Correctly mark the span of captured arguments in `format_args!()`

It should only include the identifier, or misspelling suggestions will be wrong.

15 files changed, 105 insertions, 74 deletions

diff --git a/compiler/rustc_builtin_macros/src/asm.rs b/compiler/rustc_builtin_macros/src/asm.rs
index 1a6e5694791..ac37c4973d8 100644
--- a/compiler/rustc_builtin_macros/src/asm.rs
+++ b/compiler/rustc_builtin_macros/src/asm.rs
@@ -700,11 +700,11 @@ fn expand_preparsed_asm(ecx: &mut ExtCtxt<'_>, args: AsmArgs) -> Option<ast::Inl
                                 Some(idx)
                             }
                         }
-                        parse::ArgumentNamed(name) => match args.named_args.get(&name) {
+                        parse::ArgumentNamed(name, span) => match args.named_args.get(&name) {
                             Some(&idx) => Some(idx),
                             None => {
                                 let msg = format!("there is no argument named `{}`", name);
-                                ecx.struct_span_err(span, &msg).emit();
+                                ecx.struct_span_err(template_span.from_inner(span), &msg).emit();
                                 None
                             }
                         },
diff --git a/compiler/rustc_builtin_macros/src/format.rs b/compiler/rustc_builtin_macros/src/format.rs
index 584fbd1b605..6141d00f697 100644
--- a/compiler/rustc_builtin_macros/src/format.rs
+++ b/compiler/rustc_builtin_macros/src/format.rs
@@ -11,7 +11,7 @@ use rustc_errors::{pluralize, Applicability, DiagnosticBuilder};
 use rustc_expand::base::{self, *};
 use rustc_parse_format as parse;
 use rustc_span::symbol::{sym, Ident, Symbol};
-use rustc_span::{MultiSpan, Span};
+use rustc_span::{InnerSpan, MultiSpan, Span};
 use smallvec::SmallVec;
 
 use std::borrow::Cow;
@@ -26,7 +26,7 @@ enum ArgumentType {
 enum Position {
     Exact(usize),
     Capture(usize),
-    Named(Symbol),
+    Named(Symbol, InnerSpan),
 }
 
 struct Context<'a, 'b> {
@@ -247,13 +247,13 @@ impl<'a, 'b> Context<'a, 'b> {
         match *p {
             parse::String(_) => {}
             parse::NextArgument(ref mut arg) => {
-                if let parse::ArgumentNamed(s) = arg.position {
+                if let parse::ArgumentNamed(s, _) = arg.position {
                     arg.position = parse::ArgumentIs(lookup(s));
                 }
-                if let parse::CountIsName(s) = arg.format.width {
+                if let parse::CountIsName(s, _) = arg.format.width {
                     arg.format.width = parse::CountIsParam(lookup(s));
                 }
-                if let parse::CountIsName(s) = arg.format.precision {
+                if let parse::CountIsName(s, _) = arg.format.precision {
                     arg.format.precision = parse::CountIsParam(lookup(s));
                 }
             }
@@ -276,7 +276,7 @@ impl<'a, 'b> Context<'a, 'b> {
                 // it's written second, so it should come after width/precision.
                 let pos = match arg.position {
                     parse::ArgumentIs(i) | parse::ArgumentImplicitlyIs(i) => Exact(i),
-                    parse::ArgumentNamed(s) => Named(s),
+                    parse::ArgumentNamed(s, span) => Named(s, span),
                 };
 
                 let ty = Placeholder(match arg.format.ty {
@@ -346,8 +346,8 @@ impl<'a, 'b> Context<'a, 'b> {
             parse::CountIsParam(i) => {
                 self.verify_arg_type(Exact(i), Count);
             }
-            parse::CountIsName(s) => {
-                self.verify_arg_type(Named(s), Count);
+            parse::CountIsName(s, span) => {
+                self.verify_arg_type(Named(s, span), Count);
             }
         }
     }
@@ -533,7 +533,7 @@ impl<'a, 'b> Context<'a, 'b> {
                 }
             }
 
