Bring attention to suggestions when the only difference is capitalization

src/librustc/session/config.rs

@@ -23,7 +23,7 @@ use syntax::feature_gate::UnstableFeatures;
 use syntax::source_map::SourceMap;
 
 use errors::emitter::HumanReadableErrorType;
-use errors::{ColorConfig, FatalError, Handler};
+use errors::{ColorConfig, FatalError, Handler, SourceMapperDyn};
 
 use getopts;
 
@@ -1854,6 +1854,7 @@ struct NullEmitter;
 
 impl errors::emitter::Emitter for NullEmitter {
     fn emit_diagnostic(&mut self, _: &errors::Diagnostic) {}
+    fn source_map(&self) -> Option<&Lrc<SourceMapperDyn>> { None }
 }
 
 // Converts strings provided as `--cfg [cfgspec]` into a `crate_cfg`.

src/librustc_codegen_ssa/back/write.rs

@@ -22,7 +22,8 @@ use rustc::util::common::{time_depth, set_time_depth, print_time_passes_entry};
 use rustc::util::profiling::SelfProfilerRef;
 use rustc_fs_util::link_or_copy;
 use rustc_data_structures::svh::Svh;
-use rustc_errors::{Handler, Level, FatalError, DiagnosticId};
+use rustc_data_structures::sync::Lrc;
+use rustc_errors::{Handler, Level, FatalError, DiagnosticId, SourceMapperDyn};
 use rustc_errors::emitter::{Emitter};
 use rustc_target::spec::MergeFunctions;
 use syntax::attr;
@@ -1681,6 +1682,9 @@ impl Emitter for SharedEmitter {
         }
         drop(self.sender.send(SharedEmitterMessage::AbortIfErrors));
     }
+    fn source_map(&self) -> Option<&Lrc<SourceMapperDyn>> {
+        None
+    }
 }
 
 impl SharedEmitterMain {

src/librustc_errors/annotate_snippet_emitter_writer.rs

@@ -49,6 +49,10 @@ impl Emitter for AnnotateSnippetEmitterWriter {
                                    &suggestions);
     }
 
+    fn source_map(&self) -> Option<&Lrc<SourceMapperDyn>> {
+        self.source_map.as_ref()
+    }
+
     fn should_show_explain(&self) -> bool {
         !self.short_message
     }

src/librustc_errors/emitter.rs

@@ -13,7 +13,7 @@ use syntax_pos::{SourceFile, Span, MultiSpan};
 
 use crate::{
     Level, CodeSuggestion, Diagnostic, SubDiagnostic,
-    SuggestionStyle, SourceMapperDyn, DiagnosticId,
+    SuggestionStyle, SourceMapper, SourceMapperDyn, DiagnosticId,
 };
 use crate::Level::Error;
 use crate::snippet::{Annotation, AnnotationType, Line, MultilineAnnotation, StyledString, Style};
@@ -192,6 +192,8 @@ pub trait Emitter {
         true
     }
 
+    fn source_map(&self) -> Option<&Lrc<SourceMapperDyn>>;
+
     /// Formats the substitutions of the primary_span
     ///
     /// The are a lot of conditions to this method, but in short:
@@ -204,7 +206,7 @@ pub trait Emitter {
     ///   we return the original `primary_span` and the original suggestions.
     fn primary_span_formatted<'a>(
         &mut self,
-        db: &'a Diagnostic
+        db: &'a Diagnostic,
     ) -> (MultiSpan, &'a [CodeSuggestion]) {
         let mut primary_span = db.span.clone();
         if let Some((sugg, rest)) = db.suggestions.split_first() {
@@ -234,7 +236,20 @@ pub trait Emitter {
                     format!("help: {}", sugg.msg)
                 } else {
                     // Show the default suggestion text with the substitution
-                    format!("help: {}: `{}`", sugg.msg, substitution)
+                    format!(
+                        "help: {}{}: `{}`",
+                        sugg.msg,
+                        if self.source_map().map(|sm| is_case_difference(
+                            &**sm,
+                            substitution,
+                            sugg.substitutions[0].parts[0].span,
+                        )).unwrap_or(false) {
+                            " (notice the capitalization)"
+                        } else {
+                            ""
+                        },
+                        substitution,
+                    )
                 };
                 primary_span.push_span_label(sugg.substitutions[0].parts[0].span, msg);
 
