unwrap(), sess, eval)
                 },
                 _ => {
-                    span_err!(sess.span_diagnostic, cfg.span, E0537,
-                              "invalid predicate `{}`", cfg.path);
+                    span_err!(
+                        sess.span_diagnostic, cfg.span, E0537,
+                        "invalid predicate `{}`",
+                        pprust::path_to_string(&cfg.path)
+                    );
                     false
                 }
             }
@@ -653,7 +665,9 @@ fn find_deprecation_generic<'a, I>(sess: &ParseSess,
                 let get = |meta: &MetaItem, item: &mut Option<Symbol>| {
                     if item.is_some() {
                         handle_errors(
-                            sess, meta.span, AttrError::MultipleItem(meta.path.to_string())
+                            sess,
+                            meta.span,
+                            AttrError::MultipleItem(pprust::path_to_string(&meta.path)),
                         );
                         return false
                     }
@@ -691,8 +705,10 @@ fn find_deprecation_generic<'a, I>(sess: &ParseSess,
                                     handle_errors(
                                         sess,
                                         meta.span(),
-                                        AttrError::UnknownMetaItem(mi.path.to_string(),
-                                                                   &["since", "note"]),
+                                        AttrError::UnknownMetaItem(
+                                            pprust::path_to_string(&mi.path),
+                                            &["since", "note"],
+                                        ),
                                     );
                                     continue 'outer
                                 }

src/libsyntax/ext/expand.rs

@@ -13,6 +13,7 @@
 use crate::parse::{DirectoryOwnership, PResult, ParseSess};
 use crate::parse::token;
 use crate::parse::parser::Parser;
+use crate::print::pprust;
 use crate::ptr::P;
 use crate::symbol::{sym, Symbol};
 use crate::tokenstream::{TokenStream, TokenTree};
@@ -388,7 +389,8 @@ pub fn fully_expand_fragment(&mut self, input_fragment: AstFragment) -> AstFragm
                             "`derive` may only be applied to structs, enums and unions");
                         if let ast::AttrStyle::Inner = attr.style {
                             let trait_list = derives.iter()
-                                .map(|t| t.to_string()).collect::<Vec<_>>();
+                                .map(|t| pprust::path_to_string(t))
+                                .collect::<Vec<_>>();
                             let suggestion = format!("#[derive({})]", trait_list.join(", "));
                             err.span_suggestion(
                                 span, "try an outer attribute", suggestion,
@@ -587,8 +589,11 @@ fn expand_invoc(&mut self, invoc: Invocation, ext: &SyntaxExtensionKind) -> AstF
                     let result = if let Some(result) = fragment_kind.make_from(tok_result) {
                         result
                     } else {
-                        let msg = format!("non-{kind} macro in {kind} position: {path}",
-                                          kind = fragment_kind.name(), path = mac.path);
+                        let msg = format!(
+                            "non-{kind} macro in {kind} position: {path}",
+                            kind = fragment_kind.name(),
+                            path = pprust::path_to_string(&mac.path),
+                        );
                         self.cx.span_err(span, &msg);
                         self.cx.trace_macros_diag();
                         fragment_kind.dummy(span)
@@ -878,7 +883,7 @@ pub fn ensure_complete_parse(&mut self, macro_path: &Path, kind_name: &str, span
             err.span_label(span, "caused by the macro expansion here");
             let msg = format!(
                 "the usage of `{}!` is likely invalid in {} context",
-                macro_path,
+                pprust::path_to_string(&macro_path),
                 kind_name,
             );
             err.note(&msg);

src/libsyntax/parse/diagnostics.rs

@@ -854,7 +854,7 @@ fn consume_fn_args(&mut self) -> Result<(), ()> {
                 // This is a best-effort recovery.
                 path.span,
                 "try",
-                format!("<{}>::{}", ty_str, path),
+                format!("<{}>::{}", ty_str, pprust::path_to_string(&path)),
                 Applicability::MaybeIncorrect,
             )
             .emit();

src/libsyntax/parse/parser.rs

@@ -1454,12 +1454,14 @@ fn recover_incorrect_vis_restriction(&mut self) -> PResult<'a, ()> {
 `pub(super)`: visible only in the current module's parent
 `pub(in path::to::module)`: visible only on the specified path"##;
 
