Use heuristics for capitalization warning in suggestions

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};
@@ -239,11 +239,11 @@ pub trait Emitter {
                     format!(
                         "help: {}{}: `{}`",
                         sugg.msg,
-                        if self.source_map().as_ref().map(|sm| substitution.to_lowercase() == sm
+                        if self.source_map().map(|sm| is_case_difference(
-                            .span_to_snippet(sugg.substitutions[0].parts[0].span)
+                            &**sm,
-                            .unwrap()
+                            substitution,
-                            .to_lowercase()).unwrap_or(false)
+                            sugg.substitutions[0].parts[0].span,
-                        {
+                        )).unwrap_or(false) {
                             " (notice the capitalization)"
                         } else {
                             ""
@@ -2058,3 +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'];
+    // There are ASCII chars that are confusable (above) and differ in capitalization:
+    let confusable = found.chars().zip(suggested.chars()).any(|(f, s)| {
+        (ascii_confusables.contains(&f) || ascii_confusables.contains(&s)) && f != 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};
@@ -239,8 +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 = buf.clone().to_lowercase()
+            let only_capitalization = is_case_difference(cm, &buf, bounding_span);
-                == cm.span_to_snippet(bounding_span).unwrap().to_lowercase();
             // 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);
@@ -250,7 +249,6 @@ impl CodeSuggestion {
                 buf.pop();
             }
             (buf, substitution.parts, only_capitalization)
-            
         }).collect()
     }
 }

src/test/ui/hygiene/extern-prelude-from-opaque-fail.stderr

@@ -5,7 +5,7 @@ LL |     use my_core;
    |         ^^^^^^^
    |         |
    |         no `my_core` in the root
-   |         help: a similar name exists in the module (notice the capitalization): `my_core`
+   |         help: a similar name exists in the module: `my_core`
 
 error[E0432]: unresolved import `my_core`
   --> $DIR/extern-prelude-from-opaque-fail.rs:7:13

src/test/ui/lint/lint-group-nonstandard-style.stderr

@@ -41,7 +41,7 @@ error: static variable `bad` should have an upper case name
   --> $DIR/lint-group-nonstandard-style.rs:14:16
    |
 LL |         static bad: isize = 1;
-   |                ^^^ help: convert the identifier to upper case (notice the capitalization): `BAD`
+   |                ^^^ help: convert the identifier to upper case: `BAD`
    |
 note: lint level defined here
   --> $DIR/lint-group-nonstandard-style.rs:10:14

src/test/ui/lint/lint-lowercase-static-const-pattern.stderr

@@ -2,7 +2,7 @@ error: constant in pattern `a` should have an upper case name
   --> $DIR/lint-lowercase-static-const-pattern.rs:11:13
    |
 LL |         (0, a) => 0,
-   |             ^ help: convert the identifier to upper case (notice the capitalization): `A`
+   |             ^ help: convert the identifier to upper case: `A`
    |
 note: lint level defined here
   --> $DIR/lint-lowercase-static-const-pattern.rs:4:9
@@ -14,7 +14,7 @@ error: constant in pattern `aha` should have an upper case name
   --> $DIR/lint-lowercase-static-const-pattern.rs:26:13
    |
 LL |         (0, aha) => 0,
-   |             ^^^ help: convert the identifier to upper case (notice the capitalization): `AHA`
+   |             ^^^ help: convert the identifier to upper case: `AHA`
 
 error: constant in pattern `not_okay` should have an upper case name
   --> $DIR/lint-lowercase-static-const-pattern.rs:40:13

src/test/ui/lint/lint-non-camel-case-types.stderr

@@ -38,7 +38,7 @@ error: variant `bar` should have an upper camel case name
   --> $DIR/lint-non-camel-case-types.rs:22:5
    |
 LL |     bar
-   |     ^^^ help: convert the identifier to upper camel case (notice the capitalization): `Bar`
+   |     ^^^ help: convert the identifier to upper camel case: `Bar`
 