@@ -382,6 +397,10 @@ pub trait Emitter {
 }
 
 impl Emitter for EmitterWriter {
+    fn source_map(&self) -> Option<&Lrc<SourceMapperDyn>> {
+        self.sm.as_ref()
+    }
+
     fn emit_diagnostic(&mut self, db: &Diagnostic) {
         let mut children = db.children.clone();
         let (mut primary_span, suggestions) = self.primary_span_formatted(&db);
@@ -1461,7 +1480,9 @@ impl EmitterWriter {
         let suggestions = suggestion.splice_lines(&**sm);
 
         let mut row_num = 2;
-        for &(ref complete, ref parts) in suggestions.iter().take(MAX_SUGGESTIONS) {
+        let mut notice_capitalization = false;
+        for (complete, parts, only_capitalization) in suggestions.iter().take(MAX_SUGGESTIONS) {
+            notice_capitalization |= only_capitalization;
             // Only show underline if the suggestion spans a single line and doesn't cover the
             // entirety of the code output. If you have multiple replacements in the same line
             // of code, show the underline.
@@ -1552,7 +1573,10 @@ impl EmitterWriter {
         }
         if suggestions.len() > MAX_SUGGESTIONS {
             let msg = format!("and {} other candidates", suggestions.len() - MAX_SUGGESTIONS);
-            buffer.puts(row_num, 0, &msg, Style::NoStyle);
+            buffer.puts(row_num, max_line_num_len + 3, &msg, Style::NoStyle);
+        } else if notice_capitalization {
+            let msg = "notice the capitalization difference";
+            buffer.puts(row_num, max_line_num_len + 3, &msg, Style::NoStyle);
         }
         emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message)?;
         Ok(())
@@ -2034,3 +2058,18 @@ impl<'a> Drop for WritableDst<'a> {
         }
     }
 }
+
+/// Whether the original and suggested code are visually similar enough to warrant extra wording.
+pub fn is_case_difference(sm: &dyn SourceMapper, suggested: &str, sp: Span) -> bool {
+    // FIXME: this should probably be extended to also account for `FO0` → `FOO` and unicode.
+    let found = sm.span_to_snippet(sp).unwrap();
+    let ascii_confusables = &['c', 'f', 'i', 'k', 'o', 's', 'u', 'v', 'w', 'x', 'y', 'z'];
+    // All the chars that differ in capitalization are confusable (above):
+    let confusable = found.chars().zip(suggested.chars()).filter(|(f, s)| f != s).all(|(f, s)| {
+        (ascii_confusables.contains(&f) || ascii_confusables.contains(&s))
+    });
+    confusable && found.to_lowercase() == suggested.to_lowercase()
+            // FIXME: We sometimes suggest the same thing we already have, which is a
+            //        bug, but be defensive against that here.
+            && found != suggested
+}

src/librustc_errors/lib.rs

@@ -13,7 +13,7 @@ pub use emitter::ColorConfig;
 
 use Level::*;
 
-use emitter::{Emitter, EmitterWriter};
+use emitter::{Emitter, EmitterWriter, is_case_difference};
 use registry::Registry;
 
 use rustc_data_structures::sync::{self, Lrc, Lock};
@@ -37,13 +37,16 @@ pub mod registry;
 mod styled_buffer;
 mod lock;
 
-use syntax_pos::{BytePos,
-                 Loc,
-                 FileLinesResult,
-                 SourceFile,
-                 FileName,
-                 MultiSpan,
-                 Span};
+use syntax_pos::{
+    BytePos,
+    FileLinesResult,
+    FileName,
+    Loc,
+    MultiSpan,
+    SourceFile,
+    Span,
+    SpanSnippetError,
+};
 