+        let path_str = pprust::path_to_string(&path);
+
         struct_span_err!(self.sess.span_diagnostic, path.span, E0704, "{}", msg)
             .help(suggestion)
             .span_suggestion(
                 path.span,
-                &format!("make this visible only to module `{}` with `in`", path),
-                format!("in {}", path),
+                &format!("make this visible only to module `{}` with `in`", path_str),
+                format!("in {}", path_str),
                 Applicability::MachineApplicable,
             )
             .emit();

src/libsyntax/parse/parser/expr.rs

@@ -552,8 +552,11 @@ fn parse_assoc_op_cast(&mut self, lhs: P<Expr>, lhs_span: Span,
 
                         // Report non-fatal diagnostics, keep `x as usize` as an expression
                         // in AST and continue parsing.
-                        let msg = format!("`<` is interpreted as a start of generic \
-                                           arguments for `{}`, not a {}", path, op_noun);
+                        let msg = format!(
+                            "`<` is interpreted as a start of generic arguments for `{}`, not a {}",
+                            pprust::path_to_string(&path),
+                            op_noun,
+                        );
                         let span_after_type = parser_snapshot_after_type.token.span;
                         let expr = mk_expr(self, P(Ty {
                             span: path.span,

src/libsyntax_ext/proc_macro_harness.rs

@@ -7,6 +7,7 @@
 use syntax::ext::expand::{AstFragment, ExpansionConfig};
 use syntax::ext::proc_macro::is_proc_macro_attr;
 use syntax::parse::ParseSess;
+use syntax::print::pprust;
 use syntax::ptr::P;
 use syntax::symbol::{kw, sym};
 use syntax::visit::{self, Visitor};
@@ -248,13 +249,20 @@ fn visit_item(&mut self, item: &'a ast::Item) {
         for attr in &item.attrs {
             if is_proc_macro_attr(&attr) {
                 if let Some(prev_attr) = found_attr {
+                    let path_str = pprust::path_to_string(&attr.path);
                     let msg = if attr.path.segments[0].ident.name ==
                                  prev_attr.path.segments[0].ident.name {
-                        format!("only one `#[{}]` attribute is allowed on any given function",
-                                attr.path)
+                        format!(
+                            "only one `#[{}]` attribute is allowed on any given function",
+                            path_str,
+                        )
                     } else {
-                        format!("`#[{}]` and `#[{}]` attributes cannot both be applied \
-                                to the same function", attr.path, prev_attr.path)
+                        format!(
+                            "`#[{}]` and `#[{}]` attributes cannot both be applied
+                            to the same function",
+                            path_str,
+                            pprust::path_to_string(&prev_attr.path),
+                        )
                     };
 
                     self.handler.struct_span_err(attr.span, &msg)
@@ -280,8 +288,10 @@ fn visit_item(&mut self, item: &'a ast::Item) {
         };
 
         if !is_fn {
-            let msg = format!("the `#[{}]` attribute may only be used on bare functions",
-                              attr.path);
+            let msg = format!(
+                "the `#[{}]` attribute may only be used on bare functions",
+                pprust::path_to_string(&attr.path),
+            );
 
             self.handler.span_err(attr.span, &msg);
             return;
@@ -292,8 +302,10 @@ fn visit_item(&mut self, item: &'a ast::Item) {
         }
 
         if !self.is_proc_macro_crate {
-            let msg = format!("the `#[{}]` attribute is only usable with crates of the \
-                              `proc-macro` crate type", attr.path);
+            let msg = format!(
+                "the `#[{}]` attribute is only usable with crates of the `proc-macro` crate type",
+                pprust::path_to_string(&attr.path),
+            );
 
             self.handler.span_err(attr.span, &msg);
             return;