-            Named(name) => {
+            Named(name, span) => {
                 match self.names.get(&name) {
                     Some(&idx) => {
                         // Treat as positional arg.
@@ -548,7 +548,7 @@ impl<'a, 'b> Context<'a, 'b> {
                             self.arg_types.push(Vec::new());
                             self.arg_unique_types.push(Vec::new());
                             let span = if self.is_literal {
-                                *self.arg_spans.get(self.curpiece).unwrap_or(&self.fmtsp)
+                                self.fmtsp.from_inner(span)
                             } else {
                                 self.fmtsp
                             };
@@ -559,7 +559,7 @@ impl<'a, 'b> Context<'a, 'b> {
                         } else {
                             let msg = format!("there is no argument named `{}`", name);
                             let sp = if self.is_literal {
-                                *self.arg_spans.get(self.curpiece).unwrap_or(&self.fmtsp)
+                                self.fmtsp.from_inner(span)
                             } else {
                                 self.fmtsp
                             };
@@ -629,7 +629,7 @@ impl<'a, 'b> Context<'a, 'b> {
             }
             parse::CountImplied => count(sym::Implied, None),
             // should never be the case, names are already resolved
-            parse::CountIsName(_) => panic!("should never happen"),
+            parse::CountIsName(..) => panic!("should never happen"),
         }
     }
 
@@ -676,7 +676,7 @@ impl<'a, 'b> Context<'a, 'b> {
 
                         // should never be the case, because names are already
                         // resolved.
-                        parse::ArgumentNamed(_) => panic!("should never happen"),
+                        parse::ArgumentNamed(..) => panic!("should never happen"),
                     }
                 };
 
diff --git a/compiler/rustc_parse_format/src/lib.rs b/compiler/rustc_parse_format/src/lib.rs
index 9d653de910f..a6a2cbc277c 100644
--- a/compiler/rustc_parse_format/src/lib.rs
+++ b/compiler/rustc_parse_format/src/lib.rs
@@ -95,7 +95,7 @@ pub enum Position {
     /// The argument is located at a specific index given in the format
     ArgumentIs(usize),
     /// The argument has a name.
-    ArgumentNamed(Symbol),
+    ArgumentNamed(Symbol, InnerSpan),
 }
 
 impl Position {
@@ -147,7 +147,7 @@ pub enum Count {
     /// The count is specified explicitly.
     CountIs(usize),
     /// The count is specified by the argument with the given name.
-    CountIsName(Symbol),
+    CountIsName(Symbol, InnerSpan),
     /// The count is specified by the argument at the given index.
     CountIsParam(usize),
     /// The count is implied and cannot be explicitly specified.
@@ -494,8 +494,11 @@ impl<'a> Parser<'a> {
             Some(ArgumentIs(i))
         } else {
             match self.cur.peek() {
-                Some(&(_, c)) if rustc_lexer::is_id_start(c) => {
-                    Some(ArgumentNamed(Symbol::intern(self.word())))
+                Some(&(start, c)) if rustc_lexer::is_id_start(c) => {
+                    let word = self.word();
+                    let end = start + word.len();
+                    let span = self.to_span_index(start).to(self.to_span_index(end));
+                    Some(ArgumentNamed(Symbol::intern(word), span))
                 }
 