 error: trait `foo6` should have an upper camel case name
   --> $DIR/lint-non-camel-case-types.rs:25:7
@@ -50,7 +50,7 @@ error: type parameter `ty` should have an upper camel case name
   --> $DIR/lint-non-camel-case-types.rs:29:6
    |
 LL | fn f<ty>(_: ty) {}
-   |      ^^ help: convert the identifier to upper camel case (notice the capitalization): `Ty`
+   |      ^^ help: convert the identifier to upper camel case: `Ty`
 
 error: aborting due to 8 previous errors
 

src/test/ui/lint/lint-non-snake-case-functions.stderr

@@ -26,7 +26,7 @@ error: method `render_HTML` should have a snake case name
   --> $DIR/lint-non-snake-case-functions.rs:17:8
    |
 LL |     fn render_HTML() {}
-   |        ^^^^^^^^^^^ help: convert the identifier to snake case (notice the capitalization): `render_html`
+   |        ^^^^^^^^^^^ help: convert the identifier to snake case: `render_html`
 
 error: trait method `ABC` should have a snake case name
   --> $DIR/lint-non-snake-case-functions.rs:22:8

src/test/ui/lint/lint-non-uppercase-statics.stderr

@@ -14,7 +14,7 @@ error: static variable `bar` should have an upper case name
   --> $DIR/lint-non-uppercase-statics.rs:6:12
    |
 LL | static mut bar: isize = 1;
-   |            ^^^ help: convert the identifier to upper case (notice the capitalization): `BAR`
+   |            ^^^ help: convert the identifier to upper case: `BAR`
 
 error: aborting due to 2 previous errors
 

src/test/ui/lint/lint-uppercase-variables.stderr

@@ -39,7 +39,7 @@ error: variable `Test` should have a snake case name
   --> $DIR/lint-uppercase-variables.rs:18:9
    |
 LL |     let Test: usize = 0;
-   |         ^^^^ help: convert the identifier to snake case (notice the capitalization): `test`
+   |         ^^^^ help: convert the identifier to snake case: `test`
 
 error: variable `Foo` should have a snake case name
   --> $DIR/lint-uppercase-variables.rs:22:9

src/test/ui/resolve/issue-39226.stderr

@@ -8,7 +8,7 @@ LL |         handle: Handle
    |                 ^^^^^^
    |                 |
    |                 did you mean `Handle { /* fields */ }`?
-   |                 help: a local variable with a similar name exists (notice the capitalization): `handle`
+   |                 help: a local variable with a similar name exists: `handle`
 
 error: aborting due to previous error
 

src/test/ui/test-attrs/inaccessible-test-modules.stderr

@@ -5,7 +5,7 @@ LL | use main as x;
    |     ----^^^^^
    |     |
    |     no `main` in the root
-   |     help: a similar name exists in the module (notice the capitalization): `main`
+   |     help: a similar name exists in the module: `main`
 
 error[E0432]: unresolved import `test`
   --> $DIR/inaccessible-test-modules.rs:6:5
@@ -14,7 +14,7 @@ LL | use test as y;
    |     ----^^^^^
    |     |
    |     no `test` in the root
-   |     help: a similar name exists in the module (notice the capitalization): `test`
+   |     help: a similar name exists in the module: `test`
 
 error: aborting due to 2 previous errors
 

src/test/ui/traits/trait-impl-for-module.stderr

@@ -2,7 +2,7 @@ error[E0573]: expected type, found module `a`
   --> $DIR/trait-impl-for-module.rs:7:12
    |
 LL | impl A for a {
-   |            ^ help: a trait with a similar name exists (notice the capitalization): `A`
+   |            ^ help: a trait with a similar name exists: `A`
 
 error: aborting due to previous error
 

src/test/ui/utf8_idents.stderr

@@ -38,7 +38,7 @@ warning: type parameter `γ` should have an upper camel case name
   --> $DIR/utf8_idents.rs:3:5
    |
 LL |     γ
-   |     ^ help: convert the identifier to upper camel case (notice the capitalization): `Γ`
+   |     ^ help: convert the identifier to upper camel case: `Γ`
    |
    = note: `#[warn(non_camel_case_types)]` on by default