suggestion applicabilities for libsyntax and librustc, run-rustfix tests

Consider this a down payment on rust-lang#50723. To recap, an `Applicability`
enum was recently (rust-lang#50204) added, to convey to Rustfix and other tools
whether we think it's OK for them to blindly apply the suggestion, or
whether to prompt a human for guidance (because the suggestion might
contain placeholders that we can't infer, or because we think it has a
sufficiently high probability of being wrong even though it's—
presumably—right often enough to be worth emitting in the first place).

When a suggestion is marked as `MaybeIncorrect`, we try to use comments
to indicate precisely why (although there are a few places where we just
say `// speculative` because the present author's subjective judgement
balked at the idea that the suggestion has no false positives).

The `run-rustfix` directive is opporunistically set on some relevant UI
tests (and a couple tests that were in the `test/ui/suggestions`
directory, even if the suggestions didn't originate in librustc or
libsyntax). This is less trivial than it sounds, because a surprising
number of test files aren't equipped to be tested as fixed even when
they contain successfully fixable errors, because, e.g., there are more,
not-directly-related errors after fixing. Some test files need an
attribute or underscore to avoid unused warnings tripping up the "fixed
code is still producing diagnostics" check despite the fixes being
correct; this is an interesting contrast-to/inconsistency-with the
behavior of UI tests (which secretly pass `-A unused`), a behavior which
we probably ought to resolve one way or the other (filed issue rust-lang#50926).

A few suggestion labels are reworded (e.g., to avoid phrasing it as a
question, which which is discouraged by the style guidelines listed in
`.span_suggestion`'s doc-comment).

src/librustc/infer/error_reporting/mod.rs

@@ -70,7 +70,7 @@ use ty::{self, Region, Ty, TyCtxt, TypeFoldable, TypeVariants};
 use ty::error::TypeError;
 use syntax::ast::DUMMY_NODE_ID;
 use syntax_pos::{Pos, Span};
-use errors::{DiagnosticBuilder, DiagnosticStyledString};
+use errors::{Applicability, DiagnosticBuilder, DiagnosticStyledString};
 
 use rustc_data_structures::indexed_vec::Idx;
 
@@ -1097,7 +1097,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             if let Some((sp, has_lifetimes)) = type_param_span {
                 let tail = if has_lifetimes { " + " } else { "" };
                 let suggestion = format!("{}: {}{}", bound_kind, sub, tail);
-                err.span_suggestion_short(sp, consider, suggestion);
+                err.span_suggestion_short_with_applicability(
+                    sp, consider, suggestion,
+                    Applicability::MaybeIncorrect // Issue #41966
+                );
             } else {
                 err.help(consider);
             }

src/librustc/lint/levels.rs

@@ -10,7 +10,7 @@
 
 use std::cmp;
 
-use errors::DiagnosticBuilder;
+use errors::{Applicability, DiagnosticBuilder};
 use hir::HirId;
 use ich::StableHashingContext;
 use lint::builtin;
@@ -265,10 +265,11 @@ impl<'a> LintLevelsBuilder<'a> {
                                     store.check_lint_name(&name_lower) {
                                 db.emit();
                             } else {
-                                db.span_suggestion(
+                                db.span_suggestion_with_applicability(
                                     li.span,
                                     "lowercase the lint name",
-                                    name_lower
+                                    name_lower,
+                                    Applicability::MachineApplicable
                                 ).emit();
                             }
                         } else {

src/librustc/middle/liveness.rs

@@ -109,6 +109,7 @@ use self::VarKind::*;
 use hir::def::*;
 use ty::{self, TyCtxt};
 use lint;
+use errors::Applicability;
 use util::nodemap::{NodeMap, NodeSet};
 
 use std::collections::VecDeque;
@@ -1541,11 +1542,15 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                     let mut err = self.ir.tcx
                         .struct_span_lint_node(lint::builtin::UNUSED_VARIABLES, id, sp, &msg);
                     if self.ir.variable_is_shorthand(var) {
-                        err.span_suggestion(sp, "try ignoring the field",
-                                            format!("{}: _", name));
+                        err.span_suggestion_with_applicability(sp, "try ignoring the field",
+                                                               format!("{}: _", name),
+                                                               Applicability::MachineApplicable);
                     } else {
-                        err.span_suggestion_short(sp, &suggest_underscore_msg,
-                                                  format!("_{}", name));
+                        err.span_suggestion_short_with_applicability(
+                            sp, &suggest_underscore_msg,
+                            format!("_{}", name),
+                            Applicability::MachineApplicable,
+                        );
                     }
                     err.emit()
                 }

src/librustc/traits/error_reporting.rs

@@ -27,7 +27,7 @@ use super::{
     Overflow,
 };
 
-use errors::DiagnosticBuilder;
+use errors::{Applicability, DiagnosticBuilder};
 use hir;
 use hir::def_id::DefId;
 use infer::{self, InferCtxt};
@@ -856,9 +856,12 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 if let Some(ref expr) = local.init {
                     if let hir::ExprIndex(_, _) = expr.node {
                         if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(expr.span) {
-                            err.span_suggestion(expr.span,
-                                                "consider borrowing here",
-                                                format!("&{}", snippet));
+                            err.span_suggestion_with_applicability(
+                                expr.span,
+                                "consider borrowing here",
+                                format!("&{}", snippet),
+                                Applicability::MachineApplicable
+                            );
                         }
                     }
                 }
@@ -901,7 +904,9 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                         let format_str = format!("consider removing {} leading `&`-references",
                                                  remove_refs);
 
