Use strip_{prefix|suffix} instead of {starts|ends}_with+indexing

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1632,8 +1632,8 @@ fn get_mut_span_in_struct_field<'tcx>(
 /// If possible, suggest replacing `ref` with `ref mut`.
 fn suggest_ref_mut(tcx: TyCtxt<'_>, span: Span) -> Option<Span> {
     let pattern_str = tcx.sess.source_map().span_to_snippet(span).ok()?;
-    if pattern_str.starts_with("ref")
-        && pattern_str["ref".len()..].starts_with(rustc_lexer::is_whitespace)
+    if let Some(rest) = pattern_str.strip_prefix("ref")
+        && rest.starts_with(rustc_lexer::is_whitespace)
     {
         let span = span.with_lo(span.lo() + BytePos(4)).shrink_to_lo();
         Some(span)

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1538,8 +1538,13 @@ fn print_native_static_libs(
         }
         let stem = path.file_stem().unwrap().to_str().unwrap();
         // Convert library file-stem into a cc -l argument.
-        let prefix = if stem.starts_with("lib") && !sess.target.is_like_windows { 3 } else { 0 };
-        let lib = &stem[prefix..];
+        let lib = if let Some(lib) = stem.strip_prefix("lib")
+            && !sess.target.is_like_windows
+        {
+            lib
+        } else {
+            stem
+        };
         let path = parent.unwrap_or_else(|| Path::new(""));
         if sess.target.is_like_msvc {
             // When producing a dll, the MSVC linker may not actually emit a

compiler/rustc_driver_impl/src/lib.rs

@@ -457,8 +457,7 @@ pub enum Compilation {
 fn handle_explain(early_dcx: &EarlyDiagCtxt, registry: Registry, code: &str, color: ColorConfig) {
     // Allow "E0123" or "0123" form.
     let upper_cased_code = code.to_ascii_uppercase();
-    let start = if upper_cased_code.starts_with('E') { 1 } else { 0 };
-    if let Ok(code) = upper_cased_code[start..].parse::<u32>()
+    if let Ok(code) = upper_cased_code.strip_prefix('E').unwrap_or(&upper_cased_code).parse::<u32>()
         && let Ok(description) = registry.try_find_description(ErrCode::from_u32(code))
     {
         let mut is_in_code_block = false;

compiler/rustc_expand/src/proc_macro_server.rs

@@ -394,8 +394,8 @@ impl ToInternal<SmallVec<[tokenstream::TokenTree; 2]>>
                 symbol,
                 suffix,
                 span,
-            }) if symbol.as_str().starts_with('-') => {
-                let symbol = Symbol::intern(&symbol.as_str()[1..]);
+            }) if let Some(symbol) = symbol.as_str().strip_prefix('-') => {
+                let symbol = Symbol::intern(symbol);
                 let integer = TokenKind::lit(token::Integer, symbol, suffix);
                 let a = tokenstream::TokenTree::token_joint_hidden(Minus, span);
                 let b = tokenstream::TokenTree::token_alone(integer, span);
@@ -406,8 +406,8 @@ impl ToInternal<SmallVec<[tokenstream::TokenTree; 2]>>
                 symbol,
                 suffix,
                 span,
-            }) if symbol.as_str().starts_with('-') => {
-                let symbol = Symbol::intern(&symbol.as_str()[1..]);
+            }) if let Some(symbol) = symbol.as_str().strip_prefix('-') => {
+                let symbol = Symbol::intern(symbol);
                 let float = TokenKind::lit(token::Float, symbol, suffix);
                 let a = tokenstream::TokenTree::token_joint_hidden(Minus, span);
                 let b = tokenstream::TokenTree::token_alone(float, span);

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -901,12 +901,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 .iter()
                 .map(|(item, _)| format!("{} = Type", item.name))
                 .collect();
-            let code = if snippet.ends_with('>') {
+            let code = if let Some(snippet) = snippet.strip_suffix('>') {
                 // The user wrote `Trait<'a>` or similar and we don't have a type we can
                 // suggest, but at least we can clue them to the correct syntax
                 // `Trait<'a, Item = Type>` while accounting for the `<'a>` in the
                 // suggestion.
-                format!("{}, {}>", &snippet[..snippet.len() - 1], types.join(", "))
+                format!("{}, {}>", snippet, types.join(", "))
             } else if in_expr_or_pat {
                 // The user wrote `Iterator`, so we don't have a type we can suggest, but at
                 // least we can clue them to the correct syntax `Iterator::<Item = Type>`.

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -142,8 +142,8 @@ pub fn suggest_arbitrary_trait_bound<'tcx>(
     if let Some((name, term)) = associated_ty {
         // FIXME: this case overlaps with code in TyCtxt::note_and_explain_type_err.
         // That should be extracted into a helper function.
-        if constraint.ends_with('>') {
-            constraint = format!("{}, {} = {}>", &constraint[..constraint.len() - 1], name, term);
+        if let Some(stripped) = constraint.strip_suffix('>') {
+            constraint = format!("{stripped}, {name} = {term}>");
         } else {
             constraint.push_str(&format!("<{name} = {term}>"));
         }

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -390,13 +390,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     if let Some((name, term)) = associated_ty {
                         // FIXME: this case overlaps with code in TyCtxt::note_and_explain_type_err.
                         // That should be extracted into a helper function.
-                        if constraint.ends_with('>') {
-                            constraint = format!(
-                                "{}, {} = {}>",
-                                &constraint[..constraint.len() - 1],
-                                name,
-                                term
-                            );
+                        if let Some(stripped) = constraint.strip_suffix('>') {
+                            constraint = format!("{stripped}, {name} = {term}>");
                         } else {
                             constraint.push_str(&format!("<{name} = {term}>"));
                         }