fix: allow_attributes false negative on attributes with whitespace (#16497)

The [`allow_attributes`] lint false-negatived (failed to trigger) on
attributes that contained internal whitespace, such as `#[ allow (
dead_code ) ]`.

This happened because `clippy_utils::is_from_proc_macro` relied on
strict string matching (e.g., expecting exactly `#[allow`), which fails
if there are spaces.

#### Solution:
I have updated `clippy_utils::is_from_proc_macro` to support flexible
whitespace matching.
1. Added `Pat::Attr(Symbol)`.
2. Updated `span_matches_pat` to strip `#[` and trim validation
whitespace before checking the name.

Verified with a new test case in `tests/ui/allow_attributes.rs`.

Fixes rust-lang/rust-clippy#16491

----

changelog: [`allow_attributes`]: correctly detect attributes with
internal whitespace

clippy_utils/src/check_proc_macro.rs

@@ -45,6 +45,8 @@ pub enum Pat {
     Sym(Symbol),
     /// Any decimal or hexadecimal digit depending on the location.
     Num,
+    /// An attribute.
+    Attr(Symbol),
 }
 
 /// Checks if the start and the end of the span's text matches the patterns. This will return false
@@ -65,12 +67,20 @@ fn span_matches_pat(sess: &Session, span: Span, start_pat: Pat, end_pat: Pat) ->
             Pat::OwnedMultiStr(texts) => texts.iter().any(|s| start_str.starts_with(s)),
             Pat::Sym(sym) => start_str.starts_with(sym.as_str()),
             Pat::Num => start_str.as_bytes().first().is_some_and(u8::is_ascii_digit),
+            Pat::Attr(sym) => {
+                let start_str = start_str
+                    .strip_prefix("#[")
+                    .or_else(|| start_str.strip_prefix("#!["))
+                    .unwrap_or(start_str);
+                start_str.trim_start().starts_with(sym.as_str())
+            },
         } && match end_pat {
             Pat::Str(text) => end_str.ends_with(text),
             Pat::MultiStr(texts) => texts.iter().any(|s| end_str.ends_with(s)),
             Pat::OwnedMultiStr(texts) => texts.iter().any(|s| end_str.ends_with(s)),
             Pat::Sym(sym) => end_str.ends_with(sym.as_str()),
             Pat::Num => end_str.as_bytes().last().is_some_and(u8::is_ascii_hexdigit),
+            Pat::Attr(_) => false,
         })
     })
 }
@@ -350,18 +360,7 @@ fn attr_search_pat(attr: &Attribute) -> (Pat, Pat) {
         AttrKind::Normal(..) => {
             if let Some(name) = attr.name() {
                 // NOTE: This will likely have false positives, like `allow = 1`
-                let ident_string = name.to_string();
-                if attr.style == AttrStyle::Outer {
-                    (
-                        Pat::OwnedMultiStr(vec!["#[".to_owned() + &ident_string, ident_string]),
-                        Pat::Str(""),
-                    )
-                } else {
-                    (
-                        Pat::OwnedMultiStr(vec!["#![".to_owned() + &ident_string, ident_string]),
-                        Pat::Str(""),
-                    )
-                }
+                (Pat::Attr(name), Pat::Str(""))
             } else {
                 (Pat::Str("#"), Pat::Str("]"))
             }

tests/ui/allow_attributes.fixed

@@ -63,6 +63,11 @@ fn msrv_1_80() {
     let x = 1;
 }
 
+#[rustfmt::skip]
+#[ expect ( dead_code ) ]
+//~^ allow_attributes
+struct Spaced;
+
 #[deny(clippy::allow_attributes)]
 fn deny_allow_attributes() -> Option<u8> {
     let allow = None;

tests/ui/allow_attributes.rs

@@ -63,6 +63,11 @@ fn msrv_1_80() {
     let x = 1;
 }
 
+#[rustfmt::skip]
+#[ allow ( dead_code ) ]
+//~^ allow_attributes
+struct Spaced;
+
 #[deny(clippy::allow_attributes)]
 fn deny_allow_attributes() -> Option<u8> {
     let allow = None;

tests/ui/allow_attributes.stderr

@@ -19,5 +19,11 @@ error: #[allow] attribute found
 LL |     #[allow(unused)]
    |       ^^^^^ help: replace it with: `expect`
 
-error: aborting due to 3 previous errors
+error: #[allow] attribute found
+  --> tests/ui/allow_attributes.rs:67:4
+   |
+LL | #[ allow ( dead_code ) ]
+   |    ^^^^^ help: replace it with: `expect`
+
+error: aborting due to 4 previous errors