1 files changed, 194 insertions, 0 deletions

diff --git a/crates/hir_ty/src/diagnostics/decl_check/case_conv.rs b/crates/hir_ty/src/diagnostics/decl_check/case_conv.rs
new file mode 100644
index 000000000..3800f2a6b
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics/decl_check/case_conv.rs
@@ -0,0 +1,194 @@
+//! Functions for string case manipulation, such as detecting the identifier case,
+//! and converting it into appropriate form.
+
+#[derive(Debug)]
+enum DetectedCase {
+    LowerCamelCase,
+    UpperCamelCase,
+    LowerSnakeCase,
+    UpperSnakeCase,
+    Unknown,
+}
+
+fn detect_case(ident: &str) -> DetectedCase {
+    let trimmed_ident = ident.trim_matches('_');
+    let first_lowercase = trimmed_ident.starts_with(|chr: char| chr.is_ascii_lowercase());
+    let mut has_lowercase = first_lowercase;
+    let mut has_uppercase = false;
+    let mut has_underscore = false;
+
+    for chr in trimmed_ident.chars() {
+        if chr == '_' {
+            has_underscore = true;
+        } else if chr.is_ascii_uppercase() {
+            has_uppercase = true;
+        } else if chr.is_ascii_lowercase() {
+            has_lowercase = true;
+        }
+    }
+
+    if has_uppercase {
+        if !has_lowercase {
+            DetectedCase::UpperSnakeCase
+        } else if !has_underscore {
+            if first_lowercase {
+                DetectedCase::LowerCamelCase
+            } else {
+                DetectedCase::UpperCamelCase
+            }
+        } else {
+            // It has uppercase, it has lowercase, it has underscore.
+            // No assumptions here
+            DetectedCase::Unknown
+        }
+    } else {
+        DetectedCase::LowerSnakeCase
+    }
+}
+
+/// Converts an identifier to an UpperCamelCase form.
+/// Returns `None` if the string is already is UpperCamelCase.
+pub fn to_camel_case(ident: &str) -> Option<String> {
+    let detected_case = detect_case(ident);
+
+    match detected_case {
+        DetectedCase::UpperCamelCase => return None,
+        DetectedCase::LowerCamelCase => {
+            let mut first_capitalized = false;
+            let output = ident
+                .chars()
+                .map(|chr| {
+                    if !first_capitalized && chr.is_ascii_lowercase() {
+                        first_capitalized = true;
+                        chr.to_ascii_uppercase()
+                    } else {
+                        chr
+                    }
+                })
+                .collect();
+            return Some(output);
+        }
+        _ => {}
+    }
+
+    let mut output = String::with_capacity(ident.len());
+
+    let mut capital_added = false;
+    for chr in ident.chars() {
+        if chr.is_alphabetic() {
+            if !capital_added {
+                output.push(chr.to_ascii_uppercase());
+                capital_added = true;
+            } else {
+                output.push(chr.to_ascii_lowercase());
+            }
+        } else if chr == '_' {
+            // Skip this character and make the next one capital.
+            capital_added = false;
+        } else {
+            // Put the characted as-is.
+            output.push(chr);
+        }
+    }
+
+    if output == ident {
+        // While we didn't detect the correct case at the beginning, there
+        // may be special cases: e.g. `A` is both valid CamelCase and UPPER_SNAKE_CASE.
+        None
+    } else {
+        Some(output)
+    }
+}
+
+/// Converts an identifier to a lower_snake_case form.
+/// Returns `None` if the string is already in lower_snake_case.
+pub fn to_lower_snake_case(ident: &str) -> Option<String> {
+    // First, assume that it's UPPER_SNAKE_CASE.
+    match detect_case(ident) {
+        DetectedCase::LowerSnakeCase => return None,
+        DetectedCase::UpperSnakeCase => {
+            return Some(ident.chars().map(|chr| chr.to_ascii_lowercase()).collect())
+        }
+        _ => {}
+    }
+
+    // Otherwise, assume that it's CamelCase.
+    let lower_snake_case = stdx::to_lower_snake_case(ident);
+
+    if lower_snake_case == ident {
+        // While we didn't detect the correct case at the beginning, there
+        // may be special cases: e.g. `a` is both valid camelCase and snake_case.
+        None
+    } else {
+        Some(lower_snake_case)
+    }
+}
+
+/// Converts an identifier to an UPPER_SNAKE_CASE form.
+/// Returns `None` if the string is already is UPPER_SNAKE_CASE.
+pub fn to_upper_snake_case(ident: &str) -> Option<String> {
+    match detect_case(ident) {
+        DetectedCase::UpperSnakeCase => return None,
+        DetectedCase::LowerSnakeCase => {
+            return Some(ident.chars().map(|chr| chr.to_ascii_uppercase()).collect())
+        }
+        _ => {}
+    }
+
+    // Normalize the string from whatever form it's in currently, and then just make it uppercase.
+    let upper_snake_case = stdx::to_upper_snake_case(ident);
+
+    if upper_snake_case == ident {
+        // While we didn't detect the correct case at the beginning, there
+        // may be special cases: e.g. `A` is both valid CamelCase and UPPER_SNAKE_CASE.
+        None
+    } else {
+        Some(upper_snake_case)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use expect_test::{expect, Expect};
+
+    fn check<F: Fn(&str) -> Option<String>>(fun: F, input: &str, expect: Expect) {
+        // `None` is translated to empty string, meaning that there is nothing to fix.
+        let output = fun(input).unwrap_or_default();
+
+        expect.assert_eq(&output);
+    }
+
+    #[test]
+    fn test_to_lower_snake_case() {
+        check(to_lower_snake_case, "lower_snake_case", expect![[""]]);
+        check(to_lower_snake_case, "UPPER_SNAKE_CASE", expect![["upper_snake_case"]]);
+        check(to_lower_snake_case, "Weird_Case", expect![["weird_case"]]);
+        check(to_lower_snake_case, "CamelCase", expect![["camel_case"]]);
+        check(to_lower_snake_case, "lowerCamelCase", expect![["lower_camel_case"]]);
+        check(to_lower_snake_case, "a", expect![[""]]);
+    }
+
+    #[test]
+    fn test_to_camel_case() {
+        check(to_camel_case, "CamelCase", expect![[""]]);
+        check(to_camel_case, "CamelCase_", expect![[""]]);
+        check(to_camel_case, "_CamelCase", expect![[""]]);
+        check(to_camel_case, "lowerCamelCase", expect![["LowerCamelCase"]]);
+        check(to_camel_case, "lower_snake_case", expect![["LowerSnakeCase"]]);
+        check(to_camel_case, "UPPER_SNAKE_CASE", expect![["UpperSnakeCase"]]);
+        check(to_camel_case, "Weird_Case", expect![["WeirdCase"]]);
+        check(to_camel_case, "name", expect![["Name"]]);
+        check(to_camel_case, "A", expect![[""]]);
+    }
+
+    #[test]
+    fn test_to_upper_snake_case() {
+        check(to_upper_snake_case, "UPPER_SNAKE_CASE", expect![[""]]);
+        check(to_upper_snake_case, "lower_snake_case", expect![["LOWER_SNAKE_CASE"]]);
+        check(to_upper_snake_case, "Weird_Case", expect![["WEIRD_CASE"]]);
+        check(to_upper_snake_case, "CamelCase", expect![["CAMEL_CASE"]]);
+        check(to_upper_snake_case, "lowerCamelCase", expect![["LOWER_CAMEL_CASE"]]);
+        check(to_upper_snake_case, "A", expect![[""]]);
+    }
+}