                 // This is an `ArgumentNext`.
@@ -662,8 +665,9 @@ impl<'a> Parser<'a> {
             if word.is_empty() {
                 self.cur = tmp;
                 (CountImplied, None)
-            } else if self.consume('$') {
-                (CountIsName(Symbol::intern(word)), None)
+            } else if let Some(end) = self.consume_pos('$') {
+                let span = self.to_span_index(start + 1).to(self.to_span_index(end));
+                (CountIsName(Symbol::intern(word), span), None)
             } else {
                 self.cur = tmp;
                 (CountImplied, None)
diff --git a/compiler/rustc_parse_format/src/tests.rs b/compiler/rustc_parse_format/src/tests.rs
index b7693a85ad9..6c960fdc72b 100644
--- a/compiler/rustc_parse_format/src/tests.rs
+++ b/compiler/rustc_parse_format/src/tests.rs
@@ -221,8 +221,8 @@ fn format_counts() {
                     fill: None,
                     align: AlignUnknown,
                     flags: 0,
-                    precision: CountIsName(Symbol::intern("b")),
-                    width: CountIsName(Symbol::intern("a")),
+                    precision: CountIsName(Symbol::intern("b"), InnerSpan::new(6, 7)),
+                    width: CountIsName(Symbol::intern("a"), InnerSpan::new(4, 4)),
                     precision_span: None,
                     width_span: None,
                     ty: "?",
diff --git a/compiler/rustc_trait_selection/src/traits/on_unimplemented.rs b/compiler/rustc_trait_selection/src/traits/on_unimplemented.rs
index bdf677a63b6..b05dbbe898a 100644
--- a/compiler/rustc_trait_selection/src/traits/on_unimplemented.rs
+++ b/compiler/rustc_trait_selection/src/traits/on_unimplemented.rs
@@ -309,23 +309,23 @@ impl<'tcx> OnUnimplementedFormatString {
                 Piece::String(_) => (), // Normal string, no need to check it
                 Piece::NextArgument(a) => match a.position {
                     // `{Self}` is allowed
-                    Position::ArgumentNamed(s) if s == kw::SelfUpper => (),
+                    Position::ArgumentNamed(s, _) if s == kw::SelfUpper => (),
                     // `{ThisTraitsName}` is allowed
-                    Position::ArgumentNamed(s) if s == name => (),
+                    Position::ArgumentNamed(s, _) if s == name => (),
                     // `{from_method}` is allowed
-                    Position::ArgumentNamed(s) if s == sym::from_method => (),
+                    Position::ArgumentNamed(s, _) if s == sym::from_method => (),
                     // `{from_desugaring}` is allowed
-                    Position::ArgumentNamed(s) if s == sym::from_desugaring => (),
+                    Position::ArgumentNamed(s, _) if s == sym::from_desugaring => (),
                     // `{ItemContext}` is allowed
-                    Position::ArgumentNamed(s) if s == sym::ItemContext => (),
+                    Position::ArgumentNamed(s, _) if s == sym::ItemContext => (),
                     // `{integral}` and `{integer}` and `{float}` are allowed
-                    Position::ArgumentNamed(s)
+                    Position::ArgumentNamed(s, _)
                         if s == sym::integral || s == sym::integer_ || s == sym::float =>
                     {
                         ()
                     }
                     // So is `{A}` if A is a type parameter
-                    Position::ArgumentNamed(s) => {
+                    Position::ArgumentNamed(s, _) => {
                         match generics.params.iter().find(|param| param.name == s) {
                             Some(_) => (),
                             None => {
@@ -392,7 +392,7 @@ impl<'tcx> OnUnimplementedFormatString {
             .map(|p| match p {
                 Piece::String(s) => s,
                 Piece::NextArgument(a) => match a.position {
-                    Position::ArgumentNamed(s) => match generic_map.get(&s) {
+                    Position::ArgumentNamed(s, _) => match generic_map.get(&s) {
                         Some(val) => val,
                         None if s == name => &trait_str,
                         None => {
diff --git a/src/test/ui/asm/bad-template.aarch64_mirunsafeck.stderr b/src/test/ui/asm/bad-template.aarch64_mirunsafeck.stderr
index 3b1d922a7f7..11c4e01f418 100644
--- a/src/test/ui/asm/bad-template.aarch64_mirunsafeck.stderr
+++ b/src/test/ui/asm/bad-template.aarch64_mirunsafeck.stderr
@@ -23,10 +23,10 @@ LL |         asm!("{1}", in(reg) foo);
    = help: if this argument is intentionally unused, consider using it in an asm comment: `"/* {0} */"`
 
 error: there is no argument named `a`
-  --> $DIR/bad-template.rs:36:15
+  --> $DIR/bad-template.rs:36:16
    |
 LL |         asm!("{a}");
-   |               ^^^
+   |                ^
 
 error: invalid reference to argument at index 0
   --> $DIR/bad-template.rs:38:15
@@ -123,10 +123,10 @@ LL | global_asm!("{1}", const FOO);
    = help: if this argument is intentionally unused, consider using it in an asm comment: `"/* {0} */"`
 
 error: there is no argument named `a`
-  --> $DIR/bad-template.rs:63:14
+  --> $DIR/bad-template.rs:63:15
    |
 LL | global_asm!("{a}");
-   |              ^^^
+   |               ^
 