-                        err.span_suggestion_short(sp, &format_str, String::from(""));
+                        err.span_suggestion_short_with_applicability(
+                            sp, &format_str, String::from(""), Applicability::MachineApplicable
+                        );
                         break;
                     }
                 } else {
@@ -1046,10 +1051,11 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     let sugg = fields.iter()
                         .map(|(name, _)| name.to_owned())
                         .collect::<Vec<String>>().join(", ");
-                    err.span_suggestion(found_span,
-                                        "change the closure to take multiple arguments instead of \
-                                         a single tuple",
-                                        format!("|{}|", sugg));
+                    err.span_suggestion_with_applicability(found_span,
+                                                           "change the closure to take multiple \
+                                                            arguments instead of a single tuple",
+                                                           format!("|{}|", sugg),
+                                                           Applicability::MachineApplicable);
                 }
             }
             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
@@ -1077,10 +1083,13 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                             "".to_owned()
                         },
                     );
-                    err.span_suggestion(found_span,
-                                        "change the closure to accept a tuple instead of \
-                                         individual arguments",
-                                        sugg);
+                    err.span_suggestion_with_applicability(
+                        found_span,
+                        "change the closure to accept a tuple instead of \
+                         individual arguments",
+                        sugg,
+                        Applicability::MachineApplicable
+                    );
                 }
             }
         }

src/libsyntax/attr.rs

@@ -20,7 +20,7 @@ use ast::{MetaItem, MetaItemKind, NestedMetaItem, NestedMetaItemKind};
 use ast::{Lit, LitKind, Expr, ExprKind, Item, Local, Stmt, StmtKind};
 use codemap::{BytePos, Spanned, respan, dummy_spanned};
 use syntax_pos::Span;
-use errors::Handler;
+use errors::{Applicability, Handler};
 use feature_gate::{Features, GatedCfg};
 use parse::lexer::comments::{doc_comment_style, strip_doc_comment_decoration};
 use parse::parser::Parser;
@@ -1067,14 +1067,20 @@ pub fn find_repr_attrs(diagnostic: &Handler, attr: &Attribute) -> Vec<ReprAttr>
                                     "incorrect `repr(align)` attribute format");
                                 match value.node {
                                     ast::LitKind::Int(int, ast::LitIntType::Unsuffixed) => {
-                                        err.span_suggestion(item.span,
-                                                            "use parentheses instead",
-                                                            format!("align({})", int));
+                                        err.span_suggestion_with_applicability(
+                                            item.span,
+                                            "use parentheses instead",
+                                            format!("align({})", int),
+                                            Applicability::MachineApplicable
+                                        );
                                     }
                                     ast::LitKind::Str(s, _) => {
-                                        err.span_suggestion(item.span,
-                                                            "use parentheses instead",
-                                                            format!("align({})", s));
+                                        err.span_suggestion_with_applicability(
+                                            item.span,
+                                            "use parentheses instead",
+                                            format!("align({})", s),
+                                            Applicability::MachineApplicable
+                                        );
                                     }
                                     _ => {}
                                 }

src/libsyntax/ext/expand.rs

@@ -13,7 +13,7 @@ use ast::{MacStmtStyle, StmtKind, ItemKind};
 use attr::{self, HasAttrs};
 use codemap::{ExpnInfo, NameAndSpan, MacroBang, MacroAttribute, dummy_spanned, respan};
 use config::{is_test_or_bench, StripUnconfigured};
-use errors::FatalError;
+use errors::{Applicability, FatalError};
 use ext::base::*;
 use ext::derive::{add_derived_markers, collect_derives};
 use ext::hygiene::{self, Mark, SyntaxContext};
@@ -331,7 +331,11 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                             let trait_list = traits.iter()
                                 .map(|t| format!("{}", t)).collect::<Vec<_>>();
                             let suggestion = format!("#[derive({})]", trait_list.join(", "));
-                            err.span_suggestion(span, "try an outer attribute", suggestion);
+                            err.span_suggestion_with_applicability(
+                                span, "try an outer attribute", suggestion,
+                                // We don't 𝑘𝑛𝑜𝑤 that the following item is an ADT
+                                Applicability::MaybeIncorrect
+                            );
                         }
                         err.emit();
                     }

src/libsyntax/parse/lexer/mod.rs

@@ -11,7 +11,7 @@
 use ast::{self, Ident};
 use syntax_pos::{self, BytePos, CharPos, Pos, Span, NO_EXPANSION};
 use codemap::{CodeMap, FilePathMapping};
-use errors::{FatalError, DiagnosticBuilder};
+use errors::{Applicability, FatalError, DiagnosticBuilder};
 use parse::{token, ParseSess};
 use str::char_at;
 use symbol::{Symbol, keywords};
@@ -1345,11 +1345,12 @@ impl<'a> StringReader<'a> {
                             self.sess.span_diagnostic
                                 .struct_span_err(span,
                                                  "character literal may only contain one codepoint")
-                                .span_suggestion(span,
-                                                 "if you meant to write a `str` literal, \
-                                                  use double quotes",
-                                                 format!("\"{}\"", &self.src[start..end]))
-                                .emit();
+                                .span_suggestion_with_applicability(
+                                    span,
+                                    "if you meant to write a `str` literal, use double quotes",
+                                    format!("\"{}\"", &self.src[start..end]),
+                                    Applicability::MachineApplicable
+                                ).emit();
                             return Ok(token::Literal(token::Str_(Symbol::intern("??")), None))
                         }
                         if self.ch_is('\n') || self.is_eof() || self.ch_is('/') {