Add suggestion for captured and passed arg case in fmt macro

compiler/rustc_builtin_macros/src/format.rs

@@ -1,3 +1,5 @@
+use parse::Piece::NextArgument;
+use parse::Position::ArgumentNamed;
 use rustc_ast::ptr::P;
 use rustc_ast::token;
 use rustc_ast::tokenstream::TokenStream;
@@ -34,6 +36,14 @@ enum PositionUsedAs {
     Precision,
     Width,
 }
+
+#[derive(Clone, Copy, Debug)]
+struct RedundantArgDiagBuilder<'a> {
+    arg_span: Span,
+    fmt_span: InnerSpan,
+    fmt_str: &'a str,
+}
+
 use PositionUsedAs::*;
 
 struct MacroInput {
@@ -335,8 +345,8 @@ fn make_format_args(
     let mut unfinished_literal = String::new();
     let mut placeholder_index = 0;
 
-    for piece in pieces {
-        match piece {
+    for piece in &pieces {
+        match *piece {
             parse::Piece::String(s) => {
                 unfinished_literal.push_str(s);
             }
@@ -470,6 +480,20 @@ fn make_format_args(
         report_invalid_references(ecx, &invalid_refs, &template, fmt_span, &args, parser);
     }
 
+    // Vec that contains all redundant arg, will be printed for suggestion
+    let mut redundant_expl_args = Vec::new();
+
+    // `placeholder` contains pieces that may be linked to one of redundant explicit argument
+    let _placeholders: Vec<&parse::Piece<'_>> = pieces
+        .iter()
+        .filter(|p| {
+            if let NextArgument(arg) = *p && let ArgumentNamed(_) =  arg.position {
+            return true;
+        }
+            false
+        })
+        .collect();
+
     let unused = used
         .iter()
         .enumerate()
@@ -478,6 +502,25 @@ fn make_format_args(
             let msg = if let FormatArgumentKind::Named(_) = args.explicit_args()[i].kind {
                 "named argument never used"
             } else {
+                if let Some(expr) = args.explicit_args()[i].expr.to_ty()
+                && let Some(symbol) = expr.kind.is_simple_path() {
+                    for piece in &pieces {
+                        if let NextArgument(arg) = piece
+                        && let ArgumentNamed(specifier) = arg.position {
+                            if specifier == symbol.to_string() {
+                                redundant_expl_args.push(
+                                    RedundantArgDiagBuilder {
+                                        arg_span: expr.span,
+                                        fmt_span: InnerSpan {
+                                            start: arg.position_span.start,
+                                            end: arg.position_span.end,
+                                        },
+                                        fmt_str: specifier,
+                                });
+                            }
+                        }
+                    }
+                }
                 "argument never used"
             };
             (args.explicit_args()[i].expr.span, msg)
@@ -488,7 +531,15 @@ fn make_format_args(
         // If there's a lot of unused arguments,
         // let's check if this format arguments looks like another syntax (printf / shell).
         let detect_foreign_fmt = unused.len() > args.explicit_args().len() / 2;
-        report_missing_placeholders(ecx, unused, detect_foreign_fmt, str_style, fmt_str, fmt_span);
+        report_missing_placeholders(
+            ecx,
+            unused,
+            redundant_expl_args,
+            detect_foreign_fmt,
+            str_style,
+            fmt_str,
+            fmt_span,
+        );
     }
 
     // Only check for unused named argument names if there are no other errors to avoid causing
@@ -571,6 +622,7 @@ fn invalid_placeholder_type_error(
 fn report_missing_placeholders(
     ecx: &mut ExtCtxt<'_>,
     unused: Vec<(Span, &str)>,
+    redundant_expl_args: Vec<RedundantArgDiagBuilder<'_>>,
     detect_foreign_fmt: bool,
     str_style: Option<usize>,
     fmt_str: &str,
@@ -676,6 +728,29 @@ fn report_missing_placeholders(
         diag.span_label(fmt_span, "formatting specifier missing");
     }
 
+    // Diag message buileder
+    if !redundant_expl_args.is_empty() {
+        let mut fmt_spans = Vec::new();
+        let mut arg_spans = Vec::new();
+        let mut arg = Vec::new();
+        for rd in &redundant_expl_args {
+            fmt_spans.push(Span::from_inner(fmt_span, rd.fmt_span));
+            arg_spans.push(rd.arg_span);
+            arg.push(rd.fmt_str);
+        }
+        let mut m_span: MultiSpan = fmt_spans.clone().into();
+        for i in 0..fmt_spans.len() {
+            m_span.push_span_label(
+                fmt_spans[i],
+                format!("this formatting argument captures `{}` directly", arg[i]),
+            );
+        }
+        for arg_span in arg_spans {
+            m_span.push_span_label(arg_span, "this can be removed".to_string());
+        }
+        diag.span_help(m_span, "the formatting string captures that binding directly, it doesn't need to be included in the argument list");
+    }
+
     diag.emit();
 }
 