 error: invalid reference to argument at index 0
   --> $DIR/bad-template.rs:65:14
diff --git a/src/test/ui/asm/bad-template.aarch64_thirunsafeck.stderr b/src/test/ui/asm/bad-template.aarch64_thirunsafeck.stderr
index 3b1d922a7f7..11c4e01f418 100644
--- a/src/test/ui/asm/bad-template.aarch64_thirunsafeck.stderr
+++ b/src/test/ui/asm/bad-template.aarch64_thirunsafeck.stderr
@@ -23,10 +23,10 @@ LL |         asm!("{1}", in(reg) foo);
    = help: if this argument is intentionally unused, consider using it in an asm comment: `"/* {0} */"`
 
 error: there is no argument named `a`
-  --> $DIR/bad-template.rs:36:15
+  --> $DIR/bad-template.rs:36:16
    |
 LL |         asm!("{a}");
-   |               ^^^
+   |                ^
 
 error: invalid reference to argument at index 0
   --> $DIR/bad-template.rs:38:15
@@ -123,10 +123,10 @@ LL | global_asm!("{1}", const FOO);
    = help: if this argument is intentionally unused, consider using it in an asm comment: `"/* {0} */"`
 
 error: there is no argument named `a`
-  --> $DIR/bad-template.rs:63:14
+  --> $DIR/bad-template.rs:63:15
    |
 LL | global_asm!("{a}");
-   |              ^^^
+   |               ^
 
 error: invalid reference to argument at index 0
   --> $DIR/bad-template.rs:65:14
diff --git a/src/test/ui/asm/bad-template.x86_64_mirunsafeck.stderr b/src/test/ui/asm/bad-template.x86_64_mirunsafeck.stderr
index 3b69186f1e1..c198e0a69dd 100644
--- a/src/test/ui/asm/bad-template.x86_64_mirunsafeck.stderr
+++ b/src/test/ui/asm/bad-template.x86_64_mirunsafeck.stderr
@@ -23,10 +23,10 @@ LL |         asm!("{1}", in(reg) foo);
    = help: if this argument is intentionally unused, consider using it in an asm comment: `"/* {0} */"`
 
 error: there is no argument named `a`
-  --> $DIR/bad-template.rs:36:15
+  --> $DIR/bad-template.rs:36:16
    |
 LL |         asm!("{a}");
-   |               ^^^
+   |                ^
 
 error: invalid reference to argument at index 0
   --> $DIR/bad-template.rs:38:15
@@ -123,10 +123,10 @@ LL | global_asm!("{1}", const FOO);
    = help: if this argument is intentionally unused, consider using it in an asm comment: `"/* {0} */"`
 
 error: there is no argument named `a`
-  --> $DIR/bad-template.rs:63:14
+  --> $DIR/bad-template.rs:63:15
    |
 LL | global_asm!("{a}");
-   |              ^^^
+   |               ^
 
 error: invalid reference to argument at index 0
   --> $DIR/bad-template.rs:65:14
diff --git a/src/test/ui/asm/bad-template.x86_64_thirunsafeck.stderr b/src/test/ui/asm/bad-template.x86_64_thirunsafeck.stderr
index 3b69186f1e1..c198e0a69dd 100644
--- a/src/test/ui/asm/bad-template.x86_64_thirunsafeck.stderr
+++ b/src/test/ui/asm/bad-template.x86_64_thirunsafeck.stderr
@@ -23,10 +23,10 @@ LL |         asm!("{1}", in(reg) foo);
    = help: if this argument is intentionally unused, consider using it in an asm comment: `"/* {0} */"`
 
 error: there is no argument named `a`
-  --> $DIR/bad-template.rs:36:15
+  --> $DIR/bad-template.rs:36:16
    |
 LL |         asm!("{a}");
-   |               ^^^
+   |                ^
 
 error: invalid reference to argument at index 0
   --> $DIR/bad-template.rs:38:15
@@ -123,10 +123,10 @@ LL | global_asm!("{1}", const FOO);
    = help: if this argument is intentionally unused, consider using it in an asm comment: `"/* {0} */"`
 
 error: there is no argument named `a`
-  --> $DIR/bad-template.rs:63:14
+  --> $DIR/bad-template.rs:63:15
    |
 LL | global_asm!("{a}");
-   |              ^^^
+   |               ^
 