 /// Indicates the confidence in the correctness of a suggestion.
 ///
@@ -147,6 +150,7 @@ pub trait SourceMapper {
     fn lookup_char_pos(&self, pos: BytePos) -> Loc;
     fn span_to_lines(&self, sp: Span) -> FileLinesResult;
     fn span_to_string(&self, sp: Span) -> String;
+    fn span_to_snippet(&self, sp: Span) -> Result<String, SpanSnippetError>;
     fn span_to_filename(&self, sp: Span) -> FileName;
     fn merge_spans(&self, sp_lhs: Span, sp_rhs: Span) -> Option<Span>;
     fn call_span_if_macro(&self, sp: Span) -> Span;
@@ -155,9 +159,12 @@ pub trait SourceMapper {
 }
 
 impl CodeSuggestion {
-    /// Returns the assembled code suggestions and whether they should be shown with an underline.
-    pub fn splice_lines(&self, cm: &SourceMapperDyn)
-                        -> Vec<(String, Vec<SubstitutionPart>)> {
+    /// Returns the assembled code suggestions, whether they should be shown with an underline
+    /// and whether the substitution only differs in capitalization.
+    pub fn splice_lines(
+        &self,
+        cm: &SourceMapperDyn,
+    ) -> Vec<(String, Vec<SubstitutionPart>, bool)> {
         use syntax_pos::{CharPos, Pos};
 
         fn push_trailing(buf: &mut String,
@@ -232,6 +239,7 @@ impl CodeSuggestion {
                 prev_hi = cm.lookup_char_pos(part.span.hi());
                 prev_line = fm.get_line(prev_hi.line - 1);
             }
+            let only_capitalization = is_case_difference(cm, &buf, bounding_span);
             // if the replacement already ends with a newline, don't print the next line
             if !buf.ends_with('\n') {
                 push_trailing(&mut buf, prev_line.as_ref(), &prev_hi, None);
@@ -240,7 +248,7 @@ impl CodeSuggestion {
             while buf.ends_with('\n') {
                 buf.pop();
             }
-            (buf, substitution.parts)
+            (buf, substitution.parts, only_capitalization)
         }).collect()
     }
 }

src/libsyntax/json.rs

@@ -12,7 +12,7 @@
 use crate::source_map::{SourceMap, FilePathMapping};
 
 use errors::registry::Registry;
-use errors::{SubDiagnostic, CodeSuggestion, SourceMapper};
+use errors::{SubDiagnostic, CodeSuggestion, SourceMapper, SourceMapperDyn};
 use errors::{DiagnosticId, Applicability};
 use errors::emitter::{Emitter, HumanReadableErrorType};
 
@@ -113,6 +113,10 @@ impl Emitter for JsonEmitter {
         }
     }
 
+    fn source_map(&self) -> Option<&Lrc<SourceMapperDyn>> {
+        Some(&self.sm)
+    }
+
     fn should_show_explain(&self) -> bool {
         match self.json_rendered {
             HumanReadableErrorType::Short(_) => false,

src/libsyntax/source_map.rs

@@ -987,6 +987,9 @@ impl SourceMapper for SourceMap {
     fn span_to_string(&self, sp: Span) -> String {
         self.span_to_string(sp)
     }
+    fn span_to_snippet(&self, sp: Span) -> Result<String, SpanSnippetError> {
+        self.span_to_snippet(sp)
+    }
     fn span_to_filename(&self, sp: Span) -> FileName {
         self.span_to_filename(sp)
     }

src/test/ui/const-generics/const-parameter-uppercase-lint.stderr

@@ -10,7 +10,7 @@ error: const parameter `x` should have an upper case name
   --> $DIR/const-parameter-uppercase-lint.rs:6:15
    |
 LL | fn noop<const x: u32>() {
-   |               ^ help: convert the identifier to upper case: `X`
+   |               ^ help: convert the identifier to upper case (notice the capitalization): `X`
    |
 note: lint level defined here
   --> $DIR/const-parameter-uppercase-lint.rs:4:9

src/test/ui/did_you_mean/issue-43871-enum-instead-of-variant.stderr

@@ -55,7 +55,7 @@ LL |     let z = ManyVariants::Three();
    |             ^^^^^^^^^^^^^^^^^^^
 LL |     let z = ManyVariants::Four();
    |             ^^^^^^^^^^^^^^^^^^
-and 6 other candidates
+     and 6 other candidates
 
 error: aborting due to 5 previous errors
 

src/test/ui/did_you_mean/issue-56028-there-is-an-enum-variant.stderr

@@ -13,7 +13,7 @@ LL | fn setup() -> Determine { Set }
    |               ^^^^^^^^^
 LL | fn setup() -> PutDown { Set }
    |               ^^^^^^^
-and 3 other candidates
+     and 3 other candidates
 
