Rollup merge of #128762 - fmease:use-more-slice-pats, r=compiler-errors

Use more slice patterns inside the compiler

Nothing super noteworthy. Just replacing the common 'fragile' pattern of "length check followed by indexing or unwrap" with slice patterns for legibility and 'robustness'.

r? ghost

2 files changed, 19 insertions, 20 deletions

diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index b8813ea4125..9ce5d77ef6c 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/compiler/rustc_errors/src/emitter.rs
@@ -226,17 +226,17 @@ pub trait Emitter: Translate {
     ) {
         if let Some((sugg, rest)) = suggestions.split_first() {
             let msg = self.translate_message(&sugg.msg, fluent_args).map_err(Report::new).unwrap();
-            if rest.is_empty() &&
+            if rest.is_empty()
                // ^ if there is only one suggestion
                // don't display multi-suggestions as labels
-               sugg.substitutions.len() == 1 &&
+               && let [substitution] = sugg.substitutions.as_slice()
                // don't display multipart suggestions as labels
-               sugg.substitutions[0].parts.len() == 1 &&
+               && let [part] = substitution.parts.as_slice()
                // don't display long messages as labels
-               msg.split_whitespace().count() < 10 &&
+               && msg.split_whitespace().count() < 10
                // don't display multiline suggestions as labels
-               !sugg.substitutions[0].parts[0].snippet.contains('\n') &&
-               ![
+               && !part.snippet.contains('\n')
+               && ![
                     // when this style is set we want the suggestion to be a message, not inline
                     SuggestionStyle::HideCodeAlways,
                     // trivial suggestion for tooling's sake, never shown
@@ -245,8 +245,8 @@ pub trait Emitter: Translate {
                     SuggestionStyle::ShowAlways,
                ].contains(&sugg.style)
             {
-                let substitution = &sugg.substitutions[0].parts[0].snippet.trim();
-                let msg = if substitution.is_empty() || sugg.style.hide_inline() {
+                let snippet = part.snippet.trim();
+                let msg = if snippet.is_empty() || sugg.style.hide_inline() {
                     // This substitution is only removal OR we explicitly don't want to show the
                     // code inline (`hide_inline`). Therefore, we don't show the substitution.
                     format!("help: {msg}")
@@ -255,19 +255,18 @@ pub trait Emitter: Translate {
                     format!(
                         "help: {}{}: `{}`",
                         msg,
-                        if self.source_map().is_some_and(|sm| is_case_difference(
-                            sm,
-                            substitution,
-                            sugg.substitutions[0].parts[0].span,
-                        )) {
+                        if self
+                            .source_map()
+                            .is_some_and(|sm| is_case_difference(sm, snippet, part.span,))
+                        {
                             " (notice the capitalization)"
                         } else {
                             ""
                         },
-                        substitution,
+                        snippet,
                     )
                 };
-                primary_span.push_span_label(sugg.substitutions[0].parts[0].span, msg);
+                primary_span.push_span_label(part.span, msg);
 
                 // We return only the modified primary_span
                 suggestions.clear();
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index ceebcd46a6f..aefbf05a1fc 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -2024,11 +2024,11 @@ pub fn a_or_an(s: &str) -> &'static str {
 ///
 /// Take a list ["a", "b", "c"] and output a display friendly version "a, b and c"
 pub fn display_list_with_comma_and<T: std::fmt::Display>(v: &[T]) -> String {
-    match v.len() {
-        0 => "".to_string(),
-        1 => v[0].to_string(),
-        2 => format!("{} and {}", v[0], v[1]),
-        _ => format!("{}, {}", v[0], display_list_with_comma_and(&v[1..])),
+    match v {
+        [] => "".to_string(),
+        [a] => a.to_string(),
+        [a, b] => format!("{a} and {b}"),
+        [a, v @ ..] => format!("{a}, {}", display_list_with_comma_and(v)),
     }
 }