 error: invalid reference to argument at index 0
   --> $DIR/bad-template.rs:65:14
diff --git a/src/test/ui/fmt/format-args-capture-issue-93378.stderr b/src/test/ui/fmt/format-args-capture-issue-93378.stderr
index 588541044fe..b8e2b2afb38 100644
--- a/src/test/ui/fmt/format-args-capture-issue-93378.stderr
+++ b/src/test/ui/fmt/format-args-capture-issue-93378.stderr
@@ -10,10 +10,10 @@ error: invalid reference to positional argument 0 (no arguments were given)
   --> $DIR/format-args-capture-issue-93378.rs:9:23
    |
 LL |     println!("{a:.n$} {b:.*}");
-   |               ------- ^^^--^
-   |               |          |
-   |               |          this precision flag adds an extra required argument at position 0, which is why there are 3 arguments expected
-   |               this parameter corresponds to the precision flag
+   |                   -   ^^^--^
+   |                   |      |
+   |                   |      this precision flag adds an extra required argument at position 0, which is why there are 3 arguments expected
+   |                   this parameter corresponds to the precision flag
    |
    = note: positional arguments are zero-based
    = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html
diff --git a/src/test/ui/fmt/format-args-capture-issue-94010.rs b/src/test/ui/fmt/format-args-capture-issue-94010.rs
new file mode 100644
index 00000000000..bd03e9c93ae
--- /dev/null
+++ b/src/test/ui/fmt/format-args-capture-issue-94010.rs
@@ -0,0 +1,7 @@
+fn main() {
+    const FOO: i32 = 123;
+    println!("{foo:X}");
+    //~^ ERROR: cannot find value `foo` in this scope
+    println!("{:.foo$}", 0);
+    //~^ ERROR: cannot find value `foo` in this scope
+}
diff --git a/src/test/ui/fmt/format-args-capture-issue-94010.stderr b/src/test/ui/fmt/format-args-capture-issue-94010.stderr
new file mode 100644
index 00000000000..ed90dc85536
--- /dev/null
+++ b/src/test/ui/fmt/format-args-capture-issue-94010.stderr
@@ -0,0 +1,20 @@
+error[E0425]: cannot find value `foo` in this scope
+  --> $DIR/format-args-capture-issue-94010.rs:3:16
+   |
+LL |     const FOO: i32 = 123;
+   |     --------------------- similarly named constant `FOO` defined here
+LL |     println!("{foo:X}");
+   |                ^^^ help: a constant with a similar name exists (notice the capitalization): `FOO`
+
+error[E0425]: cannot find value `foo` in this scope
+  --> $DIR/format-args-capture-issue-94010.rs:5:18
+   |
+LL |     const FOO: i32 = 123;
+   |     --------------------- similarly named constant `FOO` defined here
+...
+LL |     println!("{:.foo$}", 0);
+   |                  ^^^ help: a constant with a similar name exists (notice the capitalization): `FOO`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0425`.
diff --git a/src/test/ui/fmt/format-args-capture-missing-variables.stderr b/src/test/ui/fmt/format-args-capture-missing-variables.stderr
index d53c206003f..d980e7be273 100644
--- a/src/test/ui/fmt/format-args-capture-missing-variables.stderr
+++ b/src/test/ui/fmt/format-args-capture-missing-variables.stderr
@@ -7,40 +7,40 @@ LL |     format!("{valuea} {valueb}", valuea=5, valuec=7);
    |             formatting specifier missing
 
 error[E0425]: cannot find value `foo` in this scope
-  --> $DIR/format-args-capture-missing-variables.rs:2:17
+  --> $DIR/format-args-capture-missing-variables.rs:2:18
    |
 LL |     format!("{} {foo} {} {bar} {}", 1, 2, 3);
-   |                 ^^^^^ not found in this scope
+   |                  ^^^ not found in this scope
 
 error[E0425]: cannot find value `bar` in this scope
-  --> $DIR/format-args-capture-missing-variables.rs:2:26
+  --> $DIR/format-args-capture-missing-variables.rs:2:27
    |
 LL |     format!("{} {foo} {} {bar} {}", 1, 2, 3);
-   |                          ^^^^^ not found in this scope
+   |                           ^^^ not found in this scope
 
 error[E0425]: cannot find value `foo` in this scope
-  --> $DIR/format-args-capture-missing-variables.rs:6:14
+  --> $DIR/format-args-capture-missing-variables.rs:6:15
    |
 LL |     format!("{foo}");
-   |              ^^^^^ not found in this scope
+   |               ^^^ not found in this scope
 