 error[E0425]: cannot find value `Set` in this scope
   --> $DIR/issue-56028-there-is-an-enum-variant.rs:9:21
@@ -30,7 +30,7 @@ LL | use Determine::Set;
    |
 LL | use PutDown::Set;
    |
-and 3 other candidates
+     and 3 other candidates
 
 error: aborting due to 2 previous errors
 

src/test/ui/error-codes/E0423.stderr

@@ -34,7 +34,7 @@ LL |     let f = Foo();
    |             ^^^
    |             |
    |             did you mean `Foo { /* fields */ }`?
-   |             help: a function with a similar name exists: `foo`
+   |             help: a function with a similar name exists (notice the capitalization): `foo`
 
 error[E0423]: expected value, found struct `T`
   --> $DIR/E0423.rs:14:8

src/test/ui/expr_attr_paren_order.stderr

@@ -2,7 +2,7 @@ error: variable `X` should have a snake case name
   --> $DIR/expr_attr_paren_order.rs:19:17
    |
 LL |             let X = 0;
-   |                 ^ help: convert the identifier to snake case: `x`
+   |                 ^ help: convert the identifier to snake case (notice the capitalization): `x`
    |
 note: lint level defined here
   --> $DIR/expr_attr_paren_order.rs:17:17

src/test/ui/hygiene/globs.stderr

@@ -34,7 +34,7 @@ LL | use foo::test2::test::g;
    |
 LL | use foo::test::g;
    |
-and 2 other candidates
+     and 2 other candidates
 
 error[E0425]: cannot find function `f` in this scope
   --> $DIR/globs.rs:61:12

src/test/ui/hygiene/rustc-macro-transparency.stderr

@@ -2,7 +2,7 @@ error[E0425]: cannot find value `Opaque` in this scope
   --> $DIR/rustc-macro-transparency.rs:26:5
    |
 LL |     Opaque;
-   |     ^^^^^^ help: a local variable with a similar name exists: `opaque`
+   |     ^^^^^^ help: a local variable with a similar name exists (notice the capitalization): `opaque`
 
 error[E0423]: expected value, found macro `semitransparent`
   --> $DIR/rustc-macro-transparency.rs:29:5

src/test/ui/issues/issue-10200.stderr

@@ -2,7 +2,7 @@ error[E0532]: expected tuple struct/variant, found function `foo`
   --> $DIR/issue-10200.rs:6:9
    |
 LL |         foo(x)
-   |         ^^^ help: a tuple struct with a similar name exists: `Foo`
+   |         ^^^ help: a tuple struct with a similar name exists (notice the capitalization): `Foo`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-17546.stderr

@@ -27,7 +27,7 @@ LL |     use std::prelude::v1::Result;
    |
 LL |     use std::result::Result;
    |
-and 1 other candidates
+     and 1 other candidates
 
 error[E0573]: expected type, found variant `Result`
   --> $DIR/issue-17546.rs:28:13
@@ -44,7 +44,7 @@ LL | use std::prelude::v1::Result;
    |
 LL | use std::result::Result;
    |
-and 1 other candidates
+     and 1 other candidates
 
 error[E0573]: expected type, found variant `NoResult`
   --> $DIR/issue-17546.rs:33:15

src/test/ui/issues/issue-17718-const-naming.stderr

@@ -15,7 +15,7 @@ error: constant `foo` should have an upper case name
   --> $DIR/issue-17718-const-naming.rs:4:7
    |
 LL | const foo: isize = 3;
-   |       ^^^ help: convert the identifier to upper case: `FOO`
+   |       ^^^ help: convert the identifier to upper case (notice the capitalization): `FOO`
    |
 note: lint level defined here
   --> $DIR/issue-17718-const-naming.rs:2:9

src/test/ui/issues/issue-46332.stderr

@@ -2,7 +2,7 @@ error[E0422]: cannot find struct, variant or union type `TyUInt` in this scope
   --> $DIR/issue-46332.rs:9:5
    |
 LL |     TyUInt {};
-   |     ^^^^^^ help: a struct with a similar name exists: `TyUint`
+   |     ^^^^^^ help: a struct with a similar name exists (notice the capitalization): `TyUint`
 
 error: aborting due to previous error