tests/ui/macros/issue-105225.rs

@@ -0,0 +1,16 @@
+fn main() {
+    let x = 10;
+    println!("{x}", x);
+    //~^ ERROR argument never used
+    let y = 20;
+    println!("{x} {y}", x, y);
+    //~^ ERROR multiple unused formatting arguments
+    println!("{x} {y}", y, x);
+    //~^ ERROR multiple unused formatting arguments
+    println!("{} {y}", x, y);
+    //~^ ERROR argument never used
+    println!("{} {} {y} {} {}", y, y, y, y, y);
+    //~^ ERROR argument never used
+    println!("{x} {x} {y} {x} {x}", x, y, x, y, y);
+    //~^ ERROR argument never used
+}

tests/ui/macros/issue-105225.stderr

@@ -0,0 +1,107 @@
+error: argument never used
+  --> $DIR/issue-105225.rs:3:21
+   |
+LL |     println!("{x}", x);
+   |              -----  ^ argument never used
+   |              |
+   |              formatting specifier missing
+   |
+help: the formatting string captures that binding directly, it doesn't need to be included in the argument list
+  --> $DIR/issue-105225.rs:3:16
+   |
+LL |     println!("{x}", x);
+   |                ^    - this can be removed
+   |                |
+   |                this formatting argument captures `x` directly
+
+error: multiple unused formatting arguments
+  --> $DIR/issue-105225.rs:6:25
+   |
+LL |     println!("{x} {y}", x, y);
+   |              ---------  ^  ^ argument never used
+   |              |          |
+   |              |          argument never used
+   |              multiple missing formatting specifiers
+   |
+help: the formatting string captures that binding directly, it doesn't need to be included in the argument list
+  --> $DIR/issue-105225.rs:6:16
+   |
+LL |     println!("{x} {y}", x, y);
+   |                ^   ^    -  - this can be removed
+   |                |   |    |
+   |                |   |    this can be removed
+   |                |   this formatting argument captures `y` directly
+   |                this formatting argument captures `x` directly
+
+error: multiple unused formatting arguments
+  --> $DIR/issue-105225.rs:8:25
+   |
+LL |     println!("{x} {y}", y, x);
+   |              ---------  ^  ^ argument never used
+   |              |          |
+   |              |          argument never used
+   |              multiple missing formatting specifiers
+   |
+help: the formatting string captures that binding directly, it doesn't need to be included in the argument list
+  --> $DIR/issue-105225.rs:8:16
+   |
+LL |     println!("{x} {y}", y, x);
+   |                ^   ^    -  - this can be removed
+   |                |   |    |
+   |                |   |    this can be removed
+   |                |   this formatting argument captures `y` directly
+   |                this formatting argument captures `x` directly
+
+error: argument never used
+  --> $DIR/issue-105225.rs:10:27
+   |
+LL |     println!("{} {y}", x, y);
+   |              --------     ^ argument never used
+   |              |
+   |              formatting specifier missing
+   |
+help: the formatting string captures that binding directly, it doesn't need to be included in the argument list
+  --> $DIR/issue-105225.rs:10:19
+   |
+LL |     println!("{} {y}", x, y);
+   |                   ^       - this can be removed
+   |                   |
+   |                   this formatting argument captures `y` directly
+
+error: argument never used
+  --> $DIR/issue-105225.rs:12:45
+   |
+LL |     println!("{} {} {y} {} {}", y, y, y, y, y);
+   |              -----------------              ^ argument never used
+   |              |
+   |              formatting specifier missing
+   |
+help: the formatting string captures that binding directly, it doesn't need to be included in the argument list
+  --> $DIR/issue-105225.rs:12:22
+   |
+LL |     println!("{} {} {y} {} {}", y, y, y, y, y);
+   |                      ^                      - this can be removed
+   |                      |
+   |                      this formatting argument captures `y` directly
+
+error: multiple unused formatting arguments
+  --> new.rs:14:37
+   |
+14 |     println!("{x} {x} {y} {x} {x}", x, y, x, y, y);
+   |              ---------------------  ^  ^  ^  ^  ^ argument never used
+   |              |                      |  |  |  |
+   |              |                      |  |  |  argument never used
+   |              |                      |  |  argument never used
+   |              |                      |  argument never used
+   |              |                      argument never used
+   |              multiple missing formatting specifiers
+   |
+help: the formatting string captures that binding directly, it doesn't need to be included in the argument list
+  --> new.rs:14:16
+   |
+LL |     println!("{x} {x} {y} {x} {x}", x, y, x, y, y);
+   |                ^                    - this can be removed
+   |                |
+   |                this formatting argument captures `y` directly
+error: aborting due to 6 previous errors
+