 error[E0425]: cannot find value `valueb` in this scope
-  --> $DIR/format-args-capture-missing-variables.rs:8:23
+  --> $DIR/format-args-capture-missing-variables.rs:8:24
    |
 LL |     format!("{valuea} {valueb}", valuea=5, valuec=7);
-   |                       ^^^^^^^^ not found in this scope
+   |                        ^^^^^^ not found in this scope
 
 error[E0425]: cannot find value `foo` in this scope
-  --> $DIR/format-args-capture-missing-variables.rs:14:9
+  --> $DIR/format-args-capture-missing-variables.rs:14:10
    |
 LL |         {foo}
-   |         ^^^^^ not found in this scope
+   |          ^^^ not found in this scope
 
 error[E0425]: cannot find value `foo` in this scope
-  --> $DIR/format-args-capture-missing-variables.rs:19:13
+  --> $DIR/format-args-capture-missing-variables.rs:19:14
    |
 LL |     panic!("{foo} {bar}", bar=1);
-   |             ^^^^^ not found in this scope
+   |              ^^^ not found in this scope
 
 error: aborting due to 7 previous errors
 
diff --git a/src/test/ui/fmt/ifmt-bad-arg.stderr b/src/test/ui/fmt/ifmt-bad-arg.stderr
index acc4e95f5bb..3f1f1006713 100644
--- a/src/test/ui/fmt/ifmt-bad-arg.stderr
+++ b/src/test/ui/fmt/ifmt-bad-arg.stderr
@@ -263,34 +263,34 @@ LL |     println!("{:.*}");
    = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html
 
 error[E0425]: cannot find value `foo` in this scope
-  --> $DIR/ifmt-bad-arg.rs:27:17
+  --> $DIR/ifmt-bad-arg.rs:27:18
    |
 LL |     format!("{} {foo} {} {bar} {}", 1, 2, 3);
-   |                 ^^^^^ not found in this scope
+   |                  ^^^ not found in this scope
 
 error[E0425]: cannot find value `bar` in this scope
-  --> $DIR/ifmt-bad-arg.rs:27:26
+  --> $DIR/ifmt-bad-arg.rs:27:27
    |
 LL |     format!("{} {foo} {} {bar} {}", 1, 2, 3);
-   |                          ^^^^^ not found in this scope
+   |                           ^^^ not found in this scope
 
 error[E0425]: cannot find value `foo` in this scope
-  --> $DIR/ifmt-bad-arg.rs:31:14
+  --> $DIR/ifmt-bad-arg.rs:31:15
    |
 LL |     format!("{foo}");
-   |              ^^^^^ not found in this scope
+   |               ^^^ not found in this scope
 
 error[E0425]: cannot find value `valueb` in this scope
-  --> $DIR/ifmt-bad-arg.rs:45:23
+  --> $DIR/ifmt-bad-arg.rs:45:24
    |
 LL |     format!("{valuea} {valueb}", valuea=5, valuec=7);
-   |                       ^^^^^^^^ not found in this scope
+   |                        ^^^^^^ not found in this scope
 
 error[E0425]: cannot find value `foo` in this scope
-  --> $DIR/ifmt-bad-arg.rs:60:9
+  --> $DIR/ifmt-bad-arg.rs:60:10
    |
 LL |         {foo}
-   |         ^^^^^ not found in this scope
+   |          ^^^ not found in this scope
 
 error[E0308]: mismatched types
   --> $DIR/ifmt-bad-arg.rs:78:32
diff --git a/src/tools/clippy/clippy_lints/src/write.rs b/src/tools/clippy/clippy_lints/src/write.rs
index b0044695ea8..1fa6301ebd7 100644
--- a/src/tools/clippy/clippy_lints/src/write.rs
+++ b/src/tools/clippy/clippy_lints/src/write.rs
@@ -453,7 +453,7 @@ impl SimpleFormatArgs {
                     }
                 }
             },
-            ArgumentNamed(n) => {
+            ArgumentNamed(n, _) => {
                 if let Some(x) = self.named.iter_mut().find(|x| x.0 == n) {
                     match x.1.as_slice() {
                         // A non-empty format string has